TARGET-INDUCED NEUROGENESIS IN THE LEECH CNS INVOLVES EFFERENT PROJECTIONS TO THE TARGET

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.