Table of contents :

recapitulation theory / Haeckel's biogenetic law : ontogeny recapitulates phylogeny; that is, an organism in the course of its development goes through the same successive stages as did the species in developing from the lower to the higher forms of animal life

• morphogen gradients : hypotheses on the development
• free diffusion
• argosomes (vesicle-mediated gradients) : Wnt and Hedgehog family proteins are secreted signalling molecules (morphogens) that act at both long and short range to control growth and patterning during development. Both proteins are covalently modified by lipid, and the mechanism by which such hydrophobic molecules might spread over long distances is unknown. Wingless, Hedgehog and glycophosphatidylinositol-linked proteins copurify with lipoprotein particles, and co-localize with them in the developing wing epithelium of Drosophila. In larvae with reduced lipoprotein levels, Hedgehog accumulates near its site of production, and fails to signal over its normal range. Similarly, the range of Wingless signalling is narrowed. A novel function for lipoprotein particles, in which they act as vehicles for the movement of lipid-linked morphogens and glycophosphatidylinositol-linked proteins^ref
• a cell's perception of morphogen concentration is thought to be determined by the number of active receptors, with inactive receptors making little if any contribution. Patched (Ptc), the receptor for the morphogen Hedgehog (Hh), is active in the absence of ligand and blocks the expression of target genes by inhibiting Smoothened (Smo), an essential transducer of the Hh signal. Hh binding to Ptc abrogates the ability of Ptc to inhibit Smo, thereby unleashing Smo activity and inducing target gene expression. A cell's measure of ambient Hh concentration is not determined solely by the number of active (unliganded) Ptc molecules. Instead, we find that Hh-bound Ptc can titrate the inhibitory action of unbound Ptc. This effect is sufficient to allow normal reading of the Hh gradient in the presence of a form of Ptc that cannot bind the ligand but retains its ability to inhibit Smo. These results support a model in which the ratio of bound to unbound Ptc molecules determines the cellular response to Hh^ref.
• no one knows quite how animals with flashy coats get their patterns. But an equation developed by Alan Turing in 1952 is good at producing dead ringers for zebra hides and leopard spots^ref. Now, the equation has been used to model how spots can morph to more complex patterns as an animal ages. Turing supposed that coat patterns could be produced by 2 chemicals, which he called 'morphogens', interacting on the more or less two-dimensional space of the hide. If one morphogen causes hair to turn black, for example, and another causes it to remain pale, then differences in the rates at which these chemicals diffuse through the coat, along with various reactions between the two, would determine the patterns. Turing called them 'reaction-diffusion equations'. By tweaking the variables of diffusion speed and other behaviours of the morphogens, researchers have been able to use these equations to replicate familiar coat patterns. If one morphogen makes black hairs but also activates the morphogen for paleness, for example, and it diffuses faster than the pale morphogen, this could explain one type of spot seen in some coats: a random clump of black morphogen would diffuse outwards, activating and leaving slower pale morphogens in its wake, creating a black ring. But adult big cats have much more complicated patterns. While kittens have simple spots, a close examination of a leopard-print rug will show that adult leopards have patterns more like broken circles. Adult jaguars have an even more complex pattern — polygons with small dots in the centre (Liu R., et al. Phys. Rev. E., 74. 011914 (2006)). We have to assume 2 stages of spot growth with different rules: the first to get the baby cats their spots, and the second to create the final configurations. It took them a year to find a final solution. The pattern of the jaguar was the hardest part. People who try to do the pattern with one stage only will never get it. You have to separate it into 2 stages. The difficulty of the jaguar pattern intrigues him. It would be nice to know why it is impossible to get it in a single stage. No one has found a physical manifestation of morphogens, so it is unclear how the patterns created by these equations are actually made in a real cat. Adding 2 sets of rules for the morphogens complicates the picture even further. If it is a true reflection of what happens in cats, this means something to do with the animal's age must flip a switch in the biological rules by which their coat colours behave. Maybe the parameter values could be related to the size of the animal

• morphogenetic movement : a flowing of cell groups concerned with the formation of germ layers or of organ primordia
• convergent extension (CE) of the axial mesoderm of vertebrates is an intensively studied model for morphogenetic processes that rely on cell rearrangement. It involves the intercalation of polarized cells perpendicular to the antero-posterior (AP) axis, which narrows and lengthens the tissue. The chordamesoderm of Xenopus possesses an intrinsic AP polarity that is necessary for CE, functions in parallel to Wnt/planar cell polarity signalling, and determines the direction of tissue elongation. The mechanism that establishes AP polarity involves graded activin-like signalling and directly links mesoderm AP patterning to CE^ref
• morphodifferentiation : the arrangement of formative cells in the development of tissues or organs, which leads to production of the ultimate shape of the structure.
• cytomorphosis : the series of changes through which cells go in the process of formation, development, senescence, etc.
• epithelial stem cell differentiation : the first step is the alteration of the adhesive properties of an individual cell. This is achieved by systematically changing the complement of surface adhesion molecules used to attach to its neighbors. Epithelial buds are small bulges in undifferentiated embryonic epithelial layers that develop into a variety of complex structures, including teeth, limbs, and hair follicles. The formation of the epithelial bud is caused by a decrease in E-cadherin and an increase in P-cadherin levels : only through combined noggin and Wnt3a signaling can the Lef1 transcription complex repress E-cadherin expression, an intriguing outcome since Lef1 complexes typically activate transcription in response to Wnt signaling.
• emancipation : the establishment of local autonomy within restricted fields of a developing embryo.
• presumptive region : an area of the blastula which has been proved under normal conditions to develop into a specific organ or type of tissue.
• morphometry : the measurement of the forms or structures of organisms
• morphon : an individual organism or structural unit.
• morphophysics : the study of the physical and chemical causes of development.
• morphosis : the process of formation of a part or organ.
• morphogenesia / morphogenesis : the evolution and development of form, as the development of the shape of a particular organ or part of the body, or the development undergone by individuals who attain the type to which the majority of the individuals of the species approximate. The developmental cascade of pattern formation, body plan establishment, and the architecture of mirror-image bilateral symmetry of many structures and asymmetry of some, culminating in the adult form.
• inductor : a tissue elaborating a chemical substance which acts to determine the growth and differentiation of embryonic parts
• activator : a substance that stimulates the development of a particular structure in the embryo
• organizer : a part of an embryo which so influences some other part as to bring about and direct its histological and morphological differentiation. Parts developing as a result of induction, and inducing in their turn are classified as organizers of the second grade, third grade, and so on
• primary organizer : the dorsal lip region of the blastopore.
• dorsal lip region : the mesodermal tissue around the dorsal lip of the blastopore of an amphibian; an analogous region in the human embryo is the organizer which by induction initiates and controls early development.
• secondary organizer : one of second grade, such as the optic cup, which exerts influence on the developing lens.
• tertiary organizer : one of third grade, such as the tympanic ring, which exerts influence on the tympanic membrane
• defect experiment : observation of an embryo, after destruction of a region or part, to ascertain the effect on development.

Homo sapiens development (see also physiology of reproductive apparatus)

 

• capacitation : the process by which spermatozoa become capable of fertilizing an oocyte (ovum) after it reaches the ampullary portion of the uterine tube; a series of biochemical transformations the spermatozoa undergo in the female reproductive tract. Superoxide anion production output by spermatozoa, hydroxyl radical and hydrogen peroxide initiate sperm capacitation. Cholesterol efflux => elevation of intracellular Ca²⁺ and HCO₃^- => activation of adenylyl cyclase (AC) => cAMP => activates PKA to indirectly phosphorylate certain proteins on tyrosine. Uncertainties exist as to the number of forms of AC that are present in the spermatozoon, whether they are cytosolic or bound to subcellular structures, and which physiological effectors they respond to (e.g. bicarbonate, Ca²⁺, or receptor-coupled G-proteins). While net intracellular levels of cAMP in spermatozoa depend upon the relative activities of AC and PDE, there are wide between-sample variations within species, both in basal levels and in levels attained after activation of the AC (e.g. after sperm treatment with HCO₃^-). Moreover, minor changes in bulk cAMP levels can result in large changes in cAMP-dependent functions. Finally, while cAMP levels respond very rapidly to sperm treatment by effectors such as HCO₃^- and Ca²⁺ (key components of capacitating media), there are big discrepancies between the rates of functional response. For example, enhancement of motility and collapse of phospholipid asymmetry take place within a few minutes, whereas more than 1 h of exposure to capacitating conditions is needed for cAMP-dependent protein tyrosine phosphorylation to become detectable or for the sperm population to attain a capacitated state. During capacitation, there is also an increase in protein tyrosine phosphorylation dependent actin polymerization and in the membrane-bound PLC. The main tyrosine phosphorylated proteins during the course of capacitation and fertilization are localized to the flagellum, although tyrosine phosphorylation of less abundant proteins may also be regulated in the sperm head. Spermatozoa bound to the zona pellucida and fusing with the oocyte plasma membrane are characterized by a tyrosine phosphorylated flagellum.
• sperm hyperactivation => hyperactivated motility : the movement of hyperactivated sperm appears different under different physical conditions and in different species, but basically it involves an increase in flagellar bend amplitude and, usually, beat asymmetry. Hyperactivation may be critical to the success of fertilization, because it enhances the ability of sperm to detach from the wall of the oviduct, to move around in the labyrinthine lumen of the oviduct, to penetrate mucous substances and, finally, to penetrate the zona pellucida of the oocyte. Presumably, a signal or signals exist in the oviduct to initiate hyperactivation at the appropriate time; however, none has yet been identified with certainty. There is evidence that the source of the signal is follicular fluid, yet spermatozoa are known to hyperactivate before ovulation would release the fluid into the oviduct. It requires sperm-specific voltage-gated calcium channels CatSper1 and CatSper2 in the flagellum => calcium ions interact with the axoneme of the flagellum => protein phosphorylation in the flagellum => switch on hyperactivation. The process may also involve increases in intracellular cAMP, which at least is required to support motility in general. The edding of antioxidant enzymes inhibits the spontaneous and induced sperm hyperactivation : anyway high doses of reactive oxygen species block the sperm motility through the inhibition of ATP synthesis by the mitochondrial enzymes and cell membrane compounds injury. Although hyperactivation often occurs during the process of capacitation, divergent pathways regulate the 2 events. Sperm hyperactivation might be a moiety of capacitation or the result of capacitation.
• 
• conceptus : the sum of derivatives of a fertilized oocyte (ovum) at any stage of development from fertilization until birth, including extraembryonic membranes as well as the embryo or fetus
• gestational age (GA) : age of conceptus or pregnancy. In human clinical practice, pregnancy is timed from onset of the last normal menstruation and lasts 40 weeks (280 days). Elsewhere the onset may be timed from estrus, coitus, artificial insemination, vaginal plug formation, fertilization, or implantation.
• menstrual age : conceptus age defined by the time elapsed since the onset of the mother's last normal menstruation
• postovulatory age : conceptus age defined by the time elapsed since release of the oocyte from the ovary.
• coital age : the age of a conceptus defined by the time elapsed since the coitus that led to fertilization.
• conceptional or fertilization age : conceptus age defined by the time elapsed since fertilization. In human clinical practice pregnancy lasts 38 weeks (266 days).
• ovum : the term was formerly extended to include any early stage of the conceptus.
• Bryce-Teacher ovum : a human embryo that was thought to be the youngest known ovum at the time of its study in 1908; now known to be a pathological specimen.
• Miller ovum : an embryo 10 or 11 days old, first described in 1913.
• Peters' ovum : an embryo about 13 or 14 days old, first described in 1899.
• Mateer-Streeter ovum : an embryo about 18 days old, first described in 1920.
• Hertig-Rock ova : 34 fertilized ova, ranging from 1 to 17 days of age, 21 of which were normal, and 13 abnormal to one degree or another; discovered between 1938 and 1953, they constitute the only series of such early human conceptuses in existence.
• fertilization : of the 200-300 million sperms that cross the vaginal starting line, only 250 successfully hurdle the uterine cervix and uterus. The hardy survivors face a grueling swim up the effectively endless Fallopian tube thanks to thermotaxis (the upper reaches cradling the egg are around 2°C warmer than those at the entrance). Once in the right vicinity, sperm are thought to switch on chemotaxis, sniffing out chemicals secreted by the egg. Without this navigation mechanism in the female genital tract the chances of chaotic collision is negligible. Fertilization occurs in the outer third of the Fallopian tube. The egg remains able to be fertilized for 24 hours after ovulation
• oosperm : a recently fertilized oocyte
• dispermy : the penetration of 2 sperms or spermatozoa into one oocyte.

