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Little information is available on the sexual reproductive biology of anemones that provide essential habitat for anemonefish. Here we provide the first information on the surface ultrastructural and morphological changes during development of the embryos and planula larvae of Entacmaea quadricolor and Heteractis crispa, using light and scanning electron microscopy. Newly spawned eggs of E. quadricolor and H. crispa averaged 794 µm and 589 µm diameter, respectively, and were covered by many spires of microvilli that were evenly distributed over the egg surface, except for a single bare patch. Eggs of both species contained abundant zooxanthellae when spawned, indicating vertical transmission of symbionts. Fertilization was external, and the resulting embryos displayed superficial cleavage. As development continued, individual blastomeres became readily distinguishable and a round-to-ovoid blastula was formed, which flattened with further divisions. The edges of the blastula thickened, creating a concave-convex dish-shaped gastrula. The outer margins of the gastrula appeared to roll inward, leading to the formation of an oral pore and a ciliated planula larva. Larval motility and directional movement were first observed 36 h after spawning. E. quadricolor larval survival remained high during the first 4 d after spawning, then decreased rapidly., Introduction In cnidarians, fertilization may take place within the parent (i.e., brooding) or externally in the water column after the gametes are shed (i.e., broadcast spawning) (Campbell, 1974; Harrison and [...]

Abstract: Having descended from the first multicellular animals on earth, sponges are a key group in which to seek innovations that form the basis of the metazoan body plan, but [...]

Articulated embryos from the Lower Jurassic Elliot Formation of South Africa are referable to the prosauropod Massospondylus carinatus and, together with other material, provide substantial insights into the ontogenetic development in this early dinosaur. The large forelimbs and head and the horizontally held neck indicate that the hatchlings were obligate quadrupeds. In contrast, adult Massospondylus were at least facultatively bipedal. This suggests that the quadrupedal gait of giant sauropods may have evolved by retardation of postnatal negative allometry of the forelimbs. Embryonic body proportions and an absence of well-developed teeth suggest that hatchlings of this dinosaur may have required parental care., Prosauropod dinosaurs appeared during the early Late Triassic (1, 2) and became the dominant large herbivores in Late Triassic and Early Jurassic continental ecosystems (3) [220 to 183 million years [...]

During early vertebrate development, members of the transforming growth factor beta (TGF[beta]) family play important roles in a variety of processes, including germ layer specification, patterning, cell differentiation, migration, and organogenesis. The activities of TGF[beta]s need to be tightly controlled to ensure their function at the right time and place. Despite identification of multiple regulators of Bone Morphogenetic Protein (BMP) subfamily ligands, modulators of the activin/nodal class of TGF[beta] ligands are limited, and include follistatin, Cerberus, and Lefty. Recently, a membrane protein, tomoregulin-1 (TMEFF1, originally named X7365), was isolated and found to contain two follistatin modules in addition to an Epidermal Growth Factor (EGF) domain, suggesting that TMEFF1 may participate in regulation of TGF[beta] function. Here, we show that, unlike follistatin and follistatin-related gene (FLRG), TMEFF1 inhibits nodal but not activin in Xenopus. Interestingly, both the follistatin modules and the EGF motif contribute to nodal inhibition. A soluble protein containing the follistatin and the EGF domains, however, is not sufficient for nodal inhibition; the location of TMEFF1 at the membrane is essential for its function. These results suggest that TMEFF1 inhibits nodal through a novel mechanism. TMEFF1 also blocks mesodermal, but not epidermal induction by BMP2. Unlike nodal inhibition, regulation of BMP activities by TMEFF1 requires the latter's cytoplasmic tail, while deletion of either the follistatin modules or the EGF motif does not interfere with the BMP inhibitory function of TMEFF1. These results imply that TMEFF1 may employ different mechanisms in the regulation of nodal and BMP signals. In Xenopus, TMEFF1 is expressed from midgastrula stages onward and is enriched in neural tissue derivatives. This expression pattern suggests that TMEFF1 may modulate nodal and BMP activities during neural patterning. In summary, our data demonstrate that tomoregulin-1 is a novel regulator of nodal and BMP signaling during early vertebrate embryogenesis.

