Abstract
This chapter reviews the current status of the specification events leading to the differentiation of ectodermal cell types in sea urchin embryo. In sea urchins, the fates of all the major cell types are specified very early in embryogenesis, and it is the establishment of these individual cell types, rather than reiterative patterning along a body axis, which is critical for embryonic development. This chapter describes the embryological and molecular features that make ectodermal cell fate specification an attractive experimental system. The prevailing model that the activities of uniformly distributed transcription factors are regionally modulated by external signaling pathways is evaluated in light of new experiments. A comparison is made of the relevance of recent findings on sea urchin development to other organisms and how evolutionary mechanisms influence cell fate-specification processes is considered. The chapter also presents the current evidence linking extracellular signaling events with ectoderm-specific gene expression.