Journal of Neuroscience, Vol 11, 3868-3876, Copyright © 1991 by Society for Neuroscience
Normally unused positional cues guide ectopic afferents in the leech CNS
MB Passani, A Peinado, H Engelman, CA Baptista and ER Macagno
Department of Biological Sciences, Columbia University, New York, New York 10027.
Central projections from peripheral sensory neurons segregate into
distinct, ventrally positioned longitudinal tracts within the segmental
ganglia of the leech Hirudo medicinalis. As documented here, there is an
additional tract in the neuropils of the fifth and sixth body ganglia,
located at the lateral margin and formed by afferent axons (the "sex
afferents") originating from sensory neurons located in the male and female
genitalia. Ablation of the genitalia results in the complete absence of
this additional tract. We asked (1) whether segmental differences exist in
the distribution of pathway cues available to the sex afferents, and (2)
whether central pathway selection by these axons is specific. We
transplanted the primordia of the male genitalia to several ectopic
positions posterior to the sixth body segment and labeled the ectopic sex
afferents in order to examine their paths in the CNS. In about 50% of the
experimental animals, afferent axons originating in the transplanted tissue
segregated into a distinct lateral fiber bundle within the neuropil of a
nearby ganglion, in a position corresponding to their normal one in the sex
ganglia. The sex afferents therefore find their normal pathways even in the
ganglia of inappropriate segments, although these pathways are not used by
any other afferents in these ganglia. We propose, therefore, that the
positional cues employed by afferent axons to select appropriate pathways
in the ganglionic neuropil are expressed in all segments of the leech CNS,
regardless of whether such cues are normally used by afferent axons in each
segment.