Gene Expression and Early Cell Fate Specification in Embryos of the Purple Sea Urchin (Strongylocentrotus purpuratus)

This chapter focuses on cellular and molecular basis of the initial specification processes that occur in normal embryogenesis. A number of spatially regulated genes of the sea urchin embryo have relatively simple regulatory systems, consisting of a single module. Many of these are expressed in autonomously specified territories. Spatially distinct territories are established relatively early in sea urchin embryogenesis. The four main territories initially specified by blastula stage comprise the skeletogenic territory, the vegetal plate (or endomesodermal) territory, and the oral and aboral ectoderm territories. The cells of a fifth territory, the small micromeres, cease to divide early in embryogenesis but contribute to the coelomic pouches that give rise to the adult rudiment later in development. Founder cells for each territory have been identified and territory-specific genes have been cloned. The large micromeres that result from the second of two consecutive unequal divisions at the vegetal pole are the founder cells of the skeletogenic territory. The large micromeres are irreversibly committed to the skeletogenic pathway from the time they are formed, which is demonstrated by their unalterable pathway of differentiation in standard embryological tests