Distal limb development and specification of digit identities in tetrapods are under the control of a mesenchymal organizer called the polarizing region. Sonic Hedgehog (SHH) is the morphogenetic signal produced by the polarizing region in the posterior limb bud. Ectopic anterior SHH signaling induces digit duplications and has been suspected as a major cause underlying congenital malformations that result in digit polydactyly. Here, we report that the polydactyly of Gli3-deficient mice arises independently of SHH signaling. Disruption of one or both Gli3 alleles in mouse embryos lacking Shh progressively restores limb distal development and digit formation. Our genetic analysis indicates that SHH signaling counteracts GLI3-mediated repression of key regulator genes, cell survival, and distal progression of limb bud development., The Hedgehog (Hh) signaling pathway controls many key developmental processes during animal embryogenesis (1). In Drosophila embryos, all known functions of Hh signaling are mediated by the transcriptional effector Cubitus [...]

Proteolytic cleavage of the Three rows protein (THR) subunit during anaphase has been found to limit Drosophila separase function. THR cleavage is especially important during the process of cellularization, which is after completion of the last syncytial mitosis of early embryogenesis. This suggests that Drosophila separase has targets in addition to cohesin subunits.

In all eukaryotic organisms, the physical separation of two nascent cells must be coordinated with chromosome segregation and mitotic exit. In Saccharomyces cerevisiae and Schizosaccharomyces pombe this coordination depends on a number of genes that cooperate in intricate regulatory pathways termed mitotic exit network and septum initiation network, respectively. Here we have explored the function of potentially homologous genes in a metazoan organism, Caenorhabditis elegans, using RNA-mediated interference. Of all the genes tested, only depletion of CeCDC-14, the C. elegans homologue of the budding yeast dual-specificity phosphatase Cdc14p (Clp1/Flp1p in fission yeast), caused embryonic lethality. We show that CeCDC-14 is required for cytokinesis but may be dispensable for progression of the early embryonic cell cycles. In response to depletion of CeCDC-14, embryos fail to establish a central spindle, and several proteins normally found at this structure are mis-localized. CeCDC-14 itself localizes to the central spindle in anaphase and to the midbody in telophase. It colocalizes with the mitotic kinesin ZEN-4, and the two proteins depend on each other for correct localization. These findings identify the CDC14 phosphatase as an important regulator of central spindle formation and cytokinesis in a metazoan organism.

Early Xenopus embryos are large, and during the egg to gastrula stages, when there is little extracellular matrix, the cytoskeletons of the individual blastomeres are thought to maintain their spherical architecture and provide scaffolding for the cellular movements of gastrulation. We showed previously that depletion of plakoglobin protein during the egg to gastrula stages caused collapse of embryonic architecture. Here, we show that this is due to loss of the cortical actin skeleton after depletion of plakoglobin, whereas the microtubule and cytokeratin skeletons are still present. As a functional assay for the actin skeleton, we show that wound healing, an actin-based behavior in embryos, is also abrogated by plakoglobin depletion. Both wound healing and the amount of cortical actin are enhanced by overexpression of plakoglobin. To begin to identify links between plakoglobin and the cortical actin polymerization machinery, we show here that the Rho family GTPase cdc42, is required for wound healing in the Xenopus blastula. Myc-tagged cdc42 colocalizes with actin in purse-strings surrounding wounds. Overexpression of cdc42 dramatically enhances wound healing, whereas depletion of maternal cdc42 mRNA blocks it. In combinatorial experiments we show that cdc42 cannot rescue the effects of plakoglobin depletion, showing that plakoglobin is required for cdc42-mediated cortical actin assembly during wound healing. However, plakoglobin does rescue the effect of cdc42 depletion, suggesting that cdc42 somehow mediates the distribution or function of plakoglobin. Depletion of [alpha]-catenin does not remove the cortical actin skeleton, showing that plakoglobin does not mediate its effect by its known linkage through [alpha]-catenin to the actin skeleton. We conclude that in Xenopus, the actin skeleton is a major determinant of cell shape and overall architecture in the early embryo, and that plakoglobin plays an essential role in the assembly, maintenance, or organization of this cortical actin.

