Rules for the production of sensory cells.

During development and regeneration, cells divide, move, change their internal state, respond to environmental signals and die according to rules specified by their genome. These rules of cell behaviour are fundamental to any explanation of how sensory tissues are generated or repaired. An attempt is made to summarize and compare the rules underlying the development of four different sensory tissues: mammalian retina, avian auditory epithelium, mechanosensory bristle fields in Drosophila and, very briefly, Drosophila retina. There are parallels in cell lineage, in the timing of developmental decisions, in the postponement of the choice of differentiated state until the final cell division cycle, in the role of short-range lateral inhibition, and in the involvement of genes such as Notch and achaete/scute. These similarities may reflect the conservation of some basic mechanisms of sensory development that originated early in the evolution of multicellular animals. If so, studies in Drosophila, with its advantages for molecular genetics, may give important clues to the mechanisms of sensory development and regeneration in vertebrates.