The role of micromere signaling in Notch activation and mesoderm specification during sea urchin embryogenesis

Specification of secondary mesenchyme-derived cells in relation to the dorso-ventral axis in sea urchin blastulae

To learn how the dorso-ventral (DV) axis of sea urchin embryos affects the specification processes of secondary mesenchyme cells (SMC), a fluorescent dye was injected into one of the macromeres of 16-cell stage embryos, and the number of each type of labeled SMC was examined at the prism stage. A large number of labeled pigment cells was observed in embryos in which the progeny of the labeled macromere were distributed in the dorsal part of the embryo. In contrast, labeled pigment cells were scarcely noticed when the descendants of the labeled macromere occupied the ventral part. In such embryos, free mesenchyme cells (probably blastocoelar cells) were predominantly labeled. CH3COONa treatment, which is known to increase the number of pigment cells, canceled such patterned specification of pigment cells and blastocoelar cells along the DV axis. Pigment cells were also derived from the ventral blastomere in the treated embryo. In contrast, a similar number of coelomic pouch cells was derived from the labeled macromere, irrespective of the position of its descendants along the DV axis. After examination of the arrangement of blastomeres in late cleavage stage embryos, it was determined that 17-20 veg2-derived cells encircled the cluster of micromere descendants after the 9th cleavage. From this number and the numbers of SMC-derived cells in later stage embryos, it was suggested that the most vegetally positioned veg2 descendants at approximately the 9th cleavage were preferentially specified to pigment and blastocoelar cell lineages. The obtained results also suggested the existence of undescribed types of SMC scattered in the blastocoele.

Molecular patterning along the sea urchin animal-vegetal axis

The molecular regulatory mechanisms underlying primary axis formation during sea urchin development have recently been identified. Two opposing maternally inherited systems, one animalizing and one vegetalizing, set up the animal-vegetal (A-V) axis. The vegetal system relies in part on the Wnt-beta-catenin-Tcf/Lef signaling pathway and the animal system is based on a cohort of animalizing transcription factors that includes members of the Ets and Sox classes. The two systems autonomously define three zones of cell-type specification along the A-V axis. The vegetalmost zone gives rise to the skeletogenic mesenchyme lineage; the animalmost zone gives rise to ectoderm; and the zone in which the two systems overlap generates endoderm, secondary mesenchyme, and ectoderm. Patterning along the A-V also depends on cellular interactions involving Wnt, Notch, and BMP signaling. We discuss how these systems impact the formation of the second axis, the oral-aboral axis; how they connect to later developmental events; and how they lead to cell-type-specific gene expression via cis-regulatory networks associated with transcriptional control regions. We also discuss how these systems may confer on the embryo its spectacular regulatory capacity to replace missing parts.

New early zygotic regulators expressed in endomesoderm of sea urchin embryos discovered by differential array hybridization