Here, we describe one of the major maternal regulatory gradients, Dorsal, and threshold outputs of gene expression that result from the graded distribution of this transcription factor. The analysis of a large number of authentic and synthetic target genes suggests that the Dorsal gradient directly specifies at least four, and possibly as many as seven, different thresholds of gene activity and tissue differentiation. These thresholds initiate the differentiation of the three primary embryonic tissues: the mesoderm, neurogenic ectoderm, and dorsal ectoderm. Moreover, primary readouts of the Dorsal gradient create asymmetries that subdivide each tissue into multiple cell types during gastrulation. Dorsal patterning thresholds represent the culmination of one of the most complete gene regulation network known in development, which begins with the asymmetric positioning of the oocyte nucleus within the egg chamber and leads to the localized activation of the Toll-Dorsal signaling pathway in ventral regions of the early embryo. Key Words: Dorsal; gradient; Drosophila embryo; gene network.

Radial glia are specialized cells in the developing nervous system of all vertebrates, and are characterized by long radial processes. These processes facilitate the best known function of radial glia: guiding the radial migration of newborn neurons from the ventricular zone to the mantle regions. Recent data indicate further important roles for these cells as ubiquitous precursors that generate neurons and glia, and as key elements in patterning and region-specific differentiation of the CNS. Thus, from being regarded mainly as support cells, radial glia have emerged as multi-purpose cells involved in most aspects of brain development.

DNA methylation of CpG dinucleotides by DNA methyltransferase 1 is implicated in the regulation of transcription and, in particular, the transcription of imprinted genes. Although the oocyte-specific form of Dnmt1 (Dnmt1o) possesses a functional nuclear localization signal, it is predominantly localized in the cytoplasm of the oocyte and preimplantation mouse embryo but undergoes a transient nuclear localization during the eight-cell stage, when the embryos undergo compaction. We report here that Dnmt1o is likely retained in the cytoplasm by an active process, since ~70% of DNA methyltransferase activity is retained following permeabilization procedures that result in the release of ~75% of oocyte/embryo protein. Treatment of the embryos with agents that disrupt either microfilaments or microtubules has little, if any, effect on the retention of Dnmt1o in permeabilized embryos. While Dnmt1o does not colocalize with either mitochondria or endoplasmic reticulum, it does colocalize with annexin V, which is known to interact with Dnmt1o. We also report that the timing of nuclear entry of Dnmt1o during the eight-cell stage is independent of DNA replication, transcription, and protein synthesis, as well as compaction, cell contact, and cytokinesis. The time of nuclear entry, therefore, appears linked to the time following fertilization, which suggests that a molecular clock governs the time of nuclear import. Key Words: DNA methylation; DNA methyltransferase; compaction; preimplantation embryo.

The epiblast of the chick embryo gives rise to the ectoderm, mesoderm, and endoderm during gastrulation. Previous studies revealed that MyoD-positive cells were present throughout the epiblast, suggesting that skeletal muscle precursors would become incorporated into all three germ layers. The focus of the present study was to examine a variety of organs from the chicken fetus for the presence of myogenic cells. RT-PCR and in situ hybridizations demonstrated that MyoD-positive cells were present in the brain, lung, intestine, kidney, spleen, heart, and liver. When these organs were dissociated and placed in culture, a subpopulation of cells differentiated into skeletal muscle. The G8 antibody was used to label those cells that expressed MyoD in vivo and to follow their fate in vitro. Most, if not all, of the muscle that formed in culture arose from cells that expressed MyoD and G8 in vivo. Practically all of the G8-positive cells from the intestine differentiated after purification by FACS[R]. This population of ectopically located cells appears to be distinct from multipotential stem cells and myofibroblasts. They closely resemble quiescent, stably programmed skeletal myoblasts with the capacity to differentiate when placed in a permissive environment.

The RET tyrosine kinase receptor isoforms and differential activities in mammalian embryogenesis are discussed. Targeted mutagenesis was used to generate mice that express RET9 or RET51. Some RET9 transgenes can rescue kidney agenesis in RET-deficient mice or can cause kidney hypodysplasia in wild-type animals. Mice without RET9 lack enteric ganglia from the colon. Human mutations of c-RET bring on Hirschsprung's disease.

The author reviews the publications on the embryonic dorsal blastopore lip (the organizer) signaling. The topics of interest include molecular analysis of the organizer, formation of the signaling boundaries and limb patterning.

FoxH1 (FAST), a transcription factor, has been found to mediate Nodal signaling in anterior-posterior (A-P) patterning and node formation in mice. Mutant mice lacking the transcription factor were generated and characterized. They died early in embryonic development and showed various patterning defects explained by absence of Nodal signaling. Nodal is a factor related to transforming growth factor-beta (TGF-beta) and a potent signaling molecule necessary for specification of the A-P axis, formation of the primitive streak, and left-right pat terning.

