Regulatory signals and signal molecules in early neurogenesis of Drosophila melanogaster.

The ectodermal germ layer of Drosophila melanogaster gives rise to two major cell lineages, the neural and the epidermal. Progenitor cells for each of these lineages arise from groups of cells, whose elements must decide between taking on either fate. Commitment of the progenitor cells to one of the developmental fates implies two factors. One is intrinsic to the ectodermal cells and determines a propensity to take on neural fate; this factor is probably represented by the products of the so-called proneural genes, which are differentially distributed throughout the ectoderm. The other factor in the cells' decision to adopt one of the two alternative fates is intercellular communication, which is mediated by the products of the so-called neurogenic genes. Two types of interactions, one inhibiting and the other stimulating neural development, have been inferred. We discuss here the assumed role of various neurogenic genes, in particular Notch and Delta, in these processes.