• primary zona pellucida (ZP) binding :
• on egg cells : ZP glycoproteins. Sperm binding to ZP causes further activation of cAMP/PKA and PKC, respectively. PKC opens a calcium channel in the plasma membrane. PKA together with IP₃ activate Ca²⁺ channels in the outer acrosomal membrane, which leads to an increase in cytosolic Ca²⁺. The depletion of Ca²⁺ in the acrosome will activate a store-operated Ca²⁺ entry mechanism in the plasma membrane, leading to a higher increase in cytosolic Ca²⁺, resulting in F-actin dispersion which enable the outer acrosomal and the plasma membrane to come into contact and fuse completing the acrosomal reaction.
• ZP1
• ZP2
• ZP3
• ZP4
• on sperm cell
• galactosyltransferase
• SP56
• zona receptor kinase
• zonadhesin
• spermadhesins are classified according to their N-terminal sequence :
• AQN spermadhesins
• AWN spermadhesins
• acrosome reaction : a sequence of structural changes that occur in spermatozoa when in the vicinity of an oocyte in the oviduct or uterine tube, and that are believed to facilitate entry of a spermatozoon into the oocyte: the outer membrane of the acrosome fuses at multiple points with the overlying plasma membrane of the sperm head, creating openings through which the hydrolases of the acrosome are exocyted.
• secondary sperm-oocyte binding
• PH20 hyaluronidase / sperm adhesion molecule 1 (SPAM1)
• acrosin
• SP38 / zona pellucida-binding protein (ZPBP)
• SP17
• ADAMs (family of proteins with a disintegrin and metalloprotease domain), including ADAM1 / fertilin a, ADAM2 / fertilin b and cyritestin 1 / ADAM3A, and cyritestin 2 / ADAM3B, have been investigated and found to be important for binding rather than for fusion and painstaking studies have raised suspicions that their putative receptors, oocyte integrins, are necessary for the sperm-oocyte interaction.
• sperm-oocyte fusion : protein phosphorylation in the flagellum. It depends on molecules ...
• on egg membrane
• a₆b₁ integrin
• CD9, an oocyte tetraspanin^{ref1, ref2}
• glycosylphosphatidylinositol (GPI)-anchored proteins
• on sperm membrane
• Izumo (a novel protein of the immunoglobulin superfamily recognized by OBF13 mAb^ref)^ref
• DE-like / acidic epididymal glycoprotein-like 1
• fertilin b / ADAM2
Nucleoplasmin 2 (NPM2), a growing oocyte-specific nuclear protein, is crucial in chromatin organization and post-fertilization development : analysis of fertilization and post-fertilization events in vitro and in vivo revealed impaired early embryo development, mostly due to failure of the one-cell to two-cell transition, as evidenced by gross defects, including an absence of coalesced nucleolar structures and loss of heterochromatin and deacetylated histone H3 that normally circumscribe nucleoli in oocytes and early zygotes, respectively.
The existence of a complex population of mRNA in human sperm is well documented but their role is not yet elucidated. Using discontinuous density gradients, we have isolated high and low motile sperm from the same semen sample. The levels of different transcripts coding for molecules either involved in nuclear condensation (protamines 1 and 2) or in capacitation (eNOS, nNOS and c-myc) were then assessed in the 2 populations using semi-quantitative RT-PCR. Sperm viability was estimated by eosin-nigrosin staining and by hypo-osmotic swelling test (HOST); apoptosis percentage was measured by the TUNEL technique. The contamination by somatic and germ cells was assessed by looking for specific molecular markers of these cells, respectively CD45 and E-cadherin for somatic cells and c-kit for germ cells. The viability of sperm was unchanged in high and low motile fractions, as well as DNA fragmentation percentage. The amount of Prm-1 mRNA was significantly higher in low density motile than in the high motile fraction. In most of high motile sperm samples eNOS and nNOS transcripts were undetectable whereas they were present in the low motile sperm. In contrast, no significant variation was found in the c-myc/Prm-2 mRNA ratio between the two populations. Moreover, a partial or complete disappearance of c-myc transcripts was observed after capacitation. Thus analysing mRNA profiles could be helpful as a diagnostic tool and prognosis value for fertilization^ref.
The mRNA must exist before the single fertilized zygote has divided into 4 and then 8 cells : after that, you have activation of the embryonic genome, and that takes over. The egg has its own store of mRNA, and now it has been shown that the sperm has its own store of paternal mRNAs delivered to the egg at fertilization : 6 transcripts present in sperm, but not in unfertilized eggs, were identified and the delivery of 2 of them—clusterin and protamine-2—into eggs was followed using RT-PCT^ref. This explains the difficulties in human parthenogenesis. While mitochondria from the sperm are introduced into the oocyte at fertilization, almost all of the mitochondria are excluded during the first cleavage, and so these mRNAs may not be useful. When one made cloned animals, one wouldn't be delivering those mRNAs. This fingerprint technology could provide a screen for different toxicological effects to which a male may have been exposed.
• sperm aster : the centriole, with astral rays, that precedes the male pronucleus during fertilization
• fertilization membrane : a strong membrane formed around the fertilized ovum in some species of animals by adhesion of part of the contents of the cortical granules to the inner surface of the vitelline membrane; it prevents the entry of additional spermatozoa.
polyspermy / polyspermia : fertilization of 1 oocyte by > 1 spermatozoon
• physiological polyspermy : entrance of > 1 spermatozoon into an oocyte, occurring normally in certain species, but with only one spermatozoon participating fully in the development of the embryo
• pathological polyspermy : entrance of > 1 spermatozoon into a mature oocyte when entrance of only one is the rule; usually development is abnormal and the embryo is not viable. Eggs are often bombarded by innumerable sperm and all but one—the first to fuse with the egg—must be repelled from the surface by rapidly modifying its cell surface and extracellular matrix. Multiple variations to this modification are found throughout phylogeny^ref. The major factor blocking other male gametes from entering the zygote is the depolarization of the membrane. That is followed by the transformation of the vitelline layer around the embryo into a hardened fertilization envelope (FE), which insulates the egg in a more general manner from its environment. The driving force of this transition is secretion of the cortical granules. At least 3 discrete steps are involved with the formation of this extracellular barrier :
• following cortical granule exocytosis, an autoactivating serine protease severs plasma membrane attachments to the egg's vitelline layer, allowing the separation of this matrix from the egg surface^ref
• the structural components of the FE, primarily derived from the cortical granules^{ref1, ref2}, self-assemble into the vitelline layer scaffold and form a distinct pattern of interwoven fibers^ref
• the cortical granule-derived enzyme ovoperoxidase is targeted to the FE by the tethering protein proteoliaisin^ref.
As the structural proteins self-polymerize into fibers, ovoperoxidase activity increases in response to alkalization^ref as synthesis of one of its substrates, hydrogen peroxide (H₂O₂), increases^{ref1, ref2}. The catalytic activity of ovoperoxidase peaks within 5 min following fertilization, resulting in the formation of covalent o-o-di-tyrosine bonds between FE proteins^{ref1, ref2, ref3}. These cross-links brace the fibrous network into a solid structure, “hardening” it to protect the embryo. An essential substrate for ovoperoxidase, and a likely component involved with cell signaling, H₂O₂ is produced by a calcium-dependent mechanism involving the reduction of one molecule of O₂ and the oxidation of 2 proton donors—hereby referred to as “oxidase activity”—whose peak output occurs at about 7–8 min postfertilization^{ref1, ref2}. This temporal correlation between peak  H₂O₂ production and ovoperoxidase activity is not coincidental; both have likely evolved in a tightly regulated system in order to limit the potentially lethal effects of H₂O₂ on the embryo without compromising FE integrity. The existence of oxidase activity was first documented while measuring O₂ consumption following fertilization^ref. Although originally ascribed to oxidative phosphorylation—hence its misnomer as a “respiratory burst”—the majority of the O₂ consumed was later attributed to the synthesis of H₂O₂. This phenomenon was subsequently likened to the respiratory burst associated with activated neutrophils^{ref1, ref2}. Neutrophils use a complex of intramembraneous and cytosolic proteins to regulate the oxygen reductase activity of their main player, cytochrome b558, a heterodimeric complex of p22^PHOX and gp91^PHOXref. The H₂O₂ synthesized by the neutrophil is used by myeloperoxidase to synthesize the hypochlorous acid (HOCl) that kills bacteria^ref. The polymerization of this matrix is catalyzed by the enzyme ovoperoxidase, whose indispensable substrate is peroxide. Urchin dual oxidase (Udx1), expressed in the cell membrane of developing oocytes, is the enzyme responsible for the typical "respiratory burst" observed at fertilization in purple urchin (Strongylocentrotus purpuratus) and green sea urchin (Lytechinus variegatus). During this burst, extracellular O₂ is turned into 60 nM  of H₂O₂, which is released into the perivitelline space around each zygote. While other cells known to produce H₂O₂—such as neutrophils, which release peroxide to kill invading organisms—usually do not survive this synthesis, the embryo remains immune to peroxidase. That's why the enzyme in the egg has a dual role: at one end it's making peroxide, and at the other it's destroying it. So it pours the peroxide outside of the cell but also doesn't let it get inside the cell. A similar peroxidase activity can be found in a variety of animal eggs, such as those of mice, amphibians, and mosquitoes^ref. Why would a dual oxidase be favored for zygotic oxidase activity over the multitude of other animal reductases? One key feature might be its paradoxical and modular functions. Phylogenetic analysis of each catalytic domain shows that the dual oxidase animal reductase domain is most closely related to the yeast ferric reductase FRE while its peroxidase is most similar to catalase, an enzyme responsible for the neutralization of H2O2 to H2O and O2^ref. Our tree suggests that the enzymatic domains of the dual oxidases also have opposing activities: calcium-dependent, pH-sensitive activation of the animal reductase domain converts extracellular O2 to H₂O₂^ref. Meanwhile, the catalase-like peroxidase domain could reverse this process to neutralize the toxic reactive oxygen species (ROS) before it can penetrate the cell. The benefits of this antagonism can be seen by comparing the H₂O₂ production of thyrocytes and eggs, both utilizing dual oxidases, to mammalian neutrophils, utilizing gp91PHOX/NOX cytochromes : in thyrocytes, the calcium-sensitive p138Tox/Duox2 generates H₂O₂ necessary for a lumenal thyroid peroxidase to conjugate iodide ions to tyrosine residues on thyroglobulin, generating an active hormone that regulates metabolism^ref. In the sea urchin egg, Udx1 is primarily responsible for generating H₂O₂ necessary for ovoperoxidase-dependent cross-linking of fertilization envelope (FE) proteins, thereby completing a competent block to polyspermy^{ref1, ref2}, although the H₂O₂ generated can be spermicidal alone^{ref1, ref2} or in combination with free halides and active peroxidases^{ref1, ref2}. In thyrocytes and fertilized eggs, both normally irreplaceable and nonrenewable cell types^{ref1, ref2}, the dual oxidase's catalase domain may be a first defense against H₂O₂ that diffuses toward the cell surface. A common second defense is cytoplasmic enrichment in ROS scavengers such as cytoplasmic catalase, glutathione, and glutathione peroxidase or its functional equal, ovothiol^{ref1, ref2, ref3}. Neutrophils, on the other hand, directly synthesize superoxide anions (O2?) that dismutate to H₂O₂ within the phagosome, where is it used to destroy foreign invaders^{ref1, ref2, ref3}. The intracellular concentration of H₂O₂ reaches toxic levels, resulting in self-destruction—an acceptable outcome considering that the pathogen is eradicated, the neutrophil is replenished by hematopoetic stem cells, and macrophages clear the resultant debris. The insignificance of variable cytosolic glutathione peroxidase activity on neutrophil performance is also consistent with the dispensability of these cells^ref. Thus, the net diffusion of H₂O₂ away from the cell surface by dual oxidases, versus a self-destructive phenotype, is favorable for periodically stimulated cells that must remain viable throughout an organism's life. Our model does not, however, limit the dual oxidase's peroxidase domain from other activities that might make the paired enzymes more versatile, such as protein cross-linking^ref. The maternal pool of Udx1 transcript in zygotes and the reserve pool of Udx1 protein in granules not associated with the primary, fertilization-activated population at the egg cortex suggest that this protein may be utilized later in embryogenesis. One possible role for this enzyme is as a source of cell signaling downstream of calcium. Reactive oxygen species, particularly H₂O₂, have recently been identified as essential molecules involved with autocrine and paracrine signaling^{ref1, ref2}, particularly in cell cycle regulation^{ref1, ref2} and proliferation^{ref1, ref2}. Given that calcium transients are necessary for passage through mitosis and differentiation^{ref1, ref2, ref3}, we postulate that the oxidase activity of Udx1 may also play an active role in triggering H₂O₂-dependent signaling during embryogensis. Regulation of oxidase activity would most likely occur through the dual oxidase's paired EF-hand domains that are most identical to the visinin-like protein subfamily of neuronal calcium sensing proteins, such as neurocalcin and hippocalcin^ref. Calcium sensor proteins in this calmodulin-like subfamily display free calcium affinities in the low nanomolar to micromolar concentrations and interact with other signaling proteins through hydrophobic residues exposed upon calcium chelation^{ref1, ref2}. Therefore, multiple mechanisms would ensure that H₂O₂ synthesis never achieves the same flurry as the oxidative burst at fertilization: catalase activity of Udx1, low intracellular calcium concentrations^{ref1, ref2}, constitutively elevated cytosolic pHi^{ref1, ref2}, and regulation by PKC could all serve to modulate local concentrations of H₂O₂. In turn, localized or prolonged exposure to low concentrations of H₂O₂ may act to stimulate specific cell signaling cascades^{ref1, ref2, ref3}. A survey of the heme peroxidase family identifies a cluster of ovoperoxidase-like homologs in many invertebrates^ref. In the sea urchin, this protein is expressed exclusively during oogenesis^ref and requires H₂O₂ synthesis to cross-link the FE constituents^{ref1, ref2}. Peroxidase activity has also been identified in the oocytes and eggs of many other species at fertilization: in mouse cortical granules and to the outer surface of the zona pellucida^ref, in the fertilization capsule of Discoglossus pictus^ref, in the chorion of the fish Tribolodon hakonensis^ref, and in the eggshells of mosquitoes^ref. Each of these peroxidases requires a source of H₂O₂. Extrapolating from observations here, as well as the century-old history of the “respiratory burst” at fertilization in sea urchins, we postulate that a similar dual oxidase-dependent mechanism is functional at fertilization throughout phylogeny.
Comparison of different peroxidase-NAD(P)H oxidase systems

