Cellular diversity in the developing nervous system: a temporal view from Drosophila

This article considers the evidence for temporal transitions in CNS neural precursor cell gene expression during development. In Drosophila, five prospective competence states have so far been identified, characterized by the successive expression of Hb-->Kr-->Pdm-->Cas-->Gh in many, but not all, neuroblasts. In each temporal window of transcription factor expression, the neuroblast generates sublineages whose temporal identity is determined by the competence state of the neuroblast at the time of birth of the sublineage. Although similar regulatory programs have not yet been identified in mammals, candidate regulatory genes have been identified. Further investigation of the genetic programs that guide both invertebrate and vertebrate neural precursor cell lineage development will ultimately lead to an understanding of the molecular events that control neuronal diversity.