Development of segment specificity in identified lineages of the grasshopper CNS.

The purpose of this study was to determine the factors underlying differences in population size and composition between segmentally homologous neuronal lineages. The segmental median neuroblasts (MNBs) of grasshoppers are identified stem cells that each produce a midline group of neurons. We traced the embryonic development of the group in two disparate segments, counting MNB progeny and profiles of dying cells in fixed and stained preparations of staged embryos. In the metathoracic segment (T3), about 95 MNB progeny survive embryonic development, whereas in the next posterior segment, the first abdominal (A1), only about 60 survive. In T3, the MNB arises at 29% of embryogenesis and dies at 78%, whereas in A1 the MNB arises at 30% and dies at 73%. In T3, the number of MNB progeny initially increases at a steady rate, 10 cells being added per 5% of embryogenesis. Between 70% and 78% growth tapers off; although the T3 MNB continues to divide, cells die at the same time, specifically removing last-born progeny. By contrast, in A1 the MNB progeny increase in two phases, one from 30% to 45% and the other from 60% to 73%, again at the rate of 10 cells per 5%. Between the two phases, the number of A1 progeny is stable. The A1 MNB continues to divide, but cells die at the same time, specifically removing earlier-born progeny. The episodes of cell death in A1 and T3 coincide with embryonic molts, and thus may be hormonally triggered. Cell death is greater in A1 than T3, accounting for most of the difference in population size. The difference in MNB longevity makes a lesser contribution. The present data, together with corollary anatomical data (Thompson and Siegler, 1991), support the hypothesis that progeny fated to become certain neuronal types are selectively removed from the two MNB lineages: intersegmental interneurons from T3 and efferent neurons and local interneurons from A1.