20 Patterning the Nervous System

Abstract

I. INTRODUCTION
The Caenorhabditis elegans nervous system consists of 302 neurons of 118 types that interconnect in a reproducible manner to form a variety of neural circuits and pathways (White et al. 1986). This complex array of neurons is generated by invariant patterns of cell division and migration (Sulston et al. 1983). Many of the neural types are generated in groups by neuroblast sublineages that are reiterated along the symmetry axes of the animal. Some of these iterated cell lineages are modified to generate sets of neural descendants that are variations on a theme. The correlation of neural type with pattern of cell lineage suggests that there are genetic pathways which couple neural specification to lineage. The variations on the canonical cell lineages suggest that there are also pathways that modify the activity of such modular cell-lineage-generating mechanisms.

Genes that act to couple cell lineage to neural identity as well as genes that mediate the variations in these cell lineages have been detected in genetic screens for mutants with defects in neural development or function (Finney and Ruvkun 1990; Miller et al. 1993; Wang et al. 1993; Jin et al. 1994). Many of these genes encode transcription factors that regulate the patterned generation of neurons in stereotyped cell lineages (Way and Chalfie 1988; Finney and Ruvkun 1990; Miller et al. 1992; Jin et al. 1994) or in spatial domains (Salser et al. 1993). Some of these genes also control the detailed features of neural type, such as their migration paths...