Genes that are upregulated by LiCl treatment of sea urchin embryos and/or downregulated by injection into the egg of mRNA encoding an internal fragment of cadherin (Cad) were detected in a differential macroarray screen. The method was that recently described by J. P. Rast et al. (2000, Dev. Biol. 228, 270-296). Almost 10(5) clones from a 12-h cDNA library were screened. Measurements on internal standards showed that the screening procedure was sufficiently sensitive to afford detection of differentially expressed mRNAs of the most rare class, those present in only a few copies per average cell. The injection of Cad mRNA, which specifically blocks nuclearization of beta-catenin, resulted in many-fold decreases in the levels of transcripts of a suite of marker genes expressed zygotically during endomesoderm specification. These measurements substantiated the use of Cad mRNA as the basis for a differential screen for discovery of new endomesodermal genes. By use of the newly developed BioArray software for analysis of macroarray screens, 1106 clones representing differentially expressed genes and yielding useful sequence were recovered. The 367 clones that gave significant BLASTX matches to known cellular proteins fell into 264 nonredundant sequence classes. Those of particular interest for this work were clones encoding DNA-binding transcription factors, signal transduction pathway components, proteases, kinases, and phosphatases. Quantitative PCR analysis of 66 such selected clones revealed that the large majority of these clones had been selected because they are upregulated by LiCl treatment, which affects the expression of a much greater diversity and number of genes than are involved in endomesoderm specification. Seven transcript species were identified that responded sharply to injection of Cad mRNA, and that are not represented in maternal mRNA. Six of those encode transcription factors. We focused on three transcription factor genes of this set that were previously unknown in sea urchin embryos. By whole-mount in situ hybridization, these genes are expressed in specific domains of the endomesodermal territory. They are: (1) Speve, an evenskipped orthologue expressed very early in all vegetal blastomeres and then gradually shifting to veg(1) derivatives by the mesenchyme blastula stage; (2) Spgcm, an orthologue of the fruit fly gene glial cells missing, which is first expressed specifically and exclusively in part of the prospective secondary mesenchyme (mesodermal) domain at late-cleavage blastula stage; and (3) Spfoxc, which is first expressed in the early blastula only in the four small micromeres, and later only expressed in that coelomic pouch which gives rise to the mesoderm of the ventral surface of the adult rudiment.

Specification and differentiation processes of secondary mesenchyme-derived cells in embryos of the sea urchin Hemicentrotus pulcherrimus

Four types of mesoderm cells (pigment cells, blastocoelar cells, coelomic pouch cells and circumesophageal muscle cells) are derived from secondary mesenchyme cells (SMC) in sea urchin embryos. To gain information on the specification and differentiation processes of SMC-derived cells, we studied the exact number and division cycles of each type of cell in Hemicentrotus pulcherrimus. Numbers of blastocoelar cells, coelomic pouch cells and circumesophageal muscle fibers were 18.0 +/- 2.0 (36 h post-fertilization (h.p.f.)), 23.0 +/- 2.5 (36 h.p.f.) and 9.5 +/- 1.3 (60 h.p.f.), respectively, whereas the number of pigment cells ranged from 40 to 60. From the diameters of blastocoelar cells and coelomic pouch cells, the numbers of division cycles were elucidated; these two types of cells had undertaken 11 rounds of cell division by the prism stage, somewhat earlier than pigment cells. To determine the relationship among the four types of cells, we tried to alter the number of pigment cells with chemical treatment and found that CH3COONa increased pigment cells without affecting embryo morphology. Interestingly, the number of blastocoelar cells became smaller in CH3COONa-treated embryos. In contrast, blastocoelar cells were markedly increased with NiCl2 treatment, whereas the number of pigment cells was markedly decreased. The number of coelomic pouch cells and circumesophageal muscle fibers was not affected with these treatments, indicating that coelomic pouch and muscle cells are specified independently of, or at much later stages, than pigment and blastocoelar cells.

A provisional regulatory gene network for specification of endomesoderm in the sea urchin embryo