PIE-1 has been found to be a bifunctional protein regulating maternal/zygotic gene expression in the Caenorhabditis elegans embryonic germ line. It promotes germ cell fate by two mechanisms independent of one another.

Sex reversal from male to female in mice having no Fibroblast growth factor 9 (Fgf9) is discussed. The phenomenon shows that FGF signaling in testicular embryogenesis has a novel role. Embryogenesis of several organs is directed by Fgfs, and they may act in reproductive system development and sex determination in many species.

Three WNT factors have been shown to signal through beta-catenin function as critical regulators of the FGF-8/FGF-10 loop. Induction of Fgf-8 in the limb ectoderm by FGF-10 is mediated by the induction of Wnt-a. Three WNT signals mediated by beta-catenin control both limb initiation and apical ectodermal ridge (AER) induction in the vertebrate embryo.

A novel embryonic poly(A)-binding protein has been found to regulate mRNA deadenylation in extracts of Xenopus eggs. It is apparently responsible for stabilization of poly(A) tails. Concentration of the protein, ePAB, is critical in determining the rate of AU-rich element (ARE)-mediated deadenylation. The goal of the study was to identify factors in ARE-dependent deadenylation in Xenopus early development. An in vitro system was developed from activated egg extracts to mimic in vivo ARE-mediated deadenylation.

It has been shown, using gene targeting, that two closely related proteins, MBD2 and MBD3, are not functionally redundant in mice. MBD3, based on genetic and biochemical data, seems to be a key component of the Mi-2/NuRD corepressor complex. MBD2 may be one of a number of factors that can recruit the complex to DNA. Maternal behavior is defective and a component of MeCP1 is missing in Mdb2(-/-) mice.

Krox-20 expression has been shown to be adequate to confer odd-numbered rhombomere characteristics to r6, r4 and r2 cells, likely in a cell-autonomous way. Krox-20 was expressed ectopically in the developing chick hindbrain using electroporation. It seems Krox-20 is an important determinant of odd-numbered identity in the hindbrain. Evidence for existence of a non cell-autonomous autoactivation mechanism making recruitment of Krox-20- positive cells from even-numbered territories by neighboring Krox-20-expressing cells possible has been found. Likely Krox-20 contoles multiple, intertwined steps in segmental patterning.

The retinoic acid (RA)-metabolizing enzyme, CYP26A1, has been studied by disrupting the murine Cyp26A1 gene and found to be required for normal development. Exposure to RA during formation of the antero-posterior body axis results in homeotic transformations along the vertebral column. CYP26A1 likely is needed to maintain given embryonic areas in an RA-depleted state to protect them against harmful effects of ectopic RA signaling.

The retinoic acid-inactivating enzyme CYP26 has been found to be necessary for setting up uneven distribution of retinoic acid along the anterio-posterior axis in mouse embryos. Mutant mice lacking CYP26 were generated to study the role of the enzyme. Retinoic acid is a derivative of vitamin A and plays a vital role in vertebrate development.

Context-dependent erythroid kruppel-like factor (EKLF) responsiveness has been found to define developmental specificity of the human epsilon-globin gene in erythroid cells of YAC transgenic mice. A study focused on the globin CAAT and CAC promoter motifs has been carried out with a high-affinity CAC-binding site for the EKLF placed in the epsilon-globin promoter repeat element. Suppression of epsilon globin transcription in definitive erythropoiesis is mediated by the binding of a repressor that prevents EKLF from activating the epsilon-globin gene.

Bub3 gene disruption in mice has been studied. The study shows that there is an essential mitotic spindle checkpoint function in early embryogenesis. Bub3-null embryos seem to be normal to day 3.5 postcoitus. In days 4.5-6.4 they accumulate mitotic errors. Null embryos treated with a spindle depolymerizing agent do not arrest in metaphase. Mitotic disorganization is seen.

Findings of a recent study of metazoan development suggest that specific combinations of histone deacetylases and corepressors mediate the gene-specific actions of DNA-bound repressors in multiple organ system development. It was found, surprisingly, that N-coR and a specific deacetylase were required for transcriptional activation of one class of retinoic acid response element.

