Determinants of Early Amphibian Development

The animal-vegetal organization of the amphibian egg may originatefrom the axis of organelles and cytoskeletal elements establishedin the oocyte as it divides from the oogonium. Along this axis,cytoplasmic materials are localized during oogenesis: yolk platelets,for example, are translocated toward the vegetal pole, increasingtheir amount and size in that region. In the first cell cycleafter fertilization, the egg cortex rotates 30° relativeto the cytoplasmic core, modifying animal-vegetal organization.The direction of this rotation, biased by the point of spermentry, defines the site of development of anatomical structuresof the dorsal midline of the embryo. As its immediate effect,rotation activates the cytoplasm of a subregion of the vegetalhemisphere, causing cells cleaved from this subregion to bemore effective than other vegetal parts in inducing marginalzone cells to initiate gastrulation movements. The most stronglyinduced part of the marginal zone begins gastrulation first(the dorsal lip of the blastopore) and proceeds through a seriesof cell interactions leading to its determination as the anteriordorsal mesoderm of the embryo. If these cell movements are inhibitedin the gastrula stage, or if vegetal induction is inhibitedin the blastula stage, or if cortical rotation is inhibitedin the first cell cycle after fertilization, the embryo alwaysfails to develop dorsal structures of the anterior end of itsbody axis; the more inhibition, the more posterior is the levelof truncation, until a radial ventralized embryo develops, derivedfrom the animal-vegetal organization of the oocyte.     

Regionalized Cell Division during Sea Urchin Gastrulation Contributes to Archenteron Formation and Is Correlated with the Establishment of Larval Symmetry

During gastrulation of the sea urchin, Lytechinus variegutus there is localized proliferation of cells in the vegetal plate region prior to its invagination. Cell counts show that during gastrulation the number of cells per embryo increases 60% from 1025 to 1640. Measurements of cell volumes suggest that some growth may follow these divisions. Feulgen staining shows that the greatest mitotic activity throughout gastrulation occurs in the vegetal plate region. Labelling embryos with 3H-thymidine reveals that incorporation in the vegetal plate is confined to cells that encircle the base of the archenteron. Pulse-chase experiments indicate that these labelled cells contribute descendants to the vegetal half of the archenteron. Additionally, 3-dimensional reconstructions of vegetal regions at different stages reveal that by the end of gastrulation two bilateral clusters of labelled cells lie at the future sites of the post-oral arms of the pluteus larva, thus marking the axes of bilateral and dorso-ventral symmetry. Our findings suggest that two of the principal events of sea urchin gastrulation — the formation of the archenteron and the establishment of symmetry in the larva — are accompanied by distinct patterns of cell division.     

The bases for and timing of regional specification during larval development in Phoronis

A fate map has been constructed for Phoronis vancouverensis. The animal pole of the egg gives rise to the apical plate in the hood of the actinotroch larva. The vegetal pole of the egg marks the site of gastrulation. During the initiation of gastrulation the cells of the animal pole of the embryo are directly opposite those at the vegetal pole of the embryo. The plane of the first cleavage always goes through the animal-vegetal pole of the egg. In about 70% of the cases the plane of the first cleavage is perpendicular to the future anterior-posterior axis of the actinotroch larva; in the remaining cases the plane of the first cleavage is either oblique with reference to, or occurs along, the future anterior-posterior axis of the larva. Following gastrulation catecholamine-containing cells first make their appearance in the apical plate and gut cells first produce esterase. The timing of regional specification in these embryos has been examined by isolating animal or vegetal, anterior or posterior, or lateral regions at different time periods between the initiation of cleavage and gastrulation and examining their ability to differentiate. Animal halves isolated from early cleavage through late blastula stages do not gastrulate and do not form catecholamine-containing cells. When animal halves are isolated with endoderm during gastrulation, they differentiate catecholamine-containing cells. Vegetal halves isolated at the 8- to 16-cell stage gastrulate and form normal actinotroch larvae with esterase-positive gut and catecholamine-containing apical plate cells. When this same region is isolated at blastula stages it does not gastrulate and does not differentiate these cell types. Vegetal halves isolated during gastrulation subsequently form esterase-positive gut cells, but they do not form catecholamine-containing apical plate cells. When presumptive anterior, posterior, or lateral halves are isolated from early cleavage through blastula stages, each half forms a normal actinotroch larva. Lateral halves isolated during gastrulation also form normal larvae. Anterior halves isolated during late gastrulation differentiate only the anterior end of the actinotroch larva. These isolates have a hood with catecholamine-containing apical plate cells and the first part of an esterase-positive gut but lack the anlagen of the intestine and protonephridia. Posterior halves isolated during late gastrulation differentiate only the posterior end of the actinotroch which lacks a hood with catecholamine-containing cells but has an esterase-positive gut, protonephridia, and the anlagen of the intestine.(ABSTRACT TRUNCATED AT 400 WORDS)     

