Spatiotemporal patterns of neuronal programmed cell death during postnatal development of the gerbil cochlea.

During early postnatal development, afferent neurons of the cochlear (spiral) ganglion progressively refine their projections to auditory hair cells so that, by hearing onset, most cochlear nerve fibers innervate a single hearing receptor. One mechanism that might contribute to these changes in cochlear innervation is the programmed cell death (apoptosis) of developing neurons within the spiral ganglion. In the present study, we used the TUNEL method and morphological criteria to identify apoptotic cells within the spiral ganglion of the Mongolian gerbil during the first week of postnatal life when afferent projections to the cochlea are actively refined in this species.

Developmental segregation in the efferent projections to auditory hair cells in the gerbil.

The auditory receptor epithelium of mammals receives efferent innervation from neurons within and surrounding the superior olivary complex of the brainstem (Warr [1975] J. Comp. Neurol.

Development of ganglion cell topography in the postnatal cochlea.

In mammals, the size and number of spiral ganglion cells can vary significantly along the length of the cochlea. At present, it is unclear how these topologic differences in spiral ganglion cell morphology and density emerge during development. We addressed this issue by quantifying developmental changes in the number, density, and size of auditory ganglion cells within the cochlea of Mongolian gerbils throughout the first 3 weeks of postnatal life.

Developmental segregation in the afferent projections to mammalian auditory hair cells.

The mammalian ear contains two types of auditory receptors, inner and outer hair cells, that lie in close proximity to each other within the sensory epithelium of the cochlea. In adult mammals, these two classes of auditory hair cells are innervated by separate populations of afferent neurons that differ strikingly in their cellular morphology and their pattern of arborization within the cochlea. At present, it is unclear when or how these distinctive patterns of cochlear innervation emerge and become segregated during development.

Factors that influence rate-versus-intensity relations in single cochlear nerve fibers of the gerbil.

The relationship between characteristic frequency (CF) and rate-intensity curve shape was examined in 144 cochlear nerve fibers obtained from 39 Mongolian gerbils. Quasi-steady-state firing rates were measured in response to tone bursts at the CF. From each intensity curve, estimates of slope, firing rate at saturation, and dynamic range were derived using nonlinear curve fitting.