Sea Urchin Embryo: Specification of Cell Fates


Specification of cell fate in sea urchin embryos involves initial asymmetric distribution of maternal molecules that establish posterior and anterior domains of transcription activity. Subsequently, fates of most blastomeres along the anterior–posterior and dorsal–ventral axes of the embryo are patterned by cell–cell interactions involving signalling ligands and cell surface receptors. These signalling pathways regulate the operation of networks of genes encoding transcription factors and additional signals, which guide the terminal differentiation of different cell types. Most of the signalling mechanisms that establish different embryonic territories and some of the transcription factors that specify cell types are highly conserved with those that pattern vertebrate embryos. The relative simplicity of the sea urchin embryo and the existence of tools for rapidly determining gene function provide clear advantages for understanding how early developmental processes work.

Key Concepts:

  • Sea urchin embryogenesis employs the two common mechanisms of early fate specification, inheritance of maternal molecules and signalling among cells.

  • The maternally derived initial state, in the absence of all known signals sent among the cells of the embryo, promotes development of anterior neuroectoderm, a region of ectoderm within which nerves develop.

  • Early posterior canonical Wnt signalling activates transcription factors followed by additional signals that convert posterior blastomeres to mesoderm and endoderm and restrict the anterior neuroectoderm fate to the anterior end of the embryo.

  • Patterning of anterior (neuroectoderm) and posterior (endomesoderm) fates requires mutual antagonism between the gene regulatory networks headed by Wnt and Six3, respectively.

  • Nodal signalling is necessary and sufficient for specification of fates along the dorsal–ventral axis, including oral and aboral ectoderm types.

  • Anterior–posterior (AP) and dorsal–ventral (DV) patterning are interconnected and temporally coordinated because early posterior canonical Wnt signalling is required to derepress nodal expression.

  • The components of the major tissue territory gene regulatory networks (GRNs) have been identified by studying embryos lacking signalling through canonical Wnt, Nodal or BMP, and their functional relationships determined by knocking down each of the corresponding proteins in vivo with morpholino oligonucleotides and monitoring effects on expression of other genes.

  • Individual cell types in sea urchin embryos are determined when their fates cannot be changed experimentally; this process requires stable activation of genes necessary for specification of a particular cell type and stable repression of those required for specification of other cell types.

  • The GRN devices that produce stable regulatory states are reinforcing cross‐regulatory and feedback loops among the component transcription factors, which render these states insensitive to signals.

  • The sea urchin embryo has enormous capacity before larval stages to change cell fates upon experimental challenge because the fates of many cells are only gradually specified and can be altered by signals sent from other cells.

Keywords: maternal determinants; cell–cell interactions; fate specification; transcription factors; signalling pathways

Figure 1.

Stages of sea urchin embryo development: (a) 16‐cell; (b) 60‐cell; (c) early mesenchyme blastula (24 h postfertilisation); (d) mid‐gastrula (36 h postfertilisation); (e, f) pluteus larva (72 h postfertilisation). Cell types and tissues are color‐coded. Embryo widths along dorsal–ventral axis are approximately 100 μm.

Figure 2.

(a–e) Formation of major regions of the embryo via signalling through canonical Wnt and the TGF‐β‐ligands, Nodal and BMP2/4. A wave of canonical Wnt signalling begins at fourth cleavage to specify mesoderm and endoderm and restrict the anterior neuroectoderm to the anterior end of the embryo. The remaining tissue becomes competent to form the epidermal tissues in the oral and aboral ectoderm as a result of Nodal and BMP2/4 signalling, respectively. Signalling through Nodal results in the production of Lefty, BMP2/4 and Chordin. Lefty, a Nodal antagonist, diffuses (diffusion is designated by dashed arrows) farther than Nodal and prevents its activity where the ciliary band forms. Chordin, a diffusible BMP2/4 antagonist, prevents BMP2/4 signalling in the oral ectoderm and ciliary band regions, but BMP2/4 can diffuse to the other side of the embryo where it supports aboral ectoderm differentiation. As a result of the combined activities of these TGF‐βs and their antagonists, a low level of TGF‐β signalling is created in a strip of cells between the oral and aboral ectoderm, which becomes the ciliary band, a second neuroectodermal territory. See text for details.

