Figure 1

Roles for Notch during neurogenesis: selection of a neural progenitor and specification of neuronal subtype identity. (A, B) Notch communication (in pink) is required for the selection of neural progenitors both in Drosophila (A) and vertebrates (B). Arrows indicate the directionality of Notch signalling. Note that at the beginning of the process Notch communication is bidirectional. The letters 'a' and 'b' indicate the apical and basal sides of the neural tube. Dark blue, 'Notch off' cell; light blue, 'Notch on' cell. (C, D) Notch is also required during the specification of neural subtypes, which also consists of binary fate decisions but between two different neural subtypes. These binary decisions can either involve sister cells, as is the case during the formation of Drosophila sense organs (C) or cells that are not linearly related, as in the case of the R3 and R4 photoreceptors of the Drosophila eye (D). (C) Once specified as a neural progenitor, the SOP (Sensory organ precursor; or pI) divides to generate two cells, pIIa and pIIb, which communicate via Notch. Subsequent divisions generate the four cells of the sensory organs as well as a glial cell that will undergo apoptosis [64–66].