Afferent innervation of outer and inner hair cells is normal in neonatallyde-efferented cats

It has been hypothesized that normal pruning of exuberant branching of affe rent neurons in the developing cochlea is caused by the arrival of the oliv ocochlear efferent neurons and the resulting competition for synaptic sites on hair cells. This hypothesis was supported by a report that afferent inn ervation density on mature outer hair cells (OHCs) is elevated in animals d eefferented at birth, before the olivocochlear system reaches the outer hai r cell area (Pujol and Carlier [1982] Dev. Brain Res. 3:151-154). In the cu rrent study, this claim was evaluated quantitatively at the electron micros copic level in four cats that were de-efferented at birth and allowed to su rvive for 6-11 months. A semiserial section analysis of 156 OHCs from de-ef ferented and normal ears showed that, although de-efferentation essentially was complete in all four cases, the number and distribution of afferent te rminals on OHCs was indistinguishable from normal, and the morphology of af ferent synapses was normal in both the inner hair cell area and the OHC are a. Thus, the postnatal presence of an efferent system is not; required for the normal development of cochlear afferent innervation, and the synaptic c ompetition hypothesis is not supported. (C) 2000 Wiley-Liss, Inc.