Journal of Neuroscience, Vol 7, 1107-1122, Copyright © 1987 by Society for Neuroscience
Cell lineage, cell death, and the developmental origin of identified serotonin- and dopamine-containing neurons in the leech
DK Stuart, SS Blair and DA Weisblat
The nervous system of the glossiphoniid leech includes segmentally iterated
neurons that contain serotonin (5-HT) and dopamine. These have been
investigated in Helobdella triserialis, Theromyzon rude, and Haementeria
ghilianii. Five types of 5-HT neurons are identified by immunocytochemistry
in the abdominal ganglia of the ventral nerve cord: the bilaterally paired
Retzius, anteromedial, ventrolateral and dorsolateral neurons, and the
unpaired posteromedial (pm) neuron. Three types of bilaterally paired
dopamine neurons are identified by glyoxylic acid-induced fluorescence in
the segmental body wall: MD, LD1, and LD2. Each left or right half of the
segmental complement of the leech nervous system is known to develop from 6
distinct ectodermal primary blast cells (ns, nf, o, p, qs, and qf). To
identify the blast cells of origin of the 5-HT and dopamine neurons,
fluorescent cell lineage tracers were injected into the various precursors
of the blast cells in early (stage 6) embryos. The embryos were then raised
until their 5-HT and dopamine neurons could be scored (stage 11) for the
presence or absence of lineage tracer. We find that the Retzius,
anteromedial, and posteromedial 5-HT neurons are derived from the ns blast
cell, while the ventrolateral and dorsolateral 5-HT neurons are derived
from the nf blast cell. The unpaired pm 5-HT neuron arises as one of a
bilateral pair of neurons, of which one later dies. Whether the left or
right pm neuron survives in any given ganglion is the consequence of some
form of competitive interaction between cells derived from the left and
right n primary blast cells, possibly between the left and right pm neurons
themselves. We find that, of the dopamine neurons, the LD1 neuron is
derived from the o blast cell, the LD2 neurons from the p blast cell, and
the MD neuron from one of the 2 kinds of q blast cells. These results show
that the 5-HT and dopamine neurons arise from 5 different primary blast
cells in a highly determinate manner, and they support the view that cells
of a similar phenotype need not be closely related in the developmental
cell lineage tree.
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