• (A) Unrooted phylogram of peroxidase (left) and animal reductase (right) domains of dual oxidases compared to single-domain enzymes. Numbers are bootstrap values for each node. Class colors correspond to specific enzymatic modules (B–D). Bt, B. taurus; Ce, C. elegans; Ci, C. intestinalis; Dr, D. rerio; Dm, D. melanogaster; Hr, H. roretzi; Hs, H. sapiens; Le, L. esculentum; Lv, L. variegatus; Mm, M. musculus; Nb, N. benthamiana; Oc, O. cuniculus; Rn, R. norvegicus; S. pombe, S. pombe; St, S. tuberosum; Ss, S. scrofa. Cat/CTL, catalase; Duox, dual oxidase; EosP, eosinophil peroxidase; FRE, ferric reductase (yeast); gp91, gp91^PHOX homolog; HRP, horseradish peroxidase; LacP, lactoperoxidase; MyeP, myeloperoxidase; NOX, NADPH oxidase family member; OvoP, ovoperoxidase; Per, peroxidase; Rboh, respiratory burst oxidase homolog.
• (B) Neutrophil cytochrome b558-myeloperoxidase system.
• (C) Sea urchin egg Udx1-ovoperoxidase machinery.
• (D) Thyroid NAD(P)H oxidase-thyroid peroxidase system.
Abbreviations: PKC, protein kinase C; R-Y, extracellular matrix protein with a reactive tyrosine residue; cAMP, cyclic adenosine monophosphate; PKA, protein kinase A; Tg-Y, thyroglobulin with a reactive tyrosine residue; p138Tox/Duox2, thyroid NADPH oxidase/dual oxidase. H₂O₂, hydrogen peroxide; NAD(P)+/H, nicotinamide adenine dinucleotide (phosphate) in oxidized (-+) and reduced (-H) forms. For dual oxidases, note the peroxidase domain (star) is distinct from the generic animal reductase domain (circle).
• zygote : the fertilized ovum; the cell resulting from union of a male and a female gamete (sperm and ovum). More precisely, the cell after synapsis at the completion of fertilization until first cleavage. Also, used loosely to refer to the fertilized ovum and early derivatives for an indefinite period. It remains for 2-3 days in the ampulla of Fallopian tube and, after 3-5 days, reaches the endometrium.
• nucleopetal movement : the movement of a male pronucleus toward the female pronucleus in the zygote.
• copulation path : the course taken by the male and female pronuclei as they approach each other in a fertilized ovum.
‡ morulation => elementary, cleavage or embryonic cell / blastomere : any one of the cells derived from the zygote by mitosis
• mesentomere : a blastomere not yet divided into ...
• mesomere : a blastomere of size intermediate between a macromere and a micromere or a midzone of the mesoderm between the epimere and hypomere.
• entomere : a blastomere destined to become endoderm.
• cleavage / segmentation : the mitotic segmentation of the zygote, the size of the structure remaining unchanged, as the cleavage cells, or blastomeres, become smaller and smaller with each division.
• accessory cleavage : peripheral cleavage in telolecithal oocytes due to polyspermy.
• adequal cleavage : a form in which the blastomeres are practically equal in size.
• determinate cleavage : cleavage following a precise pattern, each blastomere having a characteristic and unalterable fate, i.e., each blastomere becoming the precursor of a definite part of the embryo.
• discoidal cleavage : cleavage limited to the animal pole of highly telolecithal oocytes.
• equal cleavage : a form in which the blastomeres are equal in size.
• equatorial cleavage : cleavage that occurs in a plane passing through the equator of the oocyte.
• complete, total or holoblastic cleavage : a form in which the entire oocyte participates in cell division
• indeterminate cleavage : that following a less rigid cleavage pattern, the blastomeres having more developmental possibilities than they usually show, each of which, when isolated, being capable of developing into a normal embryo.
• latitudinal cleavage : cleavage in planes passing at right angles to the oocyte axis.
• meridional cleavage : cleavage in planes passing through the oocyte axis.
• incomplete, partial or meroblastic cleavage : a form in which only the protoplasmic portions of the oocyte participate
• progressive cleavage : in the formation of spores within a sporangium, the production of a series of cleavage planes in succession, resulting first in formation of protospores and later in formation of sporangiospores.
• radial cleavage : a cleavage pattern characteristic of vertebrates and echinoderms, in which the spindle axes are parallel or at right angles to the polar axis of the oocyte.
• spiral cleavage : a cleavage pattern characteristic of such invertebrates as annelids and mollusks, in which the cleavage planes are oriented obliquely to the polar axis of the oocyte.
• superficial cleavage : a form in which only the surface region of centrolecithal oocytes participate.
• unequal cleavage : a form in which the blastomeres about the vegetal pole remain larger in size than those nearer the animal pole
The first cleavage plane coincides with the plane defined by the 2 apposing pronuclei once they have moved to the centre of the egg : the microtubule networks that allow mixing of parental chromosomes before dividing into two may be involved in these processes. In mouse the second polar body does not mark a stationary animal pole, but instead, in half of the embryos, moves towards a first cleavage plane^ref. The embryonic–abembryonic axis of the blastocyst arises perpendicular to the first cleavage plane, and hence to the animal–vegetal axis of the zygote. In most mouse embryos the progeny of one two-cell blastomere primarily populate the embryonic part of the blastocyst and the progeny of its sister populate the abembryonic part. However, it is not known whether the embryonic–abembryonic axis is set up by the first cleavage itself, by polarity in the oocyte that then sets the first cleavage plane with respect to the animal pole, or indeed whether it can be divorced entirely from the first cleavage and established in relation to the animal pole. The importance of the orientation of the first cleavage was tested by imposing an elongated shape on the zygote so that the division no longer passes close to the animal pole, marked by the second polar body. Non-invasive lineage tracing shows that even when the first cleavage occurs along the short axis imposed by this experimental treatment, the progeny of the resulting two-cell blastomeres tend to populate the respective embryonic and abembryonic parts of the blastocyst. Thus, the first cleavage contributes to breaking the symmetry of the embryo, generating blastomeres with different developmental characteristics^ref.
• morula : the solid mass of blastomeres formed by cleavage of a zygote.
• embryogenesis (see also pregnancy for extraembryonic maternal tissues involved in development) : the production of an embryo
• embryo : in animals, those derivatives of the zygote that will eventually become the offspring, during their period of most rapid development, i.e., from the time the long axis appears until all major structures are represented. In humans, the developing organism from the fourth day after fertilization to the end of the eighth week
• embryology : the science of the development of the individual during the embryonic stage and, by extension, in several or even all preceding and subsequent stages of the life cycle.
• comparative embryology : embryology applied with a comparative view to various species studied with reference to their taxonomy and the principle that ontogeny recapitulates phylogeny.
• descriptive embryology : the study of embryos and fetuses and their components with reference to anatomical and chronological sequence so as to define stages and report the course of development.
• causal or experimental embryology : analysis of the factors and relations in development, obtained by subjecting embryos to experimental procedures
• differential susceptibility : nonhomogeneity in response by the various regions of an embryo when subjected to a diffusely applied injurious agent
• blastocyst : the mammalian conceptus in the post-morula stage; it is like a blastula in having a fluid-filled cavity, unlike it in having the surface layer not exclusively embryoblast but mainly or entirely trophoblast, in having an eccentric embryoblast, and in not being limited to one germ layer. It undergoes implantation / nidation in the endometrium
• inner cell mass / embryoblast / embryonary pole : an aggregation of cells at the embryonic pole of the blastocyst, which is destined to form the embryo proper
• formative cell : a cell of the inner cell mass of the conceptus, a blastomere destined to form a part of the embryo
• blastula / blastosphere : the usually spherical structure produced by cleavage of a zygote, consisting of a single layer of cells (blastoderm) surrounding a fluid-filled cavity (blastocoele)
• terminal sinus : a vein which encircles the vascular area in the blastoderm
• depula : the developing egg in the stage succeeding the blastula and preceding the gastrula.
• gastrula : that early embryonic stage which follows the blastula or blastocyst. The simplest type consists of 2 layers, the ectoderm and the mesentoderm, and of 2 cavities, one lying between the ectoderm and the endoderm; the other (the archenteron) formed by invagination so as to lie within the entoderm and having an opening (the blastopore). In human embryos the gastrula stage occurs during the third week, as the embryonic disc becomes trilaminar.
• exogastrula : a gastrula in which invagination is hindered and the mesentoderm bulges outward.
• entypy : a method of gastrulation in which the endoderm lies external to the amniotic ectoderm.
• exogastrulation : the evagination to the exterior (or turning inside out) of the gut due to an interference with the normal processes of gastrulation, which can occur if the morula is cut transversely below the equator. It is usually followed by a migration of mesenchyme cells into the interior.
• mesentoderm : the inner layer of an amphibian gastrula not yet separated into mesoderm and entoderm.
• archenteron / coelenteron / gastrocoele / primitive gut : the primordial digestive cavity of those embryonic forms whose blastula becomes a gastrula by invagination
• blastopore / archistome / protostoma / anus of Rusconi : the opening of the archenteron to the exterior of the embryo, at the gastrula stage
• Ecker's plug : a plug of cells in the primordial mouth of the gastrula.
• enteron : 1. digestive tract.  2. intestinum tenue
• enterocoelic pouch : a diverticulum of the enteron of the embryo
• pharyngeal septum / buccopharyngeal membrane : the transitory partition which separates the mouth cavity from the pharynx in the embryo
• tracheoesophageal septum : the septum that, during the fourth week of embryonic development, separates the trachea from the ventral surface of the foregut (the primordial esophagus).
• Douglas' septum : the septum formed by the union of Rathke's folds, forming the rectum of the fetus
• embryonic stem (ES) cells (ESCs)
• twins : one of 2 offspring produced in the same pregnancy
• dizygotic, binovular, dissimilar, false, fraternal, heterologous, hetero-ovular, two-egg, and unlike twins : 2 offspring developed from 2 zygotes that resulted from fertilization of 2 ova fertilized at the same time; they may be of the same or different sex, and they have different genomes
• monozygotic, enzygotic, identical, mono-ovular, monovular, similar, true, or uniovular twins : 2 offspring developed from 1 zygote that divided into 2 embryos at an early stage (usually during the first 8 days); the twins therefore have identical genomes
• dichorionic twins : the separate cells of the 2-celled embryo were to dissociate, each continuing on in development
• diamnionic or diamniotic twins : twins developing within separate amniotic cavities
Laboratory examinations : twin peak sign : a sonographic sign of dichorionic twinning consisting of a triangular zone whose echotexture is similar to that of the placenta, wider at the chorionic surface of the placenta and tapering to a point within the intertwin membrane.

