Cleavage and Gastrulation in Sea Urchin

Abstract

Cleavage in the sea urchin initially is highly stereotypic and during this time cells are specified transcriptionally to obtain unique molecular identities. A complex series of intercellular signals assist in this diversification of the cells and coordinate their location in the blastula stage embryo. Once the cells are programmed by these early nuclear events the process of gastrulation follows. Gastrulation is characterized by a dramatic morphogenetic rearrangement of cells to form the basic body plan of the organism. In the sea urchin embryo gastrulation occurs in two phases: ingression of the skeletogenic cells and then invagination of the archenteron or the primitive gut.

Key Concepts:

  • Cleavage is the stage encompassing the earliest cell divisions after fertilization during which cells divide without an increase in embryo mass.

  • During cleavage, in each cell a combination of transcription factors and signalling between cells establishes gene regulatory networks.

  • Gene regulatory network states become different in subpopulations of cleaving cells through asymmetric signaling and/or asymmetric inheritance of cytoplasmic determinants, and this process is causal for cell diversification.

  • Gastrulation in the sea urchin embryo involves two separable morphogenetic processes: ingression of primary mesenchyme cells and invagination of the archenteron, or the primitive gut.

  • As a consequence of gastrulation, cells of the embryo are rearranged to establish the early body plan of the larval stage.

  • Most sea urchin embryos develop indirectly. That is, embryogenesis leads to a feeding larval stage, and the larva later metamorphoses into a juvenile, which grows into an adult.

Keywords: cleavage; blastula; gastrula; sea urchin; specification; cell fate

Figure 1.

Early development of the sea urchin embryo. A highly predictable series of cleavage divisions separates the cells into several tiers at the 60‐cell stage. Several divisions later the hatched blastula is characterized by a large central blastocoel. Shortly thereafter at the vegetal pole primary mesenchyme cells ingress into the blastocoel. Later, at gastrulation the vegetal plate invaginates to produce the archenteron, or primitive gut. The gut is surrounded by primary mesechyme cells that will produce the embryonic skeleton.

Figure 2.

Fate map of the embryo at the 60‐cell stage. Blue represents future ectodermal structures, yellow represents future gut or endoderm and red predicts future mesoderm. The arrows to the right project the normal fates of these cells.

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Further Reading

Gilbert SF (2006) Developmental Biology, 8th edn. New York: Sinauer Press.

Stern CD (ed.) (2004) Gastrulation. Cold Spring Harbor, NY: Cold Spring Harbor Press.

Wolpert L (2002) Principles of Development, 2nd edn. London: Oxford Press.

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How to Cite close
McClay, David R(Sep 2009) Cleavage and Gastrulation in Sea Urchin. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0001073.pub2]