Regional differences in the lateral mobility of plasma membrane lipids in a molluscan embryo

Regional and temporal differences in plasma membrane lipid mobility have been analyzed during the first three cleavage cycles of the embryo of the polar-lobe-forming mollusc Nassarius reticulatus by the fluorescence photobleaching recovery (FPR) method, using 1,1'-ditetradecyl 3,3,3',3'-tetramethylindocarbocyanine iodide (C14diI) as a fluorescent lipid probe. During this period of development the lateral diffusion coefficient of membrane lipids is consistently greater in the vegetal polar lobe area as compared to the animal plasma membrane area (on average 30%), demonstrating the existence of an animal-vegetal polarity in plasma membrane properties. At third cleavage, the differences between animal and vegetal plasma membrane region become even more pronounced; in the four animal micromeres the diffusion coefficient (D) and mobile fraction (MF) are 2.9 +/- 0.2 X 10(-9) cm2/sec and 51 +/- 2%, respectively, while in the four vegetal macromeres D = 5.0 +/- 0.3 X 10(-9) cm2/sec and MF = 78 +/- 2%. Superimposed upon the observed animal-vegetal polarity, the lateral diffusion in the polar lobe membrane area shows a cell-cycle-dependent modulation. The highest mean values for D are reached during the S phase (ranging from 7.0 to 7.8 X 10(-9) cm2/sec in the three cycles measured), while at the end of G2 phase and during early mitosis mean values for D have decreased significantly (ranging from 5.0 to 5.9 X 10(-9) cm2/sec). Diffusion rates in the animal membranes of the embryo are constant during the three successive cell cycles (D = 4.3-5.0 X 10(-9) cm2/sec), except for a peak at the S phase of the first cell cycle (D = 6.0 X 10(-9) cm2/sec). These results are discussed in relation with previously observed ultrastructural heterogeneities in the Nassarius egg plasma membrane. It is speculated that the observed animal-vegetal polarity in the organization of the egg membrane might play an important role in the process of cell diversification during early development.     

The role of animal-vegetal interaction with respect to the determination of dorsoventral polarity in the equal-cleaving spiralian,Lymnaea palustris

Summary Spirally cleaving embryos in which the first two cleavages generate four equal-sized blastomeres remain radially symmetrical along their animal-vegetal axis until the interval between third and fourth quartet formation. At this time animal micromeres and vegetal macromeres contact each other as they elongate and occlude the central, fluid-filled cleavage cavity. The overlying micromeres focus their contacts onto one of the four macromeres, the presumptive 3D macromere, as it elongates to a central position within the embryo. We tested the hypothesis that this animal-vegetal interaction was causally involved in the determination of the symmetry properties in both the animal and vegetal hemispheres by reversibly inhibiting animal-vegetal contact at the 24 cell stage with cytochalasin-B. Embryos remained hollow throughout the treatment period and animal-vegetal interaction did not occur. After treatment, blastomere elongation occurred but no D quadrant macromere appeared and the vegetal hemisphere remained radialized. On the basis of cleavage and ciliation patterns of first quartet derivatives, treated embryos remained fully or partially radialized, showing a strong tendancy to develop as ventral quadrants. These results show that the quadrants of this equal-cleaving spiralian are not definitively determined until after the 24 cell stage and that animal-vegetal interaction is required for D quadrant determination. The mechanisms of symmetrization in the animal and vegetal hemispheres of equal-cleaving spiralians is also discussed.     