• monochorionic or monochorial twins : the inner cell mass of a normal blastocyst, which gives rise to the embryonic axis, were to split into 2, instead of 1, masses in the blastocyst, so that twins develop with a single chorion
• monoamnionic or monoamniotic twins : twins developing within a single amniotic cavity
• diamnionic or diamniotic twins : twins developing within separate amniotic cavities
• fetal placenta / placenta foetalis / pars fetalis placentae : the nonmaternal part of the placenta, derived not from the fetus but from the trophoblast that envelops the fetus; from within outward, it consists of
• trophoblast : a layer of extraembryonic ectodermal tissue on the outside of the blastocyst. It attaches the blastocyst to the endometrium of the uterine wall and supplies nutrition to the embryo. From it are derived the chorion and amnion. The mesoblast, once thought to be trophoblastic, is now traced in primates to the caudal end of the primitive streak.
• cytotrophoblast / Langhans' layer : the inner cellular layer of the trophoblast covering a chorionic villus; it consists of ...
• cytotrophoblastic cells / Langhans' cells : polygonal, mononucleate cells resembling the cells of the cytotrophoblast, having prominent nucleoli and clear, eosinophilic or cyanophilous cytoplasm; one of the 2 cell types that compose a choriocarcinoma. It secretes :
• GnRH / LHRH
• CRH
• TRH
• somatostatin (SST)
• growth hormone (GH)
• neuropeptide Y (NPY)
• dinorphins
• IL-1
• TNF-a
• EGF
• FGF
• PDGF
• syncytiotrophoblast : the outer syncytial layer, which consists of ...
• syncytiotrophoblastic cells : large, multinucleate cells resembling the syncytiotrophoblast, having hyperchromatic nuclei (syncytial knots : protuberances of nuclei along the chorionic villi) and abundant eosinophilic, sometimes vacuolated cytoplasm; one of the cell types that compose a choriocarcinoma. It secretes
• progesterone (from maternal cholesterol, then secreted into both maternal and fetal bloodstreams)
• 17b-estradiol / E₂ and estriol (due to placental lack of 17a-hydroxylase, they are synthetised from androgenetic precursors in maternal circulation up to pregnancy wk 20, then from fetal adrenal precursors (DHEA-S) processed by placental aromatase to estrone or estradiol or by hepatic 16a-dehydroxylase and placental sulfatase into dehydroxyepiandrosterone, then converted into estriol by aromatase)
• human chorionic gonadotropin (hCG)
• human placental lactogen (hPL) / human chorionic somatomammotropin (hCS)
• ACTH
• extraembryonic or fetal membranes : the trophoblastic parts of the conceptus that provide for the support of the embryo or fetus by attachment, mechanical protection, endocrine action, and the mediation of chemical exchange with the maternal circulation. They include :
• amnion : the thin but tough extraembryonic membrane of reptiles, birds, and mammals that lines the chorion and contains the embryo and later the fetus, with the amniotic fluid around it; in mammals it is derived from trophoblast by folding or splitting.
• amniotic villus : one of the irregular, flat, opaque areas of imperfect skin on the amnion near the distal end of the umbilical cord.
• amniotic sac / bag of waters : the sac formed by the amnion
• amniotic fluid / aqua amnii / liquor amnii / waters : fluid within the amniotic cavity produced by the amnion during the early embryonic period, and later by the lungs and kidneys; at first crystal clear, it later becomes cloudy. It protects the embryo and fetus from injury; fast turnover; protective mechanical function, mobility and constant thermal environment
• amniotic fluid volume (AFV) = 500-1,500 mL in the last 10 weeks of pregnancy
• in the first half of the second trimester AFV progressively increases by 10 mL/day unitl it reaches on average 500 mLat week 20, and 700 mL and 1,000 mL in the early and mid-third trimester, respectively, before decreasing to 800-900 mL at term
• 817 mL at 30-week gestation (range : 318-2,100 mL)
• oxalate = 19.0 (4.3) mmol/L
• liquor chorii : a fluid which separates the amnion from the chorion in the early stages of gestation.
• chorion / chorial mesoblast / secundina uteri : in human embryology, the cellular, outermost extraembryonic membrane, composed of trophoblast lined with mesoderm; it develops villi about 2 weeks after fertilization, is vascularized by allantoic vessels a week later, gives rise to the placenta, and persists until birth.
• chorion frondosum / shaggy or villous chorion : the region of the chorion that bears chorionic villi : one of the threadlike projections growing in tufts on the external surface of the chorion
• primitive or primordial chorion : the chorion from its inception by addition of mesoderm to trophoblast through the stage in which it has many primordial villi.
• chorionic plate : the part of the inner chorionic wall in the area of its uterine attachment, which gives rise to chorionic villi
• subchorial lake or space : the portion of the placenta, relatively free of villi, just beneath the chorionic plate; at the edge of the placenta it becomes continuous with irregular channels to form the marginal lakes : discontinuous venous lacunae, relatively free of villi, near the edge of the placenta, formed by merging of the marginal portions of the intervillous space with the subchorial lake. Called also marginal sinus, because it was thought to be circumferentially continuous and important for placental drainage.
• primary villus : one of the earliest chorionic villi, composed of trophoblast only
• secondary villus : an intermediate stage of chorionic villi, having a core of connective tissue (mesoblast) covered with trophoblast
• stem or tertiary villus : one of the definitive type of chorionic villi, having trophoblastic cover, connective tissue (mesoblastic) core, and blood vessels
• branch villus : a branch of a tertiary (stem) villus through which the main transport of substances between the mother and fetus occurs.
• free or feeding villus : a chorionic villus that projects into the intervillous space : the space of the placenta into which the chorionic villi project and through which maternal blood circulates
• intervillous or trophoblastic lacuna : one of the blood spaces of the placenta in which the fetal villi are found
• blood lacuna : any one of the blood-filled spaces in the syncytiotrophoblast of the embryo that serve hemotrophic nutrition.
• anchoring villus / villus barbicans : a chorionic villus that attaches to the decidua basalis
• placental septum : decidual tissue that divides the placenta into cotyledons : any one of the subdivisions of the uterine surface of a discoidal placenta (one of the tufted areas of a ruminant's placenta)
• fetal cotyledons : 200 at end of pregnancy
• maternal cotyledons : 15-20 at end of pregnancy
Within pregnancy week 10-12 cytotrophoblast cells invade the distal portion of spiral arteries up to the border between decidua and myometrium, replacing intimal and muscolar coats (intravascular extravillous cytotrophoblast). Since pregnancy week 14-16 a second invasion wave replaces the intimal and muscular coats of spiral arteries also in the intramyometrial tract, creating unreactive, distendible, low-resistance vessels (incomplete replacement causes preeclampsia (PE)/eclampsia) : uterine blood flow increases fromm 50 mL/min to 500 mL/min. After pregnancy week 20 cytotrophoblast disappears and only syncytiotrophoblast remains.
• chorion laeve / smooth chorion : the smooth (nonvillous) and membranous part of the chorion
• allantois : an initially tubular ventral diverticulum of the hindgut of embryos of reptiles, birds, and mammals. In reptiles and birds, it expands to a large sac for storing urine and, after fusing with the chorion which lines the shell, provides for gas exchange. The allantois is prominent in some mammals (carnivores, ungulates); in others, including humans, it is vestigial except that its blood vessels give rise to those of the umbilical cord
• yolk or vitelline sac / vitellicle  : the extraembryonic membrane that connects with the midgut; at the end of the fourth week of development it expands into the pear-shaped umbilical vesicle connected to the body of the embryo by the long narrow yolk stalk. In marsupial and placental mammals, it produces a complete vitelline circulation in the early embryo and then undergoes regression; in oviparous vertebrates, it encloses the yolk mass, breaks down yolk, and makes it available to the developing organism. In human embryos it does not serve a primary nutritive function, but it is the first hematopoietic organ of the embryo
• vitellogenesis : production of vitellus / yolk : the stored nutrient of an egg or ovum
• accessory or nutritive yolk : the part of the yolk that serves for the nutrition of the formative portion.
• egg yolk : the yellow portion of the egg of a bird.
• formative yolk : that part of the ovum from which the embryo is developed, as in birds.
• paraderm : the part of the vitellus of the ovum that furnishes cells which contribute to the body of the embryo.
• vitellin : a phosphoprotein found in the yolk of eggs
• Eternod's sinus : a loop of vessels connecting the vessels of the chorion with those in the underside of the yolk sac
• allantochorion : a compound membrane formed by fusion of the allantois and chorion.
• chorioallantois : an extraembryonic structure derived from union of the chorion and allantois which by means of vessels in the associated mesoderm serves in gas exchange. In reptiles and birds, it is a membrane apposed to the egg shell; in many mammals, it forms the placenta.
• umbilical cord / funiculus umbilicalis / chorda umbilicalis : the flexible structure connecting the umbilicus of the embryo and fetus with the placenta and giving passage to the umbilical arteries and vein immersed in Wharton's jelly. In the newborn it measures about 50-60 cm in length and has a diameter of 15-20 mm. First formed during the fifth embryonic week from the connecting stalk, it contains the omphalomesenteric duct (yolk stalk) and the allantois
• insertion
• central insertion (90%)
• paramarginal insertion (< 10%)
• homeobox (Hox) genes are a group of approximately 40 genes that control the anterior-posterior axial development in all bilaterian animals. Much interest surrounds the precise regulation of this complex patterning, which depends inter alia upon qualitative and quantitative differences in Hox gene product levels in different tissues at different times. The expression pattern of this group of genes is described as temporal and spatial colinearity as genes are expressed in time and space in the same order that they appear on the genome^ref. Co-linearity might be achieved by 2 component steps : regulated derepression of the locus by DNA replication, making it permissive to regulated expression of specific transcription factors. The spatial and temporal organization of proliferation could provide a template for developmental gene expression in which staggered DNA replication along a developing axis relieves gene expression from transcriptional silencing
• Oct-3/4 maintains the pluripotent state of inner cell mass cells—at the blastocyst stage of preimplantation development—that develop into the fetus after implantation. The precise level of Oct-3/4 governs 3 distinct fates of ES cells
• a < 2-fold increase in expression causes differentiation into primitive endoderm and mesoderm
• repression of Oct-3/4 induces loss of pluripotency and dedifferentiation to trophectoderm
A degree of Hox fragmentation has occurred in some insects, the nematode Caenorhabditis elegans, and the tunicates Ciona intestinalis and Oikopleura dioica (9 locations around the genome : 8 > 250 kb from their nearest Hox neighbor on either side^ref). Oikopleura would resemble C. elegans, which undergoes a mosaic mode of development where all genes are expressed early in development rather than in a temporal sequence. Another similarity is that both have very fast development times and a small number of cells, which might allow them to employ mechanisms that don't require strict anterior and posterior timing. The findings also lend support to the conjecture, first made by Darwin, that tunicates are simplified chordates, rather than being particularly close to the chordate ancestor. They have undergone etrograde evolution with no return : once you have broken a cluster of genes, it's very unlikely that selection will rebuild it. C. elegans also has a simplified body plan compared to its ancestors.
Web resources :
• Homeobox genes (maps, networks, expressin patterns, evolutionary relationships...)
• Hox genes in vertebrates
• Homeotic genes at COPE
• primitive or primordial wandering cell : a small mononuclear cell of the embryo that arises from the mesoderm and subsequently by differentiation gives rise to wandering cells of the body
• germinal or primitive streak :  faint white trace at the caudal end of the embryonic disc, formed by the movement of cells at the beginning of mesoderm formation; it provides the earliest evidence of the embryonic axis.
• Hensen's protochordal or primitive node or knot : a mass of cells at the cranial end of the primitive streak, related to the organization of an embryo
• embryonic line : the primitive streak in the center of the germinal area.
• primitive groove : a lengthwise median furrow in the primitive streak of the embryo. In human embryos it is clearly visible during the third week
