Journal of Neuroscience, Vol 13, 3309-3318, Copyright © 1993 by Society for Neuroscience
Development of segment specificity in identified lineages of the grasshopper CNS
KJ Thompson and MV Siegler
Department of Biology, Emory University, Atlanta, Georgia 30322.
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.
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