Figure 3.

A generic GRN in the sea urchin embryo. A gene encoding transcription factor 1 (TF1) is activated by a signal transduction pathway (Signal1) in cell type 1. It then activates expression of another gene encoding Signal2, which is secreted and binds to a receptor on another cell (Cell type 2), where it activates the GRN of that cell type. TF1 also activates genes encoding additional (TFs 2–4) that either positively or negatively regulate each other's expression, creating a stable regulatory state that is independent of Signal1. TFs2–4 regulate the expression of additional genes that ether encode proteins required for the differentiated state (Diff1, Diff2) or additional transcription factors (e.g. TF5). A TF in the Cell Type1 GRN (shown here to be TF1) represses the operation of the Cell Type2 GRN. The structure of this network describes the basic scheme that leads from an initial inducing signal to the differentiation of one cell type and the exclusion of differentiation of another cell type.



Angerer LM, Newman LA and Angerer RC (2005) SoxB1 downregulation in vegetal lineages of sea urchin embryos is achieved by both transcriptional repression and selective protein turnover. Development 132: 999–1008.

Angerer LM, Oleksyn DW, Levine AM et al. (2001) Sea urchin goosecoid function links fate specification along the animal–vegetal and oral–aboral embryonic axes. Development 128: 4393–4404.

Angerer L, Yaguchi S, Angerer R and Burke R (2011) The evolution of nervous system patterning: insights from sea urchin development. Development 138: 3613–3623.

Ben‐Tabou de‐Leon SB and Davidson EH (2010) Information processing at the foxa node of the sea urchin endomesoderm specification network. Proceedings of the National Academy of Sciences of the USA 107: 10103–10108.

Bradham CA, Oikonomou C, Kühn A et al. (2009) Chordin is required for neural by not axial development in sea urchin embryos. Developmental Biology 328: 221–233.

Burke RD, Angerer LM, Elphick MR et al. (2006) A genomic view of the sea urchin nervous system. Developmental Biology 300: 434–460.

Cameron RA, Fraser SE, Britten RJ and Davidson EH (1991) Macromere cell fates during sea urchin development. Development 113: 1085–1091.

Cameron RA, Hough‐Evans BR, Britten RJ and Davidson EH (1987) Lineage and fate of each blastomere of the eight‐cell sea urchin embryo. Genes Development 1: 75–85.

Coffman JA and Davidson EH (2001) Oral–aboral axis specification in the sea urchin embryo. I. Axis entrainment by respiratory asymmetry. Developmental Biology 230: 18–28.

Coffman JA, McCarthy JJ, Dickey‐Sims C and Robertson AJ (2004) Oral–aboral axis specification in the sea urchin embryo; II. Mitochondrial distribution and redox state contribute to establishing polarity in Strongylocentrotus purpuratus. Developmental Biology 273: 160–171.

Croce J, Range R, Wu SY et al. (2011) Wnt6 activates endoderm in the sea urchin gene regulatory network. Development 138: 3297–3306.

Davidson EH (2009) Network design principles from the sea urchin embryo. Current Opinion in Genetics and Development 19: 535–540.

Davidson EH, Rast JP, Oliveri P et al. (2002a) A genomic regulatory network for development. Science 295: 1669–1678.

Davidson EH, Rast JP, Oliveri P et al. (2002b) A provisional regulatory gene network for specification of endomesoderm in the sea urchin embryo. Developmental Biology 246: 162–190.

Duboc V, Lapraz F, Besnardeau L and Lepage T (2008) Lefty acts as an essential modulator of Nodal activity during sea urchin oral–aboral axis formation. Developmental Biology 320: 49–59.

Duboc V, Lapraz F, Saudemont A et al. (2010) Nodal and BMP2/4 pattern the mesoderm and endoderm during development of the sea urchin embryo. Development 137: 223–235.