We present the current form of a provisional DNA sequence-based regulatory gene network that explains in outline how endomesodermal specification in the sea urchin embryo is controlled. The model of the network is in a continuous process of revision and growth as new genes are added and new experimental results become available; see http://www.its.caltech.edu/~mirsky/endomeso.htm (End-mes Gene Network Update) for the latest version. The network contains over 40 genes at present, many newly uncovered in the course of this work, and most encoding DNA-binding transcriptional regulatory factors. The architecture of the network was approached initially by construction of a logic model that integrated the extensive experimental evidence now available on endomesoderm specification. The internal linkages between genes in the network have been determined functionally, by measurement of the effects of regulatory perturbations on the expression of all relevant genes in the network. Five kinds of perturbation have been applied: (1) use of morpholino antisense oligonucleotides targeted to many of the key regulatory genes in the network; (2) transformation of other regulatory factors into dominant repressors by construction of Engrailed repressor domain fusions; (3) ectopic expression of given regulatory factors, from genetic expression constructs and from injected mRNAs; (4) blockade of the beta-catenin/Tcf pathway by introduction of mRNA encoding the intracellular domain of cadherin; and (5) blockade of the Notch signaling pathway by introduction of mRNA encoding the extracellular domain of the Notch receptor. The network model predicts the cis-regulatory inputs that link each gene into the network. Therefore, its architecture is testable by cis-regulatory analysis. Strongylocentrotus purpuratus and Lytechinus variegatus genomic BAC recombinants that include a large number of the genes in the network have been sequenced and annotated. Tests of the cis-regulatory predictions of the model are greatly facilitated by interspecific computational sequence comparison, which affords a rapid identification of likely cis-regulatory elements in advance of experimental analysis. The network specifies genomically encoded regulatory processes between early cleavage and gastrula stages. These control the specification of the micromere lineage and of the initial veg(2) endomesodermal domain; the blastula-stage separation of the central veg(2) mesodermal domain (i.e., the secondary mesenchyme progenitor field) from the peripheral veg(2) endodermal domain; the stabilization of specification state within these domains; and activation of some downstream differentiation genes. Each of the temporal-spatial phases of specification is represented in a subelement of the network model, that treats regulatory events within the relevant embryonic nuclei at particular stages.

A Delta-Notch signaling border regulated by Engrailed/Invected repression specifies boundary cells in the Drosophila hindgut

The Drosophila hindgut develops three morphologically distinct regions along its anteroposterior axis: small intestine, large intestine and rectum. Single-cell rings of 'boundary cells' delimit the large intestine from the small intestine at the anterior, and the rectum at the posterior. The large intestine also forms distinct dorsal and ventral regions; these are separated by two single-cell rows of boundary cells. Boundary cells are distinguished by their elongated morphology, high level of both apical and cytoplasmic Crb protein, and gene expression program. During embryogenesis, the boundary cell rows arise at the juxtaposition of a domain of Engrailed (En)- plus Invected (Inv)-expressing cells with a domain of Delta (Dl)-expressing cells. Analysis of loss-of-function and ectopic expression phenotypes shows that the domain of Dl-expressing cells is defined by En/Inv repression. Further, Notch pathway signaling, specifically the juxtaposition of Dl-expressing and Dl-non-expressing cells, is required to specify the rows of boundary cells. This Notch-induced cell specification is distinguished by the fact that it does not appear to utilize the ligand Serrate and the modulator Fringe.

A regulatory gene network that directs micromere specification in the sea urchin embryo

Micromeres and their immediate descendants have three known developmental functions in regularly developing sea urchins: immediately after their initial segregation, they are the source of an unidentified signal to the adjacent veg(2) cells that is required for normal endomesodermal specification; a few cleavages later, they express Delta, a Notch ligand which triggers the conditional specification of the central mesodermal domain of the vegetal plate; and they exclusively give rise to the skeletogenic mesenchyme of the postgastrular embryo. We demonstrate the key components of the zygotic regulatory gene network that accounts for micromere specificity. This network is a subelement of the overall endomesoderm specification network of the Strongylocentrotus purpuratus embryo. A central role is played by a newly discovered gene encoding a paired class homeodomain transcription factor which in micromeres acts as a repressor of a repressor: the gene is named pmar1 (paired-class micromere anti-repressor). pmar1 is expressed only during cleavage and early blastula stages, and exclusively in micromeres. It is initially activated as soon as the micromeres are formed, in response to Otx and beta-Catenin/Tcf inputs. The repressive nature of the interactions mediated by the pmar1 gene product was shown by the identical effect of introducing mRNA encoding the Pmar1 factor, and mRNA encoding an Engrailed-Pmar1 (En-Pmar1) repressor domain fusion. In both cases, the effects are derepression: of the delta gene; and of skeletogenic genes, including several transcription factors normally expressed only in micromere descendants, and also a set of downstream skeletogenic differentiation genes. The spatial phenotype of embryos bearing exogenous mRNA encoding Pmar1 factor or En-Pmar1 is expansion of the domains of expression of the downstream genes over most or all of the embryo. This results in transformation of much of the embryo into skeletogenic mesenchyme cells that express skeletogenic markers. The normal role of pmarl is to prevent, exclusively in the micromeres, the expression of a repressor that is otherwise operative throughout the embryo. This function accounts for the localization of delta transcription in micromeres, and thereby for the conditional specification of the vegetal plate mesoderm. It also explains why skeletogenic differentiation gene batteries normally function only in micromere descendants. More generally, the regulatory network subelement emerging from this work shows how the specificity of micromere function depends on continuing global regulatory interactions, as well as on early localized inputs.

