POSTEMBRYONIC NEUROGENESIS IN THE CENTRAL-NERVOUS-SYSTEM OF THE TOBACCO HORNWORM, MANDUCA-SEXTA .3. SPATIAL AND TEMPORAL PATTERNS OF PROLIFERATION

Postembryonic neurogenesis leads to a dramatic increase in the number of functional neurons within the segmental ganglia of the moth, Manduc a sexta. These adult-specific neurons are generated during larval life by segment-specific arrays of individually identifiable stem cells, o r neuroblasts (Nbs). By the end of the feeding larval stage, each Nb h as generated a discrete nest of progeny, which ranges in size from les s than 10 to more than 70 progeny. The sizes of these identifiable nes ts of progeny vary in a segment-specific manner, with the thoracic nes ts containing a greater number of progeny compared with their homologu es in the simpler abdominal ganglia. In order to describe those factor s that influence the size of the post-embryonic neuronal lineages, we examined the spatial and temporal pattern of postembryonic neurogenesi s in the segmental ganglia of Manduca. The rates at which the identifi able nests accumulated progeny were estimated by counting the number o f progeny within the nests, using sectioned material isolated from ani mals at stages ranging from embryonic hatching until the end of the fe eding larval stage. All of the postembryonic Nbs began to generate pro geny at around the time of the molt to the third larval instar. Each n est added progeny at a rate that was a characteristic of its identity and segment of origin. Although all of the nests within the thorax con tinued to accumulate progeny throughout the feeding larval stage, seve ral of the abdominal nests showed little or no growth following the mo lt to the fifth larval instar. The thymidine analog 5-bromo 2-deoxyuri dine (5-BrdU) was used to estimate the mitotic rates of the identifiab le Nbs. The number of labeled progeny within a nest 24 h after applica tion of 5-BrdU ranged from a low of 1 to 2 to a high of 11 to 13 label ed cells. In some instances there was a good correlation between the e stimated mitotic rate of an identified Nb and the rate of growth of it s associated nest of progeny. However, several of the identifiable nes ts accumulated progeny at a slower rate than predicted based on the es timated mitotic rate of the Nb. Cell death appears to be responsible f or slowing the growth of the nests during the feeding larval stage. We estimate that 10% to 70% of the neurons generated during the feeding larval stage degenerate within 24 h of their birth. The level of cell death observed within a nest was dependent on both its identity and it s segment of origin. (C) 1996 John Wiley & Sons, Inc.