Duboc V, Rottinger E, Besnardeau L and Lepage T (2004) Nodal and BMP2/4 signaling organizes the oral–aboral axis of the sea urchin embryo. Developmental Cell 6: 397–410.

Duboc V, Rottinger E, Lapraz F, Besnardeau L and Lepage T (2005) Left–right asymmetry in the sea urchin embryo is regulated by nodal signaling on the right side. Developmental Cell 9: 147–158.

Duloquin L, Lhomond G and Gache C (2007) Localized VEGF signaling from ectoderm to mesenchyme cells controls morphogenesis of the sea urchin embryo skeleton. Development 134: 2293–2302.

Ettensohn CA (1990) The regulation of primary mesenchyme cell patterning. Developmental Biology 140: 261–271.

Ettensohn CA and Ingersoll EP (1992) Morphogenesis of the Sea Urchin Embryo. New York: Marcel Dekker, Inc.

Ettensohn CA and McClay DR (1988) Cell lineage conversion in the sea urchin embryo. Developmental Biology 125: 396–409.

Flowers VL, Courteau GR, Poustka AJ, Weng W and Venuti JM (2004) Nodal/activin signaling establishes oral–aboral polarity in the early sea urchin embryo. Developmental Dynamics 231: 727–740.

Hörstadius S (1973) Experimental Embryology of Echinoderms. Oxford: Clarendon Press.

Kenny AP, Kozlowski D, Oleksyn DW, Angerer LM and Angerer RC (1999) SpSoxB1, a maternally encoded transcription factor asymmetrically distributed among early sea urchin blastomeres. Development 126: 5473–5483.

Kenny AP, Oleksyn DW, Newman LA, Angerer RC and Angerer LM (2003) Tight regulation of SpSoxB factors is required for patterning and morphogenesis in sea urchin embryos. Developmental Biology 261: 412–425.

Lapraz F, Besnardeau L and Lepage T (2009) Patterning of the dorsal–ventral axis in echinoderms: insights into the evolution of the BMP‐chordin signaling network. PLoS Biology 7: e1000248.

Logan CY, Miller JR, Ferkowicz MJ and McClay DR (1999) Nuclear beta‐catenin is required to specify vegetal cell fates in the sea urchin embryo. Development 126: 345–357.

McClay DR and Logan CY (1996) Regulative capacity of the archenteron during gastrulation in the sea urchin. Development 122: 607–616.

Oliveri P, Carrick DM and Davidson EH (2002) A regulatory gene network that directs micromere specification in the sea urchin embryo. Developmental Biology 246: 209–228.

Oliveri P, Tu Q and Davidson EH (2008) Global regulatory logic for specification of an embryonic cell lineage. Proceedings of the National Academy of Sciences of the USA 105: 5955–5962.

Peter IS and Davidson EH (2010) The endoderm gene regulatory network in sea urchin embryos up to mid‐blastula stage. Developmental Biology 340: 188–199.

Peter IS and Davidson EH (2011) A gene regulatory network controlling the embryonic specification of endoderm. Nature 474: 365–369.

Revilla‐i‐Domingo R, Oliveri P and Davidson EH (2007) A missing link in the sea urchin embryo gene regulatory network: hesC and the double‐negative specification of micromeres. Proceedings of the National Academy of Sciences of the USA 104: 12383–12388.

Röttinger E, Saudemont A, Duboc V et al. (2008) FGF signals guide migration of mesenchymal cells, control skeletal morphogenesis and regulate gastrulation during sea urchin development. Development 135: 353–365.

Saudemont A, Haillot E, Mekpoh F et al. (2010) Ancestral regulatory circuits governing ectoderm patterning downstream of nodal and BMP2/4 revealed by gene regulatory network analysis in an echinoderm. PLoS Genetics 6: e1001259.

Sethi AJ, Angerer RC and Angerer LM (2009) Gene regulatory network interactions in sea urchin endomesoderm induction. PLoS Biology 7: e1000029.