Nuclear beta-catenin-dependent Wnt8 signaling in vegetal cells of the early sea urchin embryo regulates gastrulation and differentiation of endoderm and mesodermal cell lineages

The entry of beta-catenin into vegetal cell nuclei beginning at the 16-cell stage is one of the earliest known molecular asymmetries seen along the animal-vegetal axis in the sea urchin embryo. Nuclear beta-catenin activates a vegetal signaling cascade that mediates micromere specification and specification of the endomesoderm in the remaining cells of the vegetal half of the embryo. Only a few potential target genes of nuclear beta-catenin have been functionally analyzed in the sea urchin embryo. Here, we show that SpWnt8, a Wnt8 homolog from Strongylocentrotus purpuratus, is zygotically activated specifically in 16-cell-stage micromeres in a nuclear beta-catenin-dependent manner, and its expression remains restricted to the micromeres until the 60-cell stage. At the late 60-cell stage nuclear beta-catenin-dependent SpWnt8 expression expands to the veg2 cell tier. SpWnt8 is the only signaling molecule thus far identified with expression localized to the 16-60-cell stage micromeres and the veg2 tier. Overexpression of SpWnt8 by mRNA microinjection produced embryos with multiple invagination sites and showed that, consistent with its localization, SpWnt8 is a strong inducer of endoderm. Blocking SpWnt8 function using SpWnt8 morpholino antisense oligonucleotides produced embryos that formed micromeres that could transmit the early endomesoderm-inducing signal, but these cells failed to differentiate as primary mesenchyme cells. SpWnt8-morpholino embryos also did not form endoderm, or secondary mesenchyme-derived pigment and muscle cells, indicating a role for SpWnt8 in gastrulation and in the differentiation of endomesodermal lineages. These results establish SpWnt8 as a critical component of the endomesoderm regulatory network in the sea urchin embryo.     

Jun N‐terminal kinase activity is required for invagination but not differentiation of the sea urchin archenteron

Although sea urchin gastrulation is well described at the cellular level, our understanding of the molecular changes that trigger the coordinated cell movements involved is not complete. Jun N‐terminal kinase (JNK) is a component of the planar cell polarity pathway and is required for cell movements during embryonic development in several animal species. To study the role of JNK in sea urchin gastrulation, embryos were treated with JNK inhibitor SP600125 just prior to gastrulation. The inhibitor had a limited and specific effect, blocking invagination of the archenteron. Embryos treated with 2 μM SP600125 formed normal vegetal plates, but did not undergo invagination to form an archenteron. Other types of cell movements, specifically ingression of the skeletogenic mesenchyme, were not affected, although the development and pattern of the skeleton was abnormal in treated embryos. Pigment cells, derived from nonskeletogenic mesenchyme, were also present in SP600125‐treated embryos. Despite the lack of a visible archenteron in treated embryos, cells at the original vegetal plate expressed several molecular markers for endoderm differentiation. These results demonstrate that JNK activity is required for invagination of the archenteron but not its differentiation, indicating that in this case, morphogenesis and differentiation are under separate regulation. genesis 53:762–769, 2015. © 2015 Wiley Periodicals, Inc.     

Determination and morphogenesis in the sea urchin embryo

The study of the sea urchin embryo has contributed importantly to our ideas about embryogenesis. This essay re-examines some issues where the concerns of classical experimental embryology and cell and molecular biology converge. The sea urchin egg has an inherent animal-vegetal polarity. An egg fragment that contains both animal and vegetal material will produce a fairly normal larva. However, it is not clear to what extent the oral-aboral axis is specified in embryos developing from meridional fragments. Newly available markers of the oral-aboral axis allow this issue to be settled. When equatorial halves, in which animal and vegetal hemispheres are separated, are allowed to develop, the animal half forms a ciliated hollow ball. The vegetal half, however, often forms a complete embryo. This result is not in accord with the double gradient model of animal and vegetal characteristics that has been used to interpret almost all defect, isolation and transplantation experiments using sea urchin embryos. The effects of agents used to animalize and vegetalize embryos are also due for re-examination. The classical animalizing agent, Zn2+, causes developmental arrest, not expression of animal characters. On the other hand, Li+, a vegetalizing agent, probably changes the determination of animal cells. The stability of these early determinative steps may be examined in dissociation-reaggregation experiments, but this technique has not been exploited extensively. The morphogenetic movements of primary mesenchyme are complex and involve a number of interactions. It is curious that primary mesenchyme is dispensable in skeleton formation since in embryos devoid of primary mesenchyme, the secondary mesenchyme cells will form skeletal elements. It is likely that during its differentiation the primary mesenchyme provides some of its own extracellular microenvironment in the form of collagen and proteoglycans. The detailed form of spicules made by primary mesenchyme is determined by cooperation between the epithelial body wall, the extracellular material and the inherent properties of primary mesenchyme cells. Gastrulation in sea urchins is a two-step process. The first invagination is a buckling, the mechanism of which is not understood. The secondary phase in which the archenteron elongates across the blastocoel is probably driven primarily by active cell repacking. The extracellular matrix is important for this repacking to occur, but the basis of the cellular-environmental interaction is not understood.(ABSTRACT TRUNCATED AT 400 WORDS)     

