The oral-aboral axis of a sea urchin embryo is specified by first cleavage

Several lines of evidence suggest that the oral-aboral axis in Strongylocentrotus purpuratus embryos is specified at or before the 8-cell stage. Were the oral-aboral axis specified independently of the first cleavage plane, then a random association of this plane with the blastomeres of the four embryo quadrants in the oral-aboral plane (viz. oral, aboral, right and left) would be expected. Lineage tracer dye injection into one blastomere at the 2-cell stage and observation of the resultant labeling patterns demonstrates instead a strongly nonrandom association. In at least ninety percent of cases, the progeny of the aboral blastomeres are associated with those of the left lateral blastomeres and the progeny of the oral blastomeres with the right lateral ones, respectively. Thus, ninety percent of the time the oral pole of the future oral-aboral axis lies 45 degrees clockwise from the first cleavage plane as viewed from the animal pole. The nonrandom association of blastomeres after labeling of the 2-cell stage implies that there is a mechanistic relation between axis specification and the positioning of the first cleavage plane.

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Specification of endoderm and mesoderm in the sea urchin

Zygote ◽

10.1017/s0967199400130199 ◽

1999 ◽

Vol 8 (S1) ◽

pp. S41-S41 ◽

Cited By ~ 2

Author(s):

David R. McClay

Keyword(s):

Sea Urchin ◽

Wnt Pathway ◽

Special Significance ◽

Nuclear Entry ◽

Cell Stage ◽

Animal Pole ◽

Secondary Axis ◽

Sea Urchin Embryo ◽

Vegetal Pole

It has long been recognized that micromeres have special significance in early specification events in the sea urchin embryo. Micromeres have the ability to induce a secondary axis if transferred to the animal pole at the 16-cell stage of sea urchin embryos (Hörstadius, 1939). Without micromeres an isolated animal hemisphere develops into an ectodermal ball called a dauer blastula. Addition of micromeres to an animal half rescues a normal pluteus larva, including endoderm (Hörstadius, 1939). Despite these well-known experiments, however, neither the molecular basis of that induction nor the endogenous inductive role of micromeres in development was known. In recent experiments we learned that if one eliminates micromeres from the vegetal pole at the 16-cell stage the resulting embryo makes no secondary mesenchyme. Earlier it had been found that β-catenin is crucial for specification events that lead to mesoderm and endoderm (Wikra-manayake et al., 1998; Emily-Fenouil et al., 1998; Logan et al., 1999). We noticed that at the 16-cell stage β-catenin enters the nuclei of micromeres, then enters the nuclei of macromeres at the 32-cell stage (Logan et al., 1999). Since nuclear entry of β-catenin is known to be important for its signalling function in the Wnt pathway, we asked whether β-catenin functions in the micromere induction pathway.

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Early gene expression along the animal-vegetal axis in sea urchin embryoids and grafted embryos

Development ◽

10.1242/dev.122.10.3067 ◽

1996 ◽

Vol 122 (10) ◽

pp. 3067-3074 ◽

Cited By ~ 5

Author(s):

C. Ghiglione ◽

F. Emily-Fenouil ◽

P. Chang ◽

C. Gache

Keyword(s):

Gene Expression ◽

Sea Urchin ◽

Early Gene ◽

Expression Domain ◽

Cell Stage ◽

Animal Pole ◽

Early Gene Expression ◽

Sea Urchin Embryo ◽

Zygotic Gene

The HE gene is the earliest strictly zygotic gene activated during sea urchin embryogenesis. It is transiently expressed in a radially symmetrical domain covering the animal-most two-thirds of the blastula. The border of this domain, which is orthogonal to the primordial animal-vegetal axis, is shifted towards the animal pole in Li+-treated embryos. Exogenous micromeres implanted at the animal pole of whole embryos, animal or vegetal halves do not modify the extent and localization of the HE expression domain. In grafted embryos or animal halves, the Li+ effect is not affected by the presence of ectopic micromeres at the animal pole. A Li+-induced shift of the border, similar to that seen in whole embryos, occurs in embryoids developing from animal halves isolated from 8-cell stage embryos or dissected from unfertilised eggs. Therefore, the spatial restriction of the HE gene is not controlled by the inductive cascade emanating from the micromeres and the patterning along the AV-axis revealed by Li+ does not require interactions between cells from the animal and vegetal halves. This suggests that maternal primary patterning in the sea urchin embryo is not limited to a small vegetal center but extends along the entire AV axis.