Avian embryos have a remarkable capacity to regulate: when a pre-primitive streak stage embryo is cut into fragments, each fragment can spontaneously initiate formation of a complete embryonic head-tail axis, but during normal development only one such structure typically forms. An ectopic primitive streak can be induced by misexpression of vitellogenin (Vg1) in the marginal zone : Vg1 induces an inhibitor that travels across the embryo (3 mm distance) in less than 6 hours (0.5 mm per hour : up to 10-times as fast as comparable molecules). This inhibitor acts early in the cascade of events downstream of Vg1. FGF signalling is required for primitive streak formation, in cooperation with Nodal and Chordin. 3 sequential inhibitory steps ensure that a single axis develops in the normal embryo: an early inhibitor that spreads throughout the embryo (which can be induced by Vg1), a second inhibition by Cerberus from the underlying hypoblast, and finally a late inhibition from Lefty emitted by the primitive streak itself^ref.
• blastoneuropore : in certain embryos, a temporary aperture formed by the coalescence of the blastopore and neuropore.
• ectomesoblast : the layer of cells which has not yet become differentiated into ectoblast and mesoblast.
• mesoblast : mesoderm, especially in the early undifferentiated stages.
• segmental plate : a plate of mesoblast on either side of the notochord at the posterior end of the embryo, from which the mesoblastic segments are formed.
• notochord / chorda dorsalis : the rod-shaped body, composed of cells derived from the mesoblast of the primitive node of the embryo, defining the primitive axis of the body; it is the common factor of all species of the phylum Chordata. It is the center of development of the axial skeleton
• blastema : a group of cells that give rise to a new individual, in asexual reproduction, or to an organ or part, in either normal development or in regeneration.
• mesoblastema / nephric blastema : all the cells collectively which constitute the early undifferentiated mesoderm
• nephric blastema
• scleroblastema : the embryonic tissue entering into the formation of bone
• germ layer theory : the theory that the embryo develops 3 primary germ layers, each of which gives rise to definite organ derivatives.
• embryonic shield : the double-layered disk from which the embryo proper develops
• germ layer : one of the 3 primary layers of cells of the embryo (ectoderm, endoderm, or mesoderm), from which the tissues and organs develop.
• endoderm / entoderm / endoblast / entoblast : the innermost of the 3 primary germ layers of the embryo; from it are derived the epithelium of the pharynx, respiratory tract (except the nose), digestive tract, bladder, and urethra
• prechordal or prochordal plate : thickened endoderm, cephalad of the notochord, that combines with ectoderm to become the oropharyngeal membrane.
• mesoderm : the embryo, lying between the ectoderm and the endoderm  It arose approximately 40 million years after the emergence of the ectoderm and endoderm and was most probably a derivative of the latter. From it are derived the connective tissue, bone and cartilage, muscle, blood and blood vessels, lymphatics and lymphoid organs, notochord, serous membranes (pleura, pericardium, peritoneum, kidney, and gonads
• extraembryonic mesoderm : that located outside the embryo and belonging to fetal accessory organs, covering for example the amnion and yolk sac.
• gastral mesoderm : that infolded with the endoderm during gastrulation.
• head mesoderm : loose mesoderm, cranial to the somites.
• lateral mesoderm : the lateral sheets of mesoderm within which the embryonic coelom arises.
• coelom / celom / coeloma / somatic cavity : the body cavity. In the higher invertebrates, persists throughout life. In the mammalian embryo, it is situated between the somatopleure and the splanchnopleure
• intraembryonic coelom => principal cavities of the trunk
• extraembryonic coelom / exocoelom :  the portion of the coelom external to the embryo, bordered by chorionic mesoderm and the mesoderm of the amnion and yolk sac; it communicates temporarily at the umbilicus with the intraembryonic coelom
• Heuser's exocoelomic membrane : a delicate sac of mesoblastic tissue that develops as a lining of the blastocyst or chorionic cavity just after implantation, forms the exocoelomic cavity, and quickly disappears
• syncelom : the perivisceral cavities of the body considered as one structure, including the pleural, cardiac, and peritoneal cavities, and tunica vaginalis
• paraxial mesoderm : that lying alongside the notochord and neural tube.
• peristomal mesoderm : that derived from the ventral lip of the blastopore or from the primitive streak.
• somatic mesoderm : the outer of the 2 layers into which the embryonic mesoderm divides; associated with ectoderm to constitute the somatopleure.
• splanchnic mesoderm : the inner of the 2 layers into which the embryonic mesoderm divides; associated with endoderm to constitute splanchnopleure.
• somatopleure : the embryonic body wall, formed by ectoderm and somatic mesoderm.
• splanchnopleure / splanchnoderm : the layer formed by the union of the splanchnic mesoderm with endoderm; from it are developed the muscles and the connective tissue of the digestive tract
• somite / mesoblastic, mesodermic, primitive, primordial, or protovertebral segment : one of the paired, blocklike masses of mesoderm, arranged segmentally alongside the neural tube of the embryo, forming the vertebral column and segmental musculature
• dermomyotome : all but the sclerotome of a mesodermal somite; the primordium of skeletal muscle and, perhaps, of corium.
• dermomyotome plate : the portion of the embryonic somite remaining after migration of the sclerotomic tissue
• myotome / myomere : the muscle plate or portion of a somite that develops into striated (skeletal) muscle. A group of muscles innervated from a single spinal segment.
• hypomere : the ventrolateral portion of a myotome, innervated by an anterior ramus of a spinal nerve. The lateral plate of mesoderm that develops into the walls of the body cavities.
• epimere : the dorsal portion of a somite, from which is formed muscles innervated by the dorsal ramus of a spinal nerve.
• sclerotome : one of the paired masses of mesenchymal tissue, separated from the ventromedial part of a somite, which develop into vertebrae and ribs
• ethmovomerine plate : the central part of the ethmoid bone in the fetus.
• foot plate : a flattened expansion at the end of the limb end of the embryo, the precursor of the foot
• frontal plate : a fetal plate of cartilage between the sides of the ethmoid cartilage and the sphenoid bone.
• frontonasal plate : a fetal plate from which the external nose is developed
• hand plate : a flattened expansion at the end of the limb bud of the embryo, the precursor of the hand
• lateral mesoblastic plate : the thickened portion of either side of the mesoblast.
• subgerminal plate : a sheet of protoplasm forming the floor of the segmentation cavity of the ovum.
• urethral plate : an endodermal plate that gives rise to the terminal portion of the spongy urethra
• epiblast : the upper layer of the bilaminar embryonic disc present during the second week; it gives rise to ectoderm; ectoderm; the ectoderm except for the neural plate.
• ectental line : the line of junction between the ectoderm and endoderm
• ectoderm / ectoblast : the outermost layer of cells of the 3 primary germ layers of the embryo; an outer protective coat. From it are developed the epidermis and the epidermal tissues, such as the nails, hair, enamel of teeth, and glands of the skin, the nervous system, the external sense organs such as the ear and eye, and the mucous membrane of the mouth and anus
• amniotic ectoderm : the inner layer of the amnion (and covering of the umbilical cord) that is continuous with body ectoderm.
• Wharton's jelly : the soft, jellylike, homogeneous intercellular substance of the umbilical cord; it gives the reaction for mucin and contains thin collagenous fibers which increase in number with the age of the fetus.
• basal ectoderm : trophoblast covering the eroded uterine tissue that faces the placental sinuses.
• blastodermic, primitive or primordial ectoderm : the external layer of a blastula or blastodisk
• chorionic ectoderm : the trophoblast.
• extraembryonic ectoderm : a derivative of epiblast or ectoderm located outside the body of the embryo.
• dermatomere : any segment or metamere of the embryonic integument.
• dorsal flexure : one of the flexures of the embryo in the mid-dorsal region
• caudal or sacral flexure : the bend at the aboral (caudal) end of the embryo
• Rathke's craniobuccal, craniopharyngeal or hypophyseal pouch or pocket : a diverticulum from the embryonic buccal cavity, from which the adenohypophysis is developed; its lumen persists in adults as small colloid-filled cysts and clefts at the juncture of the pars distalis and the neurohypophysis
• bucconasal or oronasal membrane : a thin epithelial plate separating the nasal pits from the oral cavity of the embryo
• Seessel's pouch : a transient outpouching of the embryonic pharynx rostrad of the oropharyngeal membrane and caudal to Rathke's pouch.
• branchial or pharyngeal apparatus : the branchial (pharyngeal) arches and clefts considered as a unit (see also malformations of the branchial apparatus)
• branchial or visceral arches : paired arched columns that bear the gills in lower aquatic vertebrates and that, in the embryos of higher vertebrates, appear in comparable form before subsequent modification into structures of the head and neck. In humans they are also called pharyngeal arches because gills do not develop. Each contains a cartilaginous bar, consisting of right and left halves.
• the first pharyngeal arch (mandibular arch) is innervated by the V cranial nerve and vascularized by the external maxillary artery
• the dorsal surface differentiates into the body of stapes, the head of malleus, and the ear pavillon
• the ventral surface differentiates into the Meckel's cartilage and the malleus
• the median line of the pharynx differentates into the body of tongue
• sphenomandibular and anterior malleolar ligaments, malleus, and incus ?
• the second pharyngeal arch (hyoid arch) is innervated by the VII and VIII cranial nerves and is vascularized by the stapedial artery
• the dorsal surface differentitates into the stapes
• the ventral surface differentiates into the stylohyoid ligament, the styloid process and the lesser horn of the hyoid bone, and cranial part of the hyoid body
• the median line of the pharynx differentates into the root of the tongue, the foramen caecum, and the median part of the thyroid
• the third pharyngeal arch is innervated by the IX cranial nerve and vascularized by the carotid artery. The ventral surface differentiates into the greater horn of the hyoid bone and the caudal part of its body
• the fourth pharyngeal arch is innervated by the X cranial nerve and vascularized by the aortic arch at left and part of the subclavian artery at right. Its ventral surface differentiates into the thyroid, cuneiform and part of the epiglottic cartilages
• the sixth pharyngeal arch differentiates into the lateral regions of the thyroid
• the actual sixth pharyngeal arch is innervated by the inferior laryngeal nerve and vascularized by the pulmonary artery and the Botallo's ductus arteriosus. It differentiates into the cricoid, aritenoid and corniculate cartilages. In the human embryo, the sixth arch is actually the fifth in number but is so named for reasons of comparative anatomy and evolution; it does not appear on the surface
The second branchial arch develops far more than the caudal ones thanks to the nuchal curvature and covers the third and forth at the end of week 5. Sinus precervicalis : the depression at the side of the neck, produced in the developing embryo by the growth of the pharyngeal (branchial) arches.
• branchial or pharyngeal cleft, fissure or groove : the embryonic ectodermal cleft between successive pharyngeal arches
• first cleft differentiates into the external auditory meatus
The second branchial cleft deepens and forms the cervical sinus (a temporary depression caudal to the embryonic hyoid arch, containing the succeeding pharyngeal arches), which closes off as the cervical vesicle.
• branchial, pharyngeal or visceral pouch : a lateral diverticulum of the pharynx that meets a corresponding pharyngeal groove in the embryonic ectoderm, forming a closing membrane that may rupture and complete the gill slit as observed in lower vertebrates
• first pouch differentiates into the Eustachian tube, the cavity of middle ear, and the mastoid cells
• second pouch differentiates into the palatine tonsils and the supratonsillary fossa
• third pouch
• the dorsal surface differentiates into the lower parathyroids and the pyriform fossa
• the ventral surface differentiates into the thymus
• forth pouch
• fifth pouch differentiates into the lateral regions of the thyroid
• ultimobranchial, postbranchial or telobranchial bodies : embryonic derivatives of the fifth pharyngeal pouches, which migrate along with the parathyroid glands and are incorporated in the thyroid gland. In submammalian vertebrates they remain as discrete masses in the neck or mediastinum throughout adult life. The parafollicular cells of these bodies produce calcitonin.