Delayed transition to new cell fates during cellular reprogramming

In many embryos specification toward one cell fate can be diverted to a different cell fate through a reprogramming process. Understanding how that process works will reveal insights into the developmental regulatory logic that emerged from evolution. In the sea urchin embryo, cells at gastrulation were found to reprogram and replace missing cell types after surgical dissections of the embryo. Non-skeletogenic mesoderm (NSM) cells reprogrammed to replace missing skeletogenic mesoderm cells and animal caps reprogrammed to replace all endomesoderm. In both cases evidence of reprogramming onset was first observed at the early gastrula stage, even if the cells to be replaced were removed earlier in development. Once started however, the reprogramming occurred with compressed gene expression dynamics. The NSM did not require early contact with the skeletogenic cells to reprogram, but the animal cap cells gained the ability to reprogram early in gastrulation only after extended contact with the vegetal halves prior to that time. If the entire vegetal half was removed at early gastrula, the animal caps reprogrammed and replaced the vegetal half endomesoderm. If the animal caps carried morpholinos to either hox11/13b or foxA (endomesoderm specification genes), the isolated animal caps failed to reprogram. Together these data reveal that the emergence of a reprogramming capability occurs at early gastrulation in the sea urchin embryo and requires activation of early specification components of the target tissues.     

LvNotch signaling plays a dual role in regulating the position of the ectoderm-endoderm boundary in the sea urchin embryo

The molecular mechanisms guiding the positioning of the ectoderm-endoderm boundary along the animal-vegetal axis of the sea urchin embryo remain largely unknown. We report here a role for the sea urchin homolog of the Notch receptor, LvNotch, in mediating the position of this boundary. Overexpression of an activated form of LvNotch throughout the embryo shifts the ectoderm-endoderm boundary more animally along the animal-vegetal axis, whereas expression of a dominant negative form shifts the border vegetally. Mosaic experiments that target activated and dominant negative forms of LvNotch into individual blastomeres of the early embryo, combined with lineage analyses, further reveal that LvNotch signaling mediates the position of this boundary by distinct mechanisms within the animal versus vegetal portions of the embryo. In the animal region of the embryo, LvNotch signaling acts cell autonomously to promote endoderm formation more animally, while in the vegetal portion, LvNotch signaling also promotes the ectoderm-endoderm boundary more animally, but through a cell non-autonomous mechanism. We further demonstrate that vegetal LvNotch signaling controls the localization of nuclear beta-catenin at the ectoderm-endoderm boundary. Based on these results, we propose that LvNotch signaling promotes the position of the ectoderm-endoderm boundary more animally via two mechanisms: (1) a cell-autonomous function within the animal region of the embryo, and (2) a cell non-autonomous role in the vegetal region that regulates a signal(s) mediating ectoderm-endoderm position, possibly through the control of nuclear beta-catenin at the boundary.     

Regional specification during embryogenesis in the craniiform brachiopod Crania anomala

A fate map has been constructed for the embryo of Crania. The animal half of the egg forms the ectodermal epithelium of the larva's apical lobe. The vegetal half of the egg forms endoderm, mesoderm, and the ectoderm of the mantle lobe. The vegetal pole is the site of gastrulation; this site becomes the posterior ventral region of the mantle lobe of the larva. The plane of the first cleavage goes through the animal-vegetal axis of the egg; it bears no relationship to the future plane of bilateral symmetry of the larva. The timing of regional specification was examined by isolating animal, vegetal, or meridional halves from oocytes, eggs, or embryos from prior to germinal vesicle breakdown through gastrulation. Animal halves isolated from oocytes formed either the epithelium of the apical lobe or a larva with all three germ layers. Animal halves isolated from unfertilized eggs and eight-cell embryos formed only apical lobe epithelium. Beginning at the blastula stage, animal halves formed mantle in addition to apical lobe epithelium. In animal halves isolated after gastrulation, the mantle lobe was always truncated. Vegetal halves isolated at all stages prior to gastrulation gastrulated and formed apical and mantle lobes with endoderm and mesoderm; however, the relative size of the apical lobe that formed decreased substantially when vegetal halves were isolated at later developmental stages. When meridional halves were isolated from unfertilized eggs and two- to four-cell embryos, both halves frequently formed normally proportioned larvae. Beginning at the blastula stage, a number of pairs frequently had a member that lacked dorsal setae on its mantle lobe while the other member of the pair formed setae, indicating that the dorsoventral axis had been set up. The process of regional specification in Crania is compared to those of Discinisca and Glottidia in the brachiopod subphylum Linguliformea and Phoronis in the phylum Phoronida.     

