Chapter 2 Position-Dependent Cell Interactions and Commitments in the Formation of the Leech Nervous System

Abstract

This chapter discusses the cellular basis of a particular instance of positional specification—the divergence of the O and P cell lines that occur during the embryonic development of the leech. Positional specification requires that cells be able to detect and respond to positional cues by choosing one of two or more alternative developmental pathways. The means by which cells detect and respond to position is a central issue in the field of pattern formation, and by studying this process within a context of defined cell lineages, it may be possible to resolve positional cues in terms of specific interactions occurring among a manageable number of cells. The chapter describes the way in which position-dependent cell interactions become converted into cell-intrinsic states of commitment, leading to a discussion of the ways in which developmental commitment of an embryonic progenitor cell may influence the composition of its descendant clone. Position-dependent cell interactions are a fundamental part of vertebrate neurogenesis and an understanding of pattern formation in invertebrate nervous systems may lend insights there as well.