• branchial sinus : an abnormal opening between a pharyngeal groove and its corresponding pharyngeal pouch, homologous with an ancestral gill slit
• branchial cleft : any of the slitlike openings in the gills of fishes, formed between the branchial arches
• hyobranchial or posthyoidean cleft : the cleft between the hyoid and the next succeeding arch in the developing embryo
• hyomandibular or hyoid cleft : the cleft between the mandibular and hyoid arches in the developing embryo
• epithelial plug : a mass of ectodermal cells that temporarily closes the external naris of the fetus.
• neural arch : one of the cartilaginous structures surrounding the embryonic spinal cord, formed by the dorsad growth of the dorsolateral arcualia; it is the primordium of the vertebral arch
• hemal arch : one of the cartilaginous structures surrounding the caudal vein in the tail of the vertebrate embryo, formed by the ventrad growth of the ventrolateral arcualia. In fish, the arches are also present in the thoracic region
• pleuropericardial membrane : a membrane in the embryo separating the pericardial cavity from the pleural cavity
• pleuroperitoneal membrane : a membrane in the embryo separating the pleural cavity from the peritoneal cavity and developing into the posterolateral part of the diaphragmù
• neuroepithelium / neurepithelium / sensory epithelium : simple columnar epithelium made up of cells specialized to serve as sensory cells for the reception of external stimuli, as the sensory cells of the
• cochlea
• neuroepithelium macularum / neuroepithelium of maculae : the specialized epithelium of the maculae of the labyrinth, containing receptor cells from some of which cilia (sensory hairs) project into the statoconic membrane.
• vestibule
• neuroepithelium cristae ampullaris / neuroepithelium of ampullary crest : the specialized epithelium of the ampullary crest of the labyrinth, containing receptor cells from some of which cilia (sensory hairs) project into the cupula.
• nasal mucosa
• tongue
The epithelium of the ectoderm, from which the CNS is developed.
• at week 4, epiblasts migrated into hypoblast differentiate into primordial germ cells (PGCs) sized 15-20 mm : germ cell fate in mice is induced in pluripotent epiblast cells in response to signals from extraembryonic tissues. The specification of approximately 40 founder PGCs and their segregation from somatic neighbours are important events in early development. A critical event during this specification includes repression of a somatic programme that is adopted by neighbouring cells. Blimp1 (also known as Prdm1), a known transcriptional repressor, has a critical role in the foundation of the mouse germ cell lineage, as its disruption causes a block early in the process of PGC formation. Blimp1^-/- mutant embryos form a tight cluster of about 20 PGC-like cells, which fail to show the characteristic migration, proliferation and consistent repression of homeobox genes that normally accompany specification of PGCs. Furthermore, our genetic lineage-tracing experiments indicate that the Blimp1⁺ cells originating from the proximal posterior epiblast cells are indeed the lineage-restricted primordial germ cell precursors^ref. At week 5 they begin migrate from the splanchnopleure of the secondary yolk sac through the mesenterium with ameboid movements into the genital ridge. The compartmentalisation of the PGCs is completed in humans by the 7^th week of gestation
• oogenesis
• spermatogenesis
• ventrolateral mass : that portion of the primordial lateral mass of the embryo from which are developed the abdominal, thoracic, and anterior cervical muscles.
• bud : any small part of the embryo or adult metazoon more or less resembling the bud of a plant and presumed to have potential for growth and differentiation.
• bronchial bud : an outgrowth from the stem bronchus giving rise to the air passages of its respective pulmonary lobe.
• end or tail bud : the remnant of the primitive knot, from which arises the caudal portion of the trunk
• limb bud : a swelling on the trunk of the embryo that becomes a limb.
• lung bud : an outgrowth from the foregut that gives rise to the trachea, bronchi, and all the branchings that form a tracheobronchial tree.
• periosteal bud : vascular connective tissue from the periosteum growing through apertures in the periosteal bone collar into the cartilaginous matrix of the primary center of ossification.
• tail bud : the primordium of the caudal appendage
• metanephric or ureteric bud or diverticulum : an outgrowth of the mesonephric duct that gives rise to all but the nephrons of the permanent kidney
• at weeks 5 the cloacal fold divides into anal membrane and genital fold
• sexual differentiation :
• determination of chromosomal or genetic sex at fertilization
• formation of organs common to both sexes
• sacrogenital fold / plica recto-uterina / plica rectouterina [Douglasi] / rectouterine fold : a crescentic fold of peritoneum extending from the rectum to the base of the broad ligament on either side, forming the rectouterine pouch
• urogenital fold or ridge : a longitudinal ridge or fold in the embryo, lateral to the root of the dorsal mesentery, which later subdivides longitudinally into ...
• wolffian or mesonephric ridge : the more lateral portion of the urogenital ridge, which gives rise to the mesonephros.
• rete, medullary or sex cords : strands of primordial cells in the medulla of the embryonic gonads that connect with some of the mesonephric tubules, and from which the rete ovarii or the rete testis develops
• genital or germ ridge or fold (bipotential gonads) : the more medial portion of the urogenital ridge, which gives rise to determination of gonadal sex
• testes
• ovaries
• ductus paramesonephricus / gasserian, müllerian, paramesonephric or primordial duct / ductus muelleri / duct of Müller (either of the paired embryonic ducts arising as a peritoneal pocket, extending caudally to join the urogenital sinus) are created by default
• external genitalia arise as feminine
• urogenital sinus / sinus urogenitalis : an elongated sac formed by division of the cloaca in the early embryo, communicating with the mesonephric ducts and bladder, and forming the vestibule of vagina, urethra and lower vagina in the female and prostate and posterior urethra in the male
• genital tubercle : an eminence ventral to the cloaca in the early embryo, which is the primordium of the glans penis or the clitoris.
• genital or labioscrotal swelling : an elevation on each side of the embryonic phallus that becomes either a labial (labia majora) or a scrotal swelling
• determination of internal ducts => determination of phenotypic sex
• in embryos with no Y chromosome (ie genetic females)
• Müller ducts are maintained and form ...
• fallopian tubes
• uterus
• upper vagina
• external genitalia remain feminine (don't require ovarian functionality !)
• urogenital sinus become the vestibule of vagina, urethra and lower vagina
• genital tubercle becomes the clitoris
• genital swellings become the labia majora
• genital folds become the labia minora
• ovaries secrete estrogens that promote maturations of
• breast
• fallopian tubes
• uterus
• in embryos with at least 1 Y chromosome (i.e. genetic males) :
• immature Sertoli cells produce the anti-Mullerian hormone (AMH), which induces degeneration of homolateral Muller ducts, apart from vestigial appendix testis
• interstitial Leydig cells are stimulated by LH or hCG to produce testosterone, which induces
• homolateral formation of ductus mesonephricus / mesonephric duct / ductus Wolffi / wolffian duct / duct of Wolff / Leydig's duct / canal of Oken (an embryonic duct which, initiated in association with rudiments of the pronephros, is taken over as an excretory duct by the mesonephros and metanephros); later it differentiates into :
• in the male
• epididymis
• vas deferens and its ampulla
• seminal vesicles
• ejaculatory duct
• prostate
•  in the female :
• epoöphoron
• ductus longitudinalis epoöphori / Gartner's canal or duct
• paroöphoron
• ductus deferens vestigialis
• masculinization of external genitalia (mediated by DHT)
• urogenital sinus become the prostate and posterior urethra
• genital tubercle grows to become the glans penis
• genital swellings fuse to form the scrotum
• genital folds become the body of penis and anterior urethra
• urethral groove : the embryonic groove that becomes the penile urethra as the urogenital folds on each side fuse.
Testicles might be outside the body because temperature influences the sex of human children. A temperature-sensitive gender switch that makes hot sperm male could be a relic from our evolutionary past, given the evidence that more males are born in hot climates. Temperature determines sex in many reptiles. For example, alligators that develop at or below 30ºC turn out to be female, whereas hotter eggs yield males. Such a system wouldn't work in mammals: they maintain a constant body temperature and carry their young within. Instead, a gene called SRY on the male-only Y chromosome kicks off male development in embryos. This genetic switch might have been bolted on to the front of a redundant, but still functioning, temperature-driven mechanism. Male animals are sometimes born without functioning SRY, suggesting that something else - perhaps temperature - might be able to trigger male development. High temperatures make males by activating genes that speed up embryonic development : males are known to develop more quickly than females. It's usually thought that testicles are kept away from the body's hot core because sperm grow best in cool conditions. This makes little sense as inventing the ability to survive at a slightly higher temperature seems easier than having external testicles : males' cool gonads are painfully vulnerable, their descent is a dicey process that can lead to hernias, and the testicles' position might instead prevent overheated sperm going down the road to maleness. Sperm's low tolerance for heat, which develops only after birth, might be an additional safeguard to keep the sex switch inactive. There are many possible explanations for the paradoxes of maleness besides temperature : for example, it might be just as easy to move the temperature switch to a hotter point as to avoid it by moving testicles outside the body. But the idea that the sex switch is still there is testable by keeping sperm in tissue culture at different temperatures, in the hope of spotting the narrow window between activating a maleness switch and killing the sperm. The human Y chromosome contains 60 Mbp of DNA, it is haploid, and 95% of it is nonrecombining : the 23Mb euchromatic male-specific region (MSR) in the Y chromosome comprises 8 massive palindromic sequences and these regions are rich in genes that are functional and testis-specific : they contain at least 156 transcription units, all located within euchromatic sequences, and 24 MSY-specific families account for 125 of these. Half of the transcription units encode 27 distinct proteins or protein families, 12 of which are expressed ubiquitously, and 11 of which are testis-specific, confirming a previous model proposing 2 distinct functional classes of MSY genes. 3 different classes of sequences comprised the euchromatin :
• the X-transposed class
• the X-degenerate class
• the ampliconic segments, composed of sequences that demonstrated intrachromosomal identities of at least 99.9%, and containing the 8 palindromic segments. The palindrome arms range from 9 kb to 1.4 Mb, are symmetrical and identical within a palindrome, and 6 of them contain the testis-specific genes as gene pairs in the palindromic arms, as well as inverted repeats and long tandem arrays. The occurrence of MSY gene pairs that are subject to frequent gene conversion might provide a mechanism for conserving gene functions across evolutionary time in the absence of crossing over
Normal testis formation depends on 2 genes : SRY gene on the Y chromosome, and DAX1 on the X chromosome [anyway patients with a duplication of Dax1 have both X-linked congenital adrenal hypoplasia and hypogonadotropic hypogonadism (features of XY sex reversal, a condition in which individuals have the chromosomes of males but the physical attributes of females)]
• estimating the age of the embryos
• Carnegie stage : a numbered stage of human embryonic development defined by anatomical characteristics such as the appearance of the developing limbs.
• during the third and early fourth weeks. : greatest length : a dimension used to express the size of very young embryos that have not yet developed the structures permitting measurement of crown-rump length
• from the fourth to the eighth week : crown-rump length (CRL) : the length of an embryo, fetus, or infant from the crown of the head to the breech. It is the equivalent of sitting vertex height in older individuals.
• during the eighth week and of fetuses : crown-heel length (CHL) : the length of an embryo, fetus, or infant from the crown of the head to the heel. It is the equivalent of standing height in older individuals.
• foot length : a heel-toe measurement useful in estimating the age of fetuses because the foot dimensions are less subject to artifacts of curvature and shrinkage than is the fetus as a whole.
• sitting length : the distance from the crown of the head to the coccyx.
• stem length : the distance from the vertex to a line joining the ischial tuberosities.