A study of cell interactions involved in Pleurodeles waltlii epidermal differentiation

Summary A polyclonal antibody (SP-2) has been produced, which recognizes antigens expressed in epidermal cells of Pleurodeles waltlii embryos. The antigens appear first at the end of gastrulation in the external surface of the embryo and are selectively expressed in ectodermally derived epidermal structures. Ectodermal commitment was investigated using cell cultures and blastocoel graft experiments. The four animal blastomeres of the 8-cell stage as well as the animal cap explants of the early gastrula stage cultured in vitro differentiate into epidermis, and SP-2 antigens are expressed. The expression of SP-2-defined antigens is inhibited both in vivo and in vitro by the inductive interaction of chordomesoderm. Once dissociated, ectodermal cells do not react with SP-2. Conversely, the aggregation of ectodermal cells may restore the expression of SP-2 antigens. Transplantation of animal cap explants or isolated ectodermal cells into the blastocoel of a host embryo at the early gastrula stage shows that only cells integrated into the epidermis express the marker antigens. When vegetal cells were dissociated from donor embryos before the mid-blastula stage and implanted into the blastocoel of host embryos at the early gastrula stage, their progeny were found in all germ layers, cells that were found in the host epidermis were stained with SP-2, whereas those contributing to mesoderm and endoderm were not. Thus the acquisition of cell polarity in epidermal differentiation and the organization of cells into epithelial structures are essential for SP-2-defined antigen expression.     

Animal and vegetal poles of the mouse egg predict the polarity of the embryonic axis, yet are nonessential for development

Recent studies suggest early (preimplantation) events might be important in the development of polarity in mammalian embryos. We report here lineage tracing experiments with green fluorescent protein showing that cells located either near to or opposite the polar body at the 8-cell stage of the mouse embryo retain their same relative positions in the blastocyst. Thus they come to lie on either end of an axis of symmetry of the blastocyst that has recently been shown to correlate with the anterior-posterior axis of the postimplantation embryo (see R. J. Weber, R. A. Pedersen, F. Wianny, M. J. Evans and M. Zernicka-Goetz (1999). Development 126, 5591-5598). The embryonic axes of the mouse can therefore be related to the position of the polar body at the 8-cell stage, and by implication, to the animal-vegetal axis of the zygote. However, we also show that chimeric embryos constructed from 2-cell stage blastomeres from which the animal or the vegetal poles have been removed can develop into normal blastocysts and become fertile adult mice. This is also true of chimeras composed of animal or vegetal pole cells derived through normal cleavage to the 8-cell stage. We discuss that although polarity of the postimplantation embryo can be traced back to the 8-cell stage and in turn to the organisation of the egg, it is not absolutely fixed by this time.     

Electron microscopic study of the development of amphioxus,Branchiostoma belcheri tsingtauense: The gastrula

The gastrulae of amphioxus were investigated by means of scanning and transmission electron microscopy (SEM and TEM) during 7 arbitrary stages that were seen about 4 to 10 hr after fertilization. Throughout gastrulation, SEM revealed subtle differences in cells of the blastoporal lip. In fractured specimens at early and middle stages, two opposing zones different in shape, size, and connection of the component cells were found: one which consists of columnar smaller cells in close contact in animal region and the other which is composed of round or polygonal larger cells in looser association in vegetal region. The polar body was found unexpectedly on the concave vegetal surface of the early gastrula in about 25% of cases. This might be the result of migration of the polar body. A short cilium that later elongated was recognized on each cell at mid-gastrula stage. The cilia on the dorsal surface (the neural ectoderm) of the final-stage gastrula became shorter than those on the epidermal ectoderm. TEM of thin sections demonstrated that the cytoplasmic components of gastrula cells are essentially the same as those of cleavage cells. But, the homogeneous nucleus seen during cleavage changed into a heterogeneous structure in which a nucleolus and dense particles were seen. Until the late stage, regional characteristics of the gastrulae indicating definitively the anterior-posterior and dorso-ventral polarity were not detected in the present SEM and TEM study.     