Process of pigment cell specification in the sand dollar, Scaphechinus mirabilis

The process of pigment cell specification in the sand dollar Scaphechinus mirabilis was examined by manipulative methods. In half embryos, which were formed by dissociating embryos at the 2-cell stage, the number of pigment cells was significantly greater than half the number of pigment cells observed in control embryos. This relative increase might have been brought about by the change in the arrangement of blastomeres surrounding the micromere progeny. To examine whether such an increase could be induced at a later stage, embryos were bisected with a glass needle. When embryos were bisected before 7 h postfertilization, the sum of pigment cells observed in a pair of embryo fragments was greater than that in control embryos. This relative increase was not seen when embryos were bisected after 7 h postfertilization. From the size of blastomeres, it became clear that the 9th cleavage was completed by 7 h postfertilization. Aphidicolin treatment revealed that 10-15 pigment founder cells were formed. The results obtained suggest that the pigment founder cells were specified through direct cell contact with micromere progeny after the 9th cleavage, and that most of the founder cells had divided three times before they differentiated into pigment cells.

Role of cell contact in the specification process of pigment founder cells in the sea urchin embryo

Effects of LiCl on the specification process of pigment founder cells were examined in the sea urchin Hemicentrotus pulcherrimus. If embryos were treated with 30 mM LiCl during 4-7 or 7-10 hours postfertilization, pigment cells increased significantly. Aphidicolin treatment indicated that this increase was due to the increase in the pigment founder cells. Interestingly, if the embryos were treated sequentially with LiCl and Ca2+-free seawater during 4-7 and 7-10 hr, respectively, they differentiated only about the same number of pigment cells as control embryos. Further, the increase was scarcely discerned when the embryos were treated with LiCl in the absence of Ca2+ during 7-10 hr. These results suggested that effect of LiCl would be ascribed to the increase in cell adhesiveness. In fact, LiCl-treated embryos were more difficult to be dissociated into single cells. Cell electrophoresis showed that the amount of the negative cell surface charges decreased considerably in LiCl-treated embryos. It was also found that the number of pigment cells seldom exceeded 100, even if embryos were exposed to a higher concentration of LiCl. This suggested that only a subpopulation of the descendants of veg2 blastomeres received the inductive signal emanated from the micromere progeny.

A genomic regulatory network for development

Development of the body plan is controlled by large networks of regulatory genes. A gene regulatory network that controls the specification of endoderm and mesoderm in the sea urchin embryo is summarized here. The network was derived from large-scale perturbation analyses, in combination with computational methodologies, genomic data, cis-regulatory analysis, and molecular embryology. The network contains over 40 genes at present, and each node can be directly verified at the DNA sequence level by cis-regulatory analysis. Its architecture reveals specific and general aspects of development, such as how given cells generate their ordained fates in the embryo and why the process moves inexorably forward in developmental time.