• previllous embryo : the conceptus before the chorionic villi of the placenta develop.
• presomite embryo : the embryo at any stage prior to the appearance of the first pair of somites.
• somite embryo : the embryo at any stage between the appearances of the first and the last pairs of somites.
• Jano[scaron]ík's embryo : a human embryo having 3 aortic arches and 2 pharyngeal pouches.
• Spee's embryo : a 1.5 mm human embryo, horizon IX, about 20 days old as described by Spee.
Web resources :
• Embryogenesis : Homo sapiens
• The Multidimensional Human Embryo : 3D images from MRI
• Basic Embriology Review Program (BERP) at University of Pennsylvania
• Embryo Images : normal and abnormal mammalian development at University of North Carolina
• Human Embryology at University of Cincinnati
• The Visible Embryo
• Embryology
• Amniotic fluid at Weizmann Institute
• Embryonic stem cells at COPE
• fetus : the unborn offspring of any viviparous animal; specifically, the unborn offspring in the postembryonic period, after major structures have been outlined, in humans from 9 weeks after fertilization until birth
• 60% of fetus energy need is for brain development : at pregnancy week 20 human trophoblast attacks uterine wall for a second time, burrowing more deeply than in any other mammal.
• fetal blood circulation
• dome cells : the large cells that compose the epitrichium of the fetus
• fetal displacement : a group of cells which, during fetal development, has become displaced from its normal relations.
• fetal movement : that of a fetus in the uterus usually observable at about 18 to 20 weeks in a primigravida and at about 16 to 18 weeks in a multipara
• cyotrophy : nutrition of the embryo or fetus.
• morphogenesis
• body and facial symmetry as well as attractiveness are human mate choice criteria
• generation of asymmetry : around 90% of people favour their right hand for writing and throwing. Researchers argue about whether genes or learning create this preference. Most people assume that there is no single 'handedness' gene because it is not simply inherited. 2 left-handed parents, for example, will often have right-handed children. > 95% of right-handers' hair whorls clockwise on the scalp. The locks of lefties and the ambidextrous are equally likely to coil either way. A single gene with either 'right' or 'random' forms might underlie the trend and set up asymmetry throughout the body. People with 1 or 2 copies of the right version would be right-handed, with clockwise hair; those with 2 random versions would split 50/50 for handedness and hair whorls.  If children of left-handers inherit the 'random' gene, they could be left- or right-handed. This would also explain why identical twins can be right- and left-handed. The genes underlying handedness might also explain why our brains are asymmetrical. And left-handed or ambidextrous people are more likely to store language in the right side of the brain, are more prone to schizophrenia and, anecdotally, are more often creative or even geniuses
• individual stereoisomers, especially the enantiomers, of a biologically active chiral molecule may differ in potency, pharmacological action, metabolism, toxicity, plasma disposition and urine excretion kinetics
• asymmetrical organs
• heart => lungs
• spleen
• stomach
• intestines
• caecum and appendix
• thoracic duct ends at the junction of the left subclavian and internal jugular veins.
• the number of similar lymph nodes at either side is not equal: their number is greater at the right side than at the left. As a rule, the range between maximum and minimum is greater in men than in women. Asymmetry of the male lymph nodes is more pronounced than that in the female
• testes : in rodents 5 strains of mice consistently exhibited asymmetrical testes, with the right testis always being heavier than the left^ref
• asymmetrical diagnostic procedures
• laparocentesis is practiced more commonly at left
• asymmetrical diseases => symmetrical thinking in clinical imaging.
• varicocele is more common at left
• retrocaval ureter is more common at right
• Bochdalek's hernia is more common at left
• Morgagni's hernia is more common at right
• developmental asymmetries of the jaws arise as a result of unilateral disruption of mandibular development during the time of normal facial growth leading to hemiretrognathism
• severe hypoplasia of the muscles of mastication on one side
• organogenesis : the origin and development of organs.
• isauxesis : growth of a part or parts at the same rate as the growth of the whole.
• gastrointestinal organogenesis
• heart organogenesis (cardiogenesis)
• nervous system organogenesis
• skeletal muscles organogenesis
• circulatory apparatus organogenesis
• neurogenesis
• organs with fractal development :
• lungs
• bones
• liver
• blood vessels
• growth (pediatrics)
• heart rate < 160 bpm (120 bpm at age 1 yr), < 100 while sleeping
• breath rate < 60 bpm
• vagitus : the cry of an infant.
• vagitus uterinus : the crying of a child in the uterus.
• vagitus vaginalis : the crying of a child while its head is still within the vagina
• normal blood values
• WBCs : 5,000-14,500/mL
• neutrophils : 1,500-8,000/mL (30-55%) (neutrophils = lymphocytes at age 5-6; adult values achieved since age 10-12)
• lymphocytes : 1,500-7,300/mL (30-50%)

