Regulation of central neurogenesis by a peripheral target has been pre
viously demonstrated in the ventral nerve cord of the leech Hirudo med
icinalis (Baptista, C. A., Gershon, T. R. and Macagno, E. R. (1990), N
ature 346, 855-858) Specifically, innervation of the male genitalia by
the fifth and sixth segmental ganglia (the sex ganglia) was shown to
trigger the birth of several hundred central neurons (PIG neurons) in
these ganglia, As reported here, removal of the target early during in
duction shows that PIC neurons can be independently induced in each si
de of a ganglion, indicating that the inductive signal is both highly
localized and conveyed to each hemiganglion independently, Further, si
nce recent observations (Becker, T., Berliner, A. J., Nitabach, M. N.,
Gan, W.-B. and Macagno, E. R. (1995), Development, 121, 359-369) had
indicated that efferent projections are probably involved in this phen
omenon, we individually ablated all possible candidates, which led to
the identification of two central neurons that appear to play signific
ant roles in conveying the inductive signal to the CNS, Ablation of a
single ML neuron reduced cell proliferation in its own hemiganglion by
nearly 50%, on the average, In contrast, proliferation on the opposit
e side of the ganglion increased by about 25%, suggesting the possibil
ity of a compensatory response by the remaining contralateral ML neuro
n, Simultaneous ablation of both ML neurons in a sex ganglion caused s
imilar reductions in cell proliferation in each hemiganglion. Deletion
of a single AL neuron produced a weaker (7%) but nonetheless reproduc
ible reduction, Ablation of the other nine central neurons that might
have been involved in PIC neuron induction had no detectable effect, B
oth ML and AL neurons exhibit ipsilateral peripheral projections, and
both arborize mostly in the hemiganglion where they reside, Thus, we c
onclude that peripheral regulation of central neurogenesis is mediated
in the leech by inductive signals conveyed retrogradely to each hemig
anglion by specific central neurons that innervate this target and the
hemiganglion they affect.