Identification and characterization of bone morphogenetic protein 2/4 gene from the starfish Archaster typicus

A bone morphogenetic protein 2/4 (BMP2/4) gene has been cloned from the starfish, Archaster typicus, for the purpose of investigating the expression pattern of the BMP4 gene in echinoderm embryos which do not produce micromeres. The isolated gene (named AtBMP2/4) contained two exons that encoded the entire coding region. The deduced AtBMP2/4 protein sequence contained 509 amino acids. Sequence comparison showed that it shared high amino acid similarity with sea urchin BMP2/4 and Xenopus BMP2 and BMP4. Northern blot analyses indicated that AtBMP2/4 mRNA initially appears at the blastula stage and has a maximal expression level at the gastrula stage. Whole-mount in situ hybridization revealed that AtBMP2/4 mRNA is expressed in the archenteron, coelomic vesicles, and ectodermal cells of gastrula stage embryos. The observed spatial distribution pattern vastly differs from that of sea urchin SpBMP2/4, which is expressed mainly in the oral ectoderm region of the mesenchyme blastula and early gastrula embryos.

Heads or tails? Amphioxus and the evolution of anterior-posterior patterning in deuterostomes

In Xenopus, the canonical Wnt-signaling pathway acting through beta-catenin functions both in establishing the dorso-ventral axis and in patterning the anterior-posterior axis. This pathway also acts in patterning the animal-vegetal axis in sea urchins. However, because sea urchin development is typically indirect, and adult sea urchins have pentamerous symmetry and lack a longitudinal nerve cord, it has not been clear how the roles of the canonical Wnt-signaling pathway in axial patterning in sea urchins and vertebrates are evolutionarily related. The developmental expression patterns of Notch, brachyury, caudal, and eight Wnt genes have now been determined for the invertebrate chordate Amphioxus, which, like sea urchins, has an early embryo that gastrulates by invagination, but like vertebrates, has a later embryo with a dorsal hollow nerve cord that elongates posteriorly from a tail bud. Comparisons of Amphioxus with other deuterostomes suggest that patterning of the ancestral deuterostome embryo along its anterior-posterior axis during the late blastula and subsequent stages involved a posterior signaling center including Wnts, Notch, and transcription factors such as brachyury and caudal. In tunicate embryos, in which cell numbers are reduced and cell fates largely determined during cleavage stages, only vestiges of this signaling center are still apparent; these include localization of Wnt-5 mRNA to the posterior cytoplasm shortly after fertilization and localization of beta-catenin to vegetal nuclei during cleavage stages. Neither in tunicates nor in Amphioxus is there any evidence that the canonical Wnt-signaling pathway functions in establishment of the dorso-ventral axis. Thus, roles for Wnt-signaling in dorso-ventral patterning of embryos may be a vertebrate innovation that arose in connection with the evolution of yolky eggs and gastrulation by extensive involution.

Potential of veg2 blastomeres to induce endoderm differentiation in sea urchin embryos

Two different modes of gastrulation in sea urchin embryos have been reported. The first mode, reported in Hemicentrotus pulcherrimus and some other species, consists of two phases: a primary and a secondary invagination. The second mode involves gastrulation with a continuous convolution of cells near the blastopore; this mode has been reported to occur in the embryos of the sand dollar, Scaphechinus mirabilis. The rudimentary gut is comprised of fewer cells in the embryos of the former species than in the latter. We assumed that the differences in gastrulation modes could be related to the different potentials of the veg2 layer to induce endoderm differentiation in the upper layer. In the present study, we produced chimeric embryos consisting of an animal cap recombined with veg2 layer blastomere(s) to compare the inductive effect of the veg2 layer and/or the blastomere(s) in H. pulcherrimus and S. mirabilis embryos. Our results showed that the inductive effect of the veg2 layer is stronger in S. mirabilis embryos than in H. pulcherrimus embryos. Moreover, it was suggested that the difference in the strength of inductive effects of veg2 layers is related to the difference in gastrulation modes.