Age-adjusted values of lymphocytes subsets^{ref1, ref2} :

age	mo.0-3		mo.3-6		mo.6-12		1-2 yrs		2-6 yrs		6-12 yrs		12-18 yrs
percentile	10th	50th	10th	50th	10th	50th	10th	50t	10th	50th	10th	50th	10th	50th
CD3+/ml	2500	3680	2500	3930	1900	3930	2100	3550	1400	2390	1200	1820	1000	1480
CD4+/ml	1600	2610	1800	2850	1400	2670	1300	2160	700	1380	650	980	530	840
CD8+/ml	560	890	590	1050	500	1040	620	1040	490	840	370	680	330	530
CD19+/ml	300	730	430	1550	610	1520	720	1310	390	750	270	480	110	300
CD16+56+/ml	170	420	170	420	160	400	180	360	130	300	100	230	70	190

• monocytes : 0-900/mL (2-13%)
• eosinophils : 0-700/mL (0-5%)
• basophils : 0-100/mL (0-0.5%)
• RBCs : 3,700,00-5,300,000/mL
• reticulocytes : 22,000-139,000/mL (0.5-2.5%)
• HGB : 10.5-15.5 g/dL
• HCT : 33-45%
• MCV : 70-93 fL (73 fL at age 2-6; 75 fL at age 6-12 : spurious microcytemia, differential diagnosis with microcytic anemias, i.e. sideropenic, sideroblastic or thalassemias)
• MCH : 23-33 pg
• MCHC : 29-37 g/dL
• RDW : 11-16%
• PLT : 150,000-450,000/mL
• MPV : 8-11.9 fL
• glucose : 65-110 mg/dL
• creatinine : 0.5-1.1 mg/dL
• bilirubine < 1.2 mg/dL
• proteins : 6-8.7 g/dL
• calcium : 9.5-12 mEq/L
• sodium : 136-142 mEq/L
• potassium : 3.5-5.5 mEq/L
• GPT/ALT < 50 UI/mL
• amylase < 100 UI/mL
• urate : 2-7 mg/dL
• aPTT : 22-33 sec
• fibrinogen : 150-450 mg/dL
• D-dimer < 0.3 mg/L
• AT-III : 70-125%
• PT : 70-130%
• INR : 0.85-1.25
• ESR < 38 mm/hr
• CRP < 5 mg/L
• dibucaine number > 70
• PAI : 0.3-3.5 U/mL
• haptoglobins : 30-200 mg/dL
• protein S : 60-113 mg/dL
• AlkP far higher than in adults
• IgM : adult values are achieved at age 2
• IgG : adult values are achieved at age 6
• IgA : adult values are achieved at age 12 (colostrum contains IgA specific for pathogens of birth channel)
• height
• height graph
• for girls
• for boys
• weight gain rate : + 25-30 g/mo in the first trimester ; +20-25 g/mo in the second trimester (body weight is important for posology); body weight is 3 times that at birth after 1 year and 4 times that at birth after 2 years
• weight graph
• for girls
• for boys
• head circumference (at birth it measures 35-36 cm and the head represents 13-14% of body weight = 350 g; at age 1 it weighs 900 g and the circumference measures 46 cm). Useful for early diagnosis of hydrocephalus, microcephaly, craniostenosis
• head circumference graph
• for girls
• for boys
• baby grid : a direct reading control chart on infant growth.
• Wetzel grid : a direct reading chart for evaluating physical fitness in terms of body build, developmental level, and basal metabolism.
• bregmatic fontanelle closes at age 1 yr : before that age extrusion can mark endocranial hyperhidrosis or meningitis or be paraphysiological. The fontanel is a window for intracranial echography
• liver : upper border at 6th intercostal space; lower border normally palpable (1 cm, 2-3 in the breast-fed) => evalute consistence
• spleen : lower pole normally palpable in 8-10% of babies
• at present, health workers worldwide judge whether babies and children are growing fast enough based on standard curves of height and weight drawn up in the 1970s. The figures were calculated by collecting data on US children, who were mainly fed infant formula. By the 1990s, shortcomings with the benchmarks were becoming clear. The main problem is that breastfed children tend to be lighter than those fed formula. So mothers are sometimes advised to switch to solid food prematurely, even though breastfeeding is considered the healthier option because it can help fight infectious diseases. It also was not clear whether the statistics for US children would hold for those in other countries. Unlike previous studies to gauge growth, the participants were specifically selected to be in the best of health, so mothers did not smoke and children were breastfed. Well-fed babies from Brazil, Ghana, India, Norway, Oman and the United States from birth to 5 years old tend to grow at the same rate regardless of their ethnic group. Because breastfed children grow more slowly than their formula-fed peers, the new standards will push more children into the overweight category
Web resources :
• Selected pediatric reference values at Weizmann Institute
• Child nomogram at Weizmann Institute
• 2000 CDC Growth Charts: United States
• PEDINFO Pediatric Informatics
• Child Health Forum
• Neonatology on the Web
• Harriet Lane WWW Links - Pediatric Points of Interest
• The Pediatric Data Base (PEDBASE)
• Pediatric Nutrition Surveillance System (PedNSS) at CDC
• Pediatric Growth Charts at CDC
• The Virtual Library - Nursing
• Usenet
• sci.med.nursing
• sci.med.midwifery

• Gilbert, Scott F. : Developmental Biology. 6^th ed. Sunderland, Massachusetts: Sinauer Associates, Inc.; © 2000.

• dmoz Open Directory Project (ODP) : Developmental Biology
• The Virtual Library - Developmental Biology
• Interactive graphical representations of regulatory gene networks (GeNet) acting at embryogenesis
• The Virtual Embryo by Leon Browder
• Spermatology WWW Home Page
• Zygote by Scott Gilbert
• European Society for Human Reproduction and Embryology (ESHRE)
• Center for Human Reproduction
• Mammalian Development

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