The structure of the high-mobility group (HMG) globular domain and basic tail of HMG-D bound to disulfide cross-linked DNA has been studied. HMG is a high-mobility group chromosomal protein present in abundance in early embryogenesis in Drosophila melanogaster. The HMG domain of HMG-D recognized DNA with binding like that used by the sequence-specific GMG domain transcription factors. However, differences of note exist int eh structure and interactions of the C-terminal end of the DNA-binding domain and the C-terminal tail.

Morphological integration in developmental compartments in the Drosophila wing are discussed. Methods of geometric morphometrics are used to study integration in the wing with focus on the hypothesis that anterior and posterior wing compartments are separate developmental units and vary independently. It was found that variation among individuals as well as developmental perturbations responsible for fluctuating asymmetry generate shape variation mainly through developmental processes integrated across the compartments.

Activin receptor patterning is discussed relative to foregut organogenesis. Mutations of activin receptors ActRHA and Act RIIb can be seen to disrupt the development of posterior foregut-derived organs. The conclusion is that activin-receptor mediated signaling regulates axial patterning, cell differentiation, and function of foregut-derived organs.

The T helper lymphocytes and their function in the immune system are discussed in this review article. The current knowledge of signals that regulate lineage commitment in T helper cells is summarized with emphasis on three subset-specific transcription factors responsible for lineage commitment. They are T-bet, c-Maf and GATA-3. Topics include cytokines, development of the Th1 lineage and signaling pathways, interferons in the Th1 responses, APC, antigen and costimulation.

An association may exist between Bcl-2 and tissue failure related to Escherichia coli O157:H7. The molecular machinery of cell-death induction by verotxins (VTs) has been studied. Verotoxin II (VTII) subunit A has a pentameric sequence from the BH1 domain and interacts with mitochondrial Bcl-2 to bring on target cell death that is the result of caspase 3 activation. VTII interacts with Bcl-2 through each BH1 domain pentameric sequence. VTI does not. The VTII/Bcl-2 complex is required for induction of cell death in target cells.

A novel beta-catenin-interacting protein, ICAT, has been identified. It has been found to inhibit interaction of beta-catenin with TCF-4 and represses beta-catenin-TCF-4-mediated transactivation. ICAT inhibits Xenopus axis formation in that it interferes with Wnt signaling. It appears likely that ICAT negatively regulates Wnt signaling by inhibition of the interaction of beta-catenin with TCF. It is also integral in development and cell proliferation.

NF-(kappa)B activity has been found to be entirely absent in IKK1 and IKK2 double-deficient mice and an additional defect in neurulation was seen. The mice lacking these critical kinases died at E12. Data suggest the kinases, induced by stress, pathogens, and cytokines, are needed for NF-(kappa)B activation in vivo and are important in protecting neurons against apoptosis during development.

Signal transduction and transcription in the melanocyte lineage are discussed in this review article. Gene expression and how it is controlled in neural crest-derived melanocytes by interaction of specific transcription factors and signal transduction pathways are emphasized along with regulation of the expression and function of the microphthalmia-associated transcription factor, Mitf. Mouse studies and zerbrafish studies will be valuable in the study of melanocyte development. It will give novel insights into the integration of gene expression and signal transduction pathways and how they control development of a specific cell lineage.

A hairy-related transcriptional repressor and its coregulation of anterior and posterior mesendodermal development are discussed. Recent research has found the first model for vertebrate endoderm patterning where an early regulatory step at gastrulation, mediated by her5 controls cell contribution to the two regions jointly.

The importance of cytochrome c in apoptotic signaling pathways has been studied using genetically altered mice. It was found that cytochrome c is necessary for Apak-1-mediated activation of caspase -3 and for apoptosis brought on by common forms of cell stress, but not for apoptosis driven by TNF(alpha) and the death receptor pathway. Cytochrome c deficiency causes embryonic lethality. It also attenuates stress-induced apoptosis.

Embryonic polarity and the PAR-1 molecule, the first to be found to play an important and perhaps conserved role in both D. melanogaster and C. elegans, are discussed in this minireview article. It is interesting to consider whether conserved mechanisms used for establishing embryonic polarity exist. The most significant blank space in understanding the function of PAR proteins has been information that links them to downstream localization events, that explains how they mediate asymmetric distribution of other molecules.

