During nervous system development, regulating when progenitor cells stop proliferating and start differentiating produces the correct number of neurons. But is neurogenesis triggered by cell-extrinsic or cell-intrinsic signals? In the developing zebrafish retina it may be primarily the latter,report Kay and colleagues (see p. ). In Drosophila eye development, signals from newly differentiated neurons trigger neurogenesis in adjacent progenitors, and researchers have proposed that a similar sequential induction drives retinal neurogenesis in vertebrates. By manipulating the environment of developing retinoblasts, Kay et al show instead that temporally staggered, cell-intrinsic expression of the proneural gene atonal-homologue 5 (ath5) is sufficient to support ganglion cell neurogenesis in the zebrafish retina. Thus,cell-intrinsic factors alone can trigger retinal neurogenesis. However, note the researchers, midline-derived Sonic hedgehog signals are part of the mechanism that sets the neurogenic timer earlier in zebrafish development.
