Morphogenesis of an identified leech neuron: segmental specification of axonal outgrowth.

We have investigated the development of segmental diversity in an identified leech neuron, the Retzius cell. Retzius cells in the genital segments differ from those in other segments in lacking central axons and contacting different peripheral targets: the genitalia. These differences are not apparent during initial axon outgrowth, when all Retzius cells follow the same morphogenetic pattern. Rather, they first appear about the time the peripheral axons of the genital segment Retzius cells contact the genital primordia. This suggests that the pattern of central and peripheral axonal outgrowth may be modified by an interaction with peripheral targets.