DEVELOPMENT AND PATHWAY FORMATION OF PERIPHERAL NEURONS DURING LEECH EMBRYOGENESIS

By labeling the germinal plates of staged leech embryos with monoclona l antibodies to the immunoglobulin superfamily member Tractin, we have documented the distribution and initial development of peripheral neu rons in a hirudinid leech. We iind, in addition to sensillar and extra sensillar sensory neurons, that there are 21 identifiable peripheral n eurons in each hemisegment. These neurons are found in highly stereoty ped positions, and all but two of them are associated with the segment al nerves. We show that eight of the peripheral neurons have the chara cteristic morphology of stretch receptor neurons and that they form a circumferentially distributed grid aligned in such a may that each of five specialized longitudinal muscle fascicles are monitored by at lea st two stretch receptor cells co tiering ventral, lateral, and dorsal regions of the body wall. Furthermore, we show that, in contrast to th e dorsal posterior nerve, which is pioneered by central projections, t he pathways of the three remaining segmental nerves are Likely to be p ioneered or guided by peripheral neurons. J. Comp. Neurol. 397:394-402 , 1998. (C) 1998 Wiley-Liss, Inc.