Sherwood DR and McClay DR (1999) LvNotch signaling mediates secondary mesenchyme specification in the sea urchin embryo. Development 126: 1703–1713.

Sherwood DR and McClay DR (2001) LvNotch signaling plays a dual role in regulating the position of the ectoderm‐endoderm boundary in the sea urchin embryo. Development 128: 2221–2232.

Sodergren E, Weinstock GM, Davidson EH et al. (2006) The genome of the sea urchin Strongylocentrotus purpuratus. Science 314: 941–952.

Su YH, Li E, Geiss GK et al. (2009) A perturbation model of the gene regulatory network for oral and aboral ectoderm specification in the sea urchin embryo. Developmental Biology 329: 410–421.

Sweet HC, Gehring M and Ettensohn CA (2002) LvDelta is a mesoderm‐inducing signal in the sea urchin embryo and can endow blastomeres with organizer‐like properties. Development 129: 1945–1955.

Wei Z, Angerer RC and Angerer LM (2011) Direct development of neurons within foregut endoderm of sea urchin embryos. Proceedings of the National Academy of Sciences of the USA 108: 9143–9147.

Wei Z, Yaguchi J, Yaguchi S, Angerer RC and Angerer LM (2009) The sea urchin animal pole domain is a Six3‐dependent neurogenic patterning center. Development 136: 1179–1189.

Weitzel HE, Illies MR, Byrum CA et al. (2004) Differential stability of beta‐catenin along the animal‐vegetal axis of the sea urchin embryo mediated by dishevelled. Development 131: 2947–2956.

Wikramanayake AH, Huang L and Klein WH (1998) Beta‐catenin is essential for patterning the maternally specified animal‐vegetal axis in the sea urchin embryo. Proceedings of the National Academy of Sciences of the USA 95: 9343–9348.

Wikramanayake AH, Peterson R, Chen J et al. (2004) 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. Genesis 39: 194–205.

Yaguchi S, Yaguchi J, Angerer RC and Angerer LM (2008) A Wnt‐FoxQ2‐nodal pathway links primary and secondary axis specification in sea urchin embryos. Developmental Cell 14: 97–107.

Yaguchi S, Yaguchi J, Angerer RC, Angerer LM and Burke RD (2010) TGFβ signaling positions the ciliary band and patterns neurons in the sea urchin embryo. Developmental Biology 347: 71–81.

Yaguchi S, Yaguchi J and Burke RD (2006) Specification of ectoderm restricts the size of the animal plate and patterns neurogenesis in sea urchin embryos. Development 133: 2337–2346.

Yajima M and Wessel GM (2011) Small micromeres contribute to the germline in the sea urchin. Development 138: 237–243.

Further Reading

Croce JC and McClay DR (2006) The canonical Wnt pathway in embryonic axis polarity. Seminars Cell Developmental Biology 17: 168–174.

Epel D, Vacquier VD, Peeler M, Miller P and Patton C (2004) Sea urchin gametes in the teaching laboratory: good experiments and good experiences. Methods in Cell Biology 74: 797–823.

Ernst S (2011) Offerings from a Sea Urchin. Developmental Biology 358: 285–294.

Ettensohn CA (2009) Lessons from a gene regulatory network: echinoderm skeletogenesis provides insights into evolution, plasticity and morphogenesis. Development 136: 11–21.

Ettensohn CA, Wessel GM and Wray GA (2004) The invertebrate deuterostomes: an introduction to their phylogeny, reproduction, development and genomics. Methods in Cell Biology 74: 1–13.

McClay DM (2011) Evolutionary crossroads in developmental biology: sea urchins. Development 138: 2639–2648.

Peter IS and Davidson EH (2011) Evolution of gene regulatory networks controlling body plan development. Cell 144: 970–985.

Contact Editor close
Submit a note to the editor about this article by filling in the form below.

* Required Field

How to Cite close
Angerer, Robert C, and Angerer, Lynne M(Jan 2012) Sea Urchin Embryo: Specification of Cell Fates. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1002/9780470015902.a0001513.pub3]