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Autonomous and non-autonomous differentiation of ectoderm in different sea urchin species

Development ◽

10.1242/dev.121.5.1497 ◽

1995 ◽

Vol 121 (5) ◽

pp. 1497-1505 ◽

Cited By ~ 7

Author(s):

A.H. Wikramanayake ◽

B.P. Brandhorst ◽

W.H. Klein

Keyword(s):

Sea Urchin ◽

Strongylocentrotus Purpuratus ◽

Protein A ◽

Cellular Interactions ◽

V Protein ◽

Lytechinus Pictus ◽

Sea Urchin Embryo ◽

Oral Ectoderm ◽

Temporal And Spatial

During early embryogenesis, the highly regulative sea urchin embryo relies extensively on cell-cell interactions for cellular specification. Here, the role of cellular interactions in the temporal and spatial expression of markers for oral and aboral ectoderm in Strongylocentrotus purpuratus and Lytechinus pictus was investigated. When pairs of mesomeres or animal caps, which are fated to give rise to ectoderm, were isolated and cultured they developed into ciliated embryoids that were morphologically polarized. In animal explants from S. purpuratus, the aboral ectoderm-specific Spec1 gene was activated at the same time as in control embryos and at relatively high levels. The Spec1 protein was restricted to the squamous epithelial cells in the embryoids suggesting that an oral-aboral axis formed and aboral ectoderm differentiation occurred correctly. However, the Ecto V protein, a marker for oral ectoderm differentiation, was detected throughout the embryoid and no stomodeum or ciliary band formed. These results indicated that animal explants from S. purpuratus were autonomous in their ability to form an oral-aboral axis and to differentiate aboral ectoderm, but other aspects of ectoderm differentiation require interaction with vegetal blastomeres. In contrast to S. purpuratus, aboral ectoderm-specific genes were not expressed in animal explants from L. pictus even though the resulting embryoids were morphologically very similar to those of S. purpuratus. Recombination of the explants with vegetal blastomeres or exposure to the vegetalizing agent LiCl restored activity of aboral ectoderm-specific genes, suggesting the requirement of a vegetal induction for differentiation of aboral ectoderm cells.(ABSTRACT TRUNCATED AT 250 WORDS)

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GSK3beta/shaggy mediates patterning along the animal-vegetal axis of the sea urchin embryo

Development ◽

10.1242/dev.125.13.2489 ◽

1998 ◽

Vol 125 (13) ◽

pp. 2489-2498 ◽

Cited By ~ 19

Author(s):

F. Emily-Fenouil ◽

C. Ghiglione ◽

G. Lhomond ◽

T. Lepage ◽

C. Gache

Keyword(s):

Sea Urchin ◽

Wnt Pathway ◽

Dominant Negative ◽

Early Gene ◽

Expression Domain ◽

Animal Pole ◽

Axial Patterning ◽

Sea Urchin Embryo ◽

Vegetal Pole ◽

Dominant Negative Activity

In the sea urchin embryo, the animal-vegetal axis is defined before fertilization and different embryonic territories are established along this axis by mechanisms which are largely unknown. Significantly, the boundaries of these territories can be shifted by treatment with various reagents including zinc and lithium. We have isolated and characterized a sea urchin homolog of GSK3beta/shaggy, a lithium-sensitive kinase which is a component of the Wnt pathway and known to be involved in axial patterning in other embryos including Xenopus. The effects of overexpressing the normal and mutant forms of GSK3beta derived either from sea urchin or Xenopus were analyzed by observation of the morphology of 48 hour embryos (pluteus stage) and by monitoring spatial expression of the hatching enzyme (HE) gene, a very early gene whose expression is restricted to an animal domain with a sharp border roughly coinciding with the future ectoderm / endoderm boundary. Inactive forms of GSK3beta predicted to have a dominant-negative activity, vegetalized the embryo and decreased the size of the HE expression domain, apparently by shifting the boundary towards the animal pole. These effects are similar to, but even stronger than, those of lithium. Conversely, overexpression of wild-type GSK3beta animalized the embryo and caused the HE domain to enlarge towards the vegetal pole. Unlike zinc treatment, GSK3beta overexpression thus appeared to provoke a true animalization, through extension of the presumptive ectoderm territory. These results indicate that in sea urchin embryos the level of GSKbeta activity controls the position of the boundary between the presumptive ectoderm and endoderm territories and thus, the relative extent of these tissue layers in late embryos. GSK3beta and probably other downstream components of the Wnt pathway thus mediate patterning both along the primary AV axis of the sea urchin embryo and along the dorsal-ventral axis in Xenopus, suggesting a conserved basis for axial patterning between invertebrate and vertebrate in deuterostomes.

