Abstract

The generation of the correct cell types at the appropriate position and time is the first step in the assembly of functional tissues. One well studies example is the development of the vertebrate spinal cord. In this tissue, distinct neuronal subtypes are generated in a precise spatial order from progenitor cells arrayed along the dorsal-ventral axis of the neural tube. Underpinning this organization is a complex network of extrinsic and intrinsic factors. Particularly well understood is the mechanism that determines the generation of different neuronal subtypes in ventral regions of the spinal cord. In this region of the nervous system, the secreted protein Sonic Hedgehog (Shh) acts in graded fashion to organize the pattern of neurogenesis. This is a dynamic process in which exposure to Shh generates progenitors with successively more ventral identities. At the same time tissue growth alters the arrangement of cells and the proportions of cell types and contributes to the elaboration of pattern. A gene regulatory network composed of transcription factors regulated by Shh signaling play an essential role in determining the graded response of cells. Thus, accurate patterning of the neural tube and the specification of neuronal subtype identity relies on the interplay of cellular and molecular processes.