The role of Brachyury (T) during gastrulation movements in the sea urchin Lytechinus variegatus

The studies described here sought to identify and characterize genes involved in the gastrulation and morphogenetic movements that occur during sea urchin embryogenesis. An orthologue of the T-box family transcription factor, Brachyury, was cloned through a candidate gene approach. Brachyury (T) is the founding member of this T-box transcription factor family and has been implicated in gastrulation movements in Xenopus, zebrafish, and mouse embryogenesis. Polyclonal serum was generated to LvBrac in order to characterize protein expression. LvBrac initially appears at mesenchyme blastula stage in two distinct regions with embryonic expression perduring until pluteus stage. Vegetally, LvBrac expression is in endoderm and lies circumferentially around the blastopore. This torus-shaped area of LvBrac expression remains constant in size as endoderm cells express LvBrac upon moving into that circumference and cease LvBrac expression as they leave the circumference. Vegetal expression remains around the anus through pluteus stage. The second domain of LvBrac expression first appears broadly in the oral ectoderm at mesenchyme blastula stage and at later embryonic stages is refined to just the stomodael opening. Vegetal LvBrac expression depends on autonomous beta-catenin signaling in macromeres and does not require micromere or veg2-inductive signals. It was then determined that LvBrac is necessary for the morphogenetic movements occurring in both expression regions. A dominant-interfering construct was generated by fusing the DNA binding domain of LvBrac to the transcriptional repression module of the Drosophila Engrailed gene in order to perturb gene function. Microinjection of mRNA encoding this LvBrac-EN construct resulted in a block in gastrulation movements but not expression of endoderm and mesoderm marker genes. Furthermore, injection of LvBrac-EN into one of two blastomeres resulted in normal gastrulation movements of tissues derived from the injected blastomere, indicating that LvBrac downstream function may be nonautonomous during sea urchin gastrulation.

Characterization and developmental expression of the amphioxus homolog of Notch (AmphiNotch): evolutionary conservation of multiple expression domains in amphioxus and vertebrates

Notch encodes a transmembrane protein that functions in intercellular signaling. Although there is one Notch gene in Drosophila, vertebrates have three or more with overlapping patterns of embryonic expression. We cloned the entire 7575-bp coding region of an amphioxus Notch gene (AmphiNotch), encoding 2524 amino acids, and obtained the exon/intron organization from a genomic cosmid clone. Southern blot and PCR data indicate that AmphiNotch is the only Notch gene in amphioxus. AmphiNotch, like Drosophila Notch and vertebrate Notch1 and Notch2, has 36 EGF repeats, 3 Notch/lin-12 repeats, a transmembrane region, and 6 ankyrin repeats. Phylogenetic analysis places it at the base of all the vertebrate genes, suggesting it is similar to the ancestral gene from which the vertebrate Notch family genes evolved. AmphiNotch is expressed in all three embryonic germ layers in spatiotemporal patterns strikingly similar to those of all the vertebrate homologs combined. In the developing nerve cord, AmphiNotch is first expressed in the posteriormost part of the neural plate, then it becomes more broadly expressed and later is localized dorsally in the anteriormost part of the nerve cord corresponding to the diencephalon. In late embryos and larvae, AmphiNotch is also expressed in parts of the pharyngeal endoderm, in the anterior gut diverticulum, and, like AmphiPax2/5/8, in the rudiment of Hatschek's kidney. A comparison with Notch1 and Pax5 and Pax8 expression in the embryonic mouse kidney helps support homology of the amphioxus and vertebrate kidneys. AmphiNotch is also an early marker for presumptive mesoderm, transcripts first being detectable at the gastrula stage in a ring of mesendoderm just inside the blastopore and subsequently in the posterior mesoderm, notochord, and somites. As in sea urchins and vertebrates, these domains of AmphiNotch expression overlap with those of several Wnt genes and brachyury. These relationships suggest that amphioxus shares with other deuterostomes a common mechanism for patterning along the anterior/posterior axis involving a posterior signaling center in which the Notch and Wnt pathways and brachyury interact.

