Sea Urchin Embryo: Specification of Cell Fate

020068:p0001 Sea urchin larvae develop from bilaterally symmetric deuterostome embryos whose basic structure is closely related to that of vertebrates. Millions of synchronously developing sea urchin embryos can be obtained by mixing gametes from a single pair of parents. The species most commonly studied develop into free-swimming pluteus larvae in about 3 days at 158C in a defined medium, sea water. The structure of the pluteus larva is relatively simple, consisting of only five major tissues and 14 known cell types. Development can be followed continuously in real time by microscopy, because the embryos are optically transparent and no histological preparation (e.g. fixing, sectioning and staining) is required. A wealth of information from experimental manipulations of the embryo, cell biology and molecular assays has established that pattern formation and cell differentiation at the beginning of development employ the two major mechanisms of specification of cell fate – the inheritance of maternal determinants and signalling among cells. Initially, an asymmetric distribution of transcription factors along the major axes (animal–vegetal (AV, corresponding approximately to anterior–posterior), oral–aboral (OA, roughly equivalent to dorsal/ventral and left right (LR)) establishes domains of specific gene expression. This endows cells in different regions of the embryo with the capacities to send and receive signals. These signals then lead to the expression of new sets of transcription factors which regulate genes that reinforce initial cell fate specifications and are required for production of specific proteins characterize the differentiated state of different cell types. 020068:p0002 The sea urchin is an excellent system to study fundamental mechanisms underlying the development of vertebrate embryos for a number of reasons. It is evolutionarily related to these organisms as a basal deuterostome; it is relative simple; its genome has been sequenced; and the developmental functions of genes can by assessed because of the recent development of methods that permit experimental perturbations of gene expression. Together these features have led during the last 5 years to the identifications of several core gene regulatory networks that govern development of the three basic germ layers – ectoderm, endoderm and mesoderm.