Incomplete Maturation of Brainstem Auditory Nuclei in Genetically Induced Early Postnatal Cochlear Degeneration

Abstract

In the Shaker-2 mouse mutant, a cochleo-saccular type of genetically induced inner ear degeneration occurs. Morphological signs of degeneration are evident in the 3rd postnatal week and a severe and almost total degeneration has occurred by the age of 6-9 weeks. There are no qualitative differences between the brain of the normal CBA/J mouse and that of the Shaker-2 mouse. The growth of the auditory brainstem nuclei (dorsal cochlear nucleus and ventral cochlear nucleus) in the mutant Sh-2 mouse has stopped by 14 days of age--but not in the normal CBA/J mouse. The brainstem as a whole continues to grow between 14 and 140 days of age in both strains. The early cochlear degeneration coincides with the critical period when a normal input from the peripheral receptor organ is necessary for the maturation of the central auditory pathway.

Maturation of Inner Ear Ganglion Cells

“…The decrease in cell volume after the rapid phase of cell increase is considered to be a rebound effect to reach final adjustment for adult condi tions. Similar phenomena occur in the kitten for instance [22], In comparison, an opposite type of morphological adaptation with a de crease in cell volume has recently been docu mented in the cochlear nuclei in the Shaker-2 mouse, which never reaches normal hear ing (or normal electrophysiological poten tials) prior to inner ear degeneration [24], In these animals the sensory deprivation re sulted in a decrease in cell volume. Thus, an increase in cross-sectional area seems con ceivable during functional maturation.…”

The timecourse of neuronal connