Morphological changes in leech Retzius neurons after target contact during embryogenesis.

Segmental variation in identified neurons may provide an opportunity to examine extrinsic influences on neuronal phenotype, since segmentally homologous neurons must contain much the same intrinsic information, having arisen from very similar or identical precursors. Two large serotonergic Retzius (Rz) cells are found in each segmental ganglion of the leech Hirudo medicinalis. While most Rz cells innervate the body wall in their own segment and, by way of axons in the interganglionic connectives, the body wall of adjacent segments, the Rz cells in ganglia 5 and 6 [Rz(5,6)] lack interganglionic axons and innervate only the reproductive tissue (Glover and Mason, 1986). Here we describe and quantify the development of differences between Rz(5,6) and other Rz cells in peripheral innervation, neuropilar arborization, and soma size. We filled individual Rz cells with Lucifer yellow or HRP in adults and in staged embryos. During the first 72 hr of outgrowth of Rz cell processes, the morphology of Rz(5,6) was indistinguishable from that of other Rz cells. Only after the processes of Rz(5,6) reached the reproductive tissue did they begin to differ from their segmental homologs. This temporal correlation suggests that these morphological differences arise because of some interaction between Rz(5,6) and their target tissue.