A novel Drosophila gene, act up (acu), controls actin polymerization to alter cell shape and restrict hedgehog signaling in the eye disc of the insect. It appears that dynamic changes in actin filaments may change cell shape, controlling signal movement to coordinate a differentiation wave.

The progesterone-Wnt connection is of interest relative to side-branching in the mammary gland, which is a derivative of the ectoderm. The existence of transgenic and gene knockout mice has given a way to study how different hormones regulate cell growth, differentiation and death in mammary gland cells. The signaling pathways can be studied and such study may contribute to understanding of breast cancer.

The Drosophila tyrosine kinase called SH2 domain ankyrin repeat kinase (Shark) is necessary for embryonic dorsal closure. Flies with a Shark tyrosine kinase gene mutation, shark (super.1) show a DC-defective phenotype. Dpp is not expressed in shark(super.1) mutant leading edge (LE) cells. Shark regulates the NJNK signaling pathway to Dpp expression in LE cells/

The Hedgehog signaling pathway is discussed in this review article which tells that new discoveries in the pathway shed light on how shaky understanding of the moleular and cellular mechanisms underlying this vital signaling pathway is. The signaling regulates a great variety of developmental events in the fly embryo and the vertebrate one and is central for several cancers. The model now widely accepted is that Hedgehog signaling leads to ligand-dependent receptor inactivation, the likely receptor encoded by the segment polarity gene, patched. The important role of cholesterol in this signaling is related to Patched, with sequestering and binding and the difference between the two of interest.

Pan-neural Prospero has been found to end cell proliferation in Drosophila neurogenesis. Drosophila prospero encodes a vital regulator of transition from mitotically active cells to totally differentiated neurons. Loss of pros bring aberrant expression of various cell-cycle regulating genes and ectopic mitotic activity. Ectopic pros expression on the other hand brings transcriptional suppression of multiple cell-cycle regulatoring genes and early termination of cell division.

A few personal favorite examples, starting from macromolecular assembly and increasing in complexity and scale to patterning in vertebrate embryology are discussed to illustrate the nature of biological organization and explore potential underlying chemical principles. As the twentieth century comes to an end, genetics is central in biological thought. It is worthwhile to ask to what extent the postgenomic view of modern biology would convince a nineteenth century vitalist that the nature of life is now known. Further annotating the database of genes may not be the end. The physicochemical nature of living systems may need to be explored. Sequence information is often far removed from mechanisms that promote function of proteins, embryos, and cells. Nineteenth century thinking on life is reviewed.

Study results show that Mash1 and Neugenins 1 and 2 (Ngns) have a role in specification of dorsal-ventral neuronal identity as well as acting as neuronal determinants. They link pathways of neurogenesis and regional patterning in the forebrain directly. The replacement of the Ngns by Mash1 in dorsal prgenitors was used to look at the role of the neural detmination genes in neruonal-type specification in the telencephalon.

Mcl-1 deficiency brings on peri-implantaion lethality in the embryo. Mcl-1 is a Bcl-2 family member. Null blastocysts did not hatch or attach in vitro, likely an indication of a trophectoderm defect. The inner cell mass, however, could grow in culture. The blastocysts showed no evidence of greater apoptosis, but had a delay in maturation beyond precompaction.

The role of Max, of the Myc superfamily, in early embyronic growth and development is an essential one recent research has shown. Max acts as obligate dimerization-DNA-binding partner for Myc oncoproteins and Mad transcriptional repressors. A mouse losing Max function has developmental arrest of both embryonic and extra-embryonic tissues in general at early postimplantation at the same time as loss or dilution of maternal Max stores in the expanding embryo in vivo. The same happens in vivo in blastocyst outgrowth.

A complex between the death domains of Pelle and Tube is discussed relative to its 3-D structure. In vivo assays of the mutants of Pelle and Tube show that integrity of the major heterodimer interface is vital to their activity. Pelle is a serine-threonine kinase and Tube is an adaptor protein. Through their N-terminal death domains, interaction leads to the nuclear translocation of the transcription factor Dorsal and activation of zygotic patterning gene in Drosophila embryogenesis.

A PDZ domain-containing actin-binding protein, shroom (shrm) is necessary for neural tube mouse morphogenesis. A mutation that causes mouse exencephaly, acrania, spina bifida, and facial clefting has been found through use of gene trap mutagenesis. It appears that Shrm is a critical determination of the cellular architecture necessary for proper neurulation.