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A POU gene required for early cleavage and protein accumulation in the sea urchin embryo

Development ◽

10.1242/dev.120.7.1929 ◽

1994 ◽

Vol 120 (7) ◽

pp. 1929-1935 ◽

Cited By ~ 4

Author(s):

B.R. Char ◽

H. Tan ◽

R. Maxson

Keyword(s):

Sea Urchin ◽

Strongylocentrotus Purpuratus ◽

Antisense Oligodeoxynucleotides ◽

Protein Accumulation ◽

Antisense Inhibition ◽

Early Cleavage ◽

Sea Urchin Embryo ◽

Classical Work ◽

Zygotic Transcription ◽

New Protein

SpOct is a POU gene expressed during oogenesis and early embryogenesis of the sea urchin, Strongylocentrotus purpuratus. In the first use of antisense technology in the sea urchin embryo, we report that disruption of SpOct gene function in 1-cell zygotes by the injection of antisense oligodeoxynucleotides arrests development prior to the first cell division. We show that single-stranded antisense oligodeoxynucleotides specifically block cleavage, and that injection of SpOct mRNA overcomes this block. The accumulation of [35S]methionine into zygotically synthesized protein is significantly reduced in antisense-injected embryos. DNA synthesis is also reduced by the antisense regimen as expected from the antisense inhibition of protein accumulation. That protein accumulation prior to the first cleavage is retarded by antisense targeting of a transcription factor is very surprising in light of classical work showing that the activation of protein synthesis does not require zygotic transcription. We conclude that either some new transcription is obligate for the accumulation of new protein, or that the SpOct gene plays a novel, non-transcriptional role in this process.

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Early mRNAs, spatially restricted along the animal-vegetal axis of sea urchin embryos, include one encoding a protein related to tolloid and BMP-1

Development ◽

10.1242/dev.114.3.769 ◽

1992 ◽

Vol 114 (3) ◽

pp. 769-786 ◽

Cited By ~ 16

Author(s):

S.D. Reynolds ◽

L.M. Angerer ◽

J. Palis ◽

A. Nasir ◽

R.C. Angerer

Keyword(s):

Sea Urchin ◽

Gene Families ◽

Transcript Accumulation ◽

Cell Stage ◽

Multiple Transcripts ◽

Morphogenetic Protein ◽

Sea Urchin Embryo ◽

Peak Abundance ◽

Restricted Pattern ◽

Asymmetrically Distributed

The cloning and characterization of cDNAs representing four genes or small gene families that are coordinately expressed in a spatially restricted pattern during the very early blastula (VEB) stage of sea urchin development are presented. The VEB genes encode multiple transcripts that are expressed transiently in embryos of Strongylocentrotus purpuratus between 16-cell stage and hatching, with peak abundance 12 to 15 hours post-fertilization (approximately 150–250 cells). The VEB transcripts share the same spatial pattern in the early blastula embryo: they are asymmetrically distributed along the animal-vegetal axis but their distribution around this axis is uniform. Thus, the VEB transcripts are the earliest messages to reveal asymmetry along the primary axis in the sea urchin embryo. The temporal and spatial patterns of VEB transcript accumulation are not consistent with involvement of these gene products in cell division or in tissue-specific functions. Furthermore, VEB messages cannot be detected in either ovary or adult tissues, suggesting that these genes function exclusively during embryogenesis. We suggest that the VEB genes function in constructing the early blastula. Two VEB genes encode metalloendoproteases: one (SpHE) is hatching enzyme and the other (SpAN) is similar to bone morphogenetic protein-1 (BMP-1; Wozney et al., Science 242: 1528–1534, 1988) and the Tolloid gene product (tld) (Shimell et al., Cell 67: 459–482, 1991). Several lines of evidence suggest that the VEB genes are regulated directly by factors or regulatory activities localized along the maternally specificed animal-vegetal axis.

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