Polar asymmetry in the organization of the cortical cytokeratin system of Xenopus laevis oocytes and embryos

We have used whole-mount immunofluorescence microscopy of late-stage Xenopus laevis oocytes and early embryos to examine the organization of their cortical cytokeratin systems. In both mature oocytes and early embryos, there is a distinct animal-vegetal polarity in cytokeratin organization. In mature (stage-VI) oocytes, the cytokeratin filaments of the vegetal region form a unique, almost geodesic network; in the animal region, cytokeratin organization appears much more variable and irregular. In unfertilized, postgerminal vesicle breakdown eggs, the cortical cytokeratin system is disorganized throughout both animal and vegetal hemispheres. After fertilization, cytokeratin organization reappears first in a punctate pattern that is transformed into an array of oriented filaments. These cytokeratin filaments appear first in the vegetal hemisphere and are initially thin. Subsequently, they form bundles that grow thicker through the period of first to second cleavage, at which point large cytokeratin filament bundles form a loose, fishnet-like system that encompasses the vegetal portion of each blastomere. In the animal region, cytokeratin filaments do not appear to form large fibre networks, but rather appear to be organized into a system of fine filaments. The animal-vegetal polarity in cytokeratin organization persists until early blastula (stage 5); in later-stage embryos, both animal and vegetal blastomeres possess qualitatively similar cytokeratin filament systems. The entire process of cytokeratin reorganization in the egg is initiated by prick activation. These observations indicate that the cortical cytoskeleton of Xenopus oocytes and early embryos is both dynamic and asymmetric.     

Predictability of dorso-ventral asymmetry in the cleavage stage zebrafish embryo: an analysis using lithium sensitivity as a dorso-ventral marker.

The dorso-ventral axis in zebrafish first becomes apparent at gastrulation, when the future ventral side appears thinner than the dorsal side. The exact time of establishment of the dorso-ventral axis is not known. We show here that the dorso-ventral axis is specified as early as the 32 cell stage. Using lithium as a marker for dorso-ventral asymmetry, we show that lithium-sensitivity is a characteristic of future ventral cell, but not future dorsal cells, and that there is an asymmetric lithium-sensitivity along the long axis of the 32 cell stage embryo. Consequently, the dorso-ventral axis corresponds to the long axis of the embryo. Because the effect of lithium treatment is short-lived, the dorso-ventral axis must be specified in zebrafish already at the 32 cell stage.     

Early development and axis specification in the sea anemone Nematostella vectensis

We investigated the early development of the sea anemone Nematostella vectensis, an emerging model system of the Cnidaria. Early cleavage stages are characterized by substantial variability from embryo to embryo, yet invariably lead to the formation of a coeloblastula. The coeloblastula undergoes a series of unusual broad invaginations-evaginations which can be blocked by cell cycle inhibitors suggesting a causal link of the invagination cycles to the synchronized cell divisions. Blastula invagination cycles stop as cell divisions become asynchronous. Marking experiments show a clear correspondence of the animal-vegetal axis of the egg to the oral-aboral axis of the embryo. The animal pole gives rise to the concave side of the blastula and later to the blastopore of the gastrula, and hence the oral pole of the future polyp. Asymmetric distribution of granules in the unfertilized egg suggest an animal-vegetal asymmetry in the egg in addition to the localized position of the pronucleus. To determine whether this asymmetry reflects asymmetrically distributed determinants along the animal-vegetal axis, we carried out blastomere isolations and embryonic divisions at various stages. Our data strongly indicate that normal primary polyps develop only if cellular material from the animal hemisphere is included, whereas the vegetal hemisphere alone is incapable to differentiate an oral pole. Molecular marker analysis suggests that also the correct patterning of the aboral pole depends on signals from the oral half. This suggests that in Nematostella embryos the animal hemisphere contains organizing activity to form a normal polyp.