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.