During a critical period in leech embryogenesis, the sex nerves that c
onnect the 5th and 6th midbody ganglia (MG5 and MG6) to the primordium
of the male sexual organ carry a spatially localized signal that indu
ces the birth of several hundred neurons specific to these ganglia. We
examined particular cellular elements (afferents, efferents, non-neur
onal components) within these nerves as potential conveyors of the ind
uctive signal. We show that axons of peripheral sensory neurons in the
male genitalia travel along the sex nerves and into MG5 and MG6, but
reach the CNS after the critical period has elapsed and cannot, theref
ore, be involved in the induction. Of the six sex nerves, four contain
non-neuronal cells that span the entire distance between the male gen
italia and the sex ganglia. However, when male genitalia were transpla
nted to ectopic locations close to MG6, induction occurred frequently
but only in MG6, mediated by ectopic nerves that do not contain these
cells. Thus, non-neuronal cells specific to the normal sex nerves are
not necessary for induction. In addition, dye injections into the targ
et during the critical period failed to reveal migrating cells in the
sex nerves that could convey the inductive signal to the CNS. Finally,
we show that 11 pairs of central neurons in each ganglion project to
the male organ early during the critical period. In the adult, at leas
t 3 additional pairs of neurons in MG6 also innervate this target. We
conclude that the only components of the sex nerves that connect the s
ex ganglia to the target during the critical period that could be asso
ciated with induced central mitogenesis are the axons of central neuro
ns that innervate the male genitalia.