A micromere induction signal is activated by beta-catenin and acts through notch to initiate specification of secondary mesenchyme cells in the sea urchin embryo

At fourth cleavage of sea urchin embryos four micromeres at the vegetal pole separate from four macromeres just above them in an unequal cleavage. The micromeres have the capacity to induce a second axis if transplanted to the animal pole and the absence of micromeres at the vegetal pole results in the failure of macromere progeny to specify secondary mesenchyme cells (SMCs). This suggests that micromeres have the capacity to induce SMCs. We demonstrate that micromeres require nuclear beta-catenin to exhibit SMC induction activity. Transplantation studies show that much of the vegetal hemisphere is competent to receive the induction signal. The micromeres induce SMCs, most likely through direct contact with macromere progeny, or at most a cell diameter away. The induction is quantitative in that more SMCs are induced by four micromeres than by one. Temporal studies show that the induction signal is passed from the micromeres to macromere progeny between the eighth and tenth cleavage. If micromeres are removed from hosts at the fourth cleavage, SMC induction in hosts is rescued if they later receive transplanted micromeres between the eighth and tenth cleavage. After the tenth cleavage addition of induction-competent micromeres to micromereless embryos fails to specify SMCs. For macromere progeny to be competent to receive the micromere induction signal, beta-catenin must enter macromere nuclei. The macromere progeny receive the micromere induction signal through the Notch receptor. Signaling-competent micromeres fail to induce SMCs if macromeres express dominant-negative Notch. Expression of an activated Notch construct in macromeres rescues SMC specification in the absence of induction-competent micromeres. These data are consistent with a model whereby beta-catenin enters the nuclei of micromeres and, as a consequence, the micromeres produce an inductive ligand. Between the eighth and tenth cleavage micromeres induce SMCs through Notch. In order to be receptive to the micromere inductive signal the macromeres first must transport beta-catenin to their nuclei, and as one consequence the Notch pathway becomes competent to receive the micromere induction signal, and to transduce that signal. As Notch is maternally expressed in macromeres, additional components must be downstream of nuclear beta-catenin in macromeres for these cells to receive and transduce the micromere induction signal.

Body-plan evolution in the Bilateria: early antero-posterior patterning and the deuterostome-protostome dichotomy

Recent molecular analyses reveal common themes in early antero-posterior patterning in the four major groups of invertebrate deuterostomes and vertebrates in spite of large differences in the mode of gastrulation. Comparisons with Drosophila and Cnidarians suggest a scheme for evolution of the Bilaterian body plan and emphasize the pressing need for similar studies in a wider variety of organisms, especially more basal protostomes.

Animal-vegetal axis patterning mechanisms in the early sea urchin embryo

We discuss recent progress in understanding how cell fates are specified along the animal-vegetal axis of the sea urchin embryo. This process is initiated by cell-autonomous, maternally directed, mechanisms that establish three unique gene-regulatory domains. These domains are defined by distinct sets of vegetalizing (beta-catenin) and animalizing transcription factor (ATF) activities and their region of overlap in the macromeres, which specifies these cells as early mesendoderm. Subsequent signaling among cleavage-stage blastomeres further subdivides fates of macromere progeny to yield major embryonic tissues. Zygotically produced Wnt8 reinforces maternally regulated levels of nuclear beta-catenin in vegetal derivatives to down regulate ATF activity and further promote mesendoderm fates. Signaling through the Notch receptor from the vegetal micromere lineages diverts adjacent mesendoderm to secondary mesenchyme fates. Continued Wnt signaling expands the vegetal domain of beta-catenin's transcriptional regulatory activity and competes with animal signaling factors, including BMP2/4, to specify the endoderm-ectoderm border within veg(1) progeny. This model places new emphasis on the importance of the ratio of maternally regulated vegetal and animal transcription factor activities in initial specification events along the animal-vegetal axis.

Cell movements in the sea urchin embryo

Recent studies show that gastrulation in the sea urchin embryo involves movement of cells over the blastopore lip (involution). Some cells in the vegetal plate of the late blastula become bottle-shaped but they play a limited role in gastrulation. The functions of specific integrins, regulators of cell-cell adhesion, and extracellular matrix components in gastrulation are currently being analyzed. In addition, light-microscopic studies continue to provide a unique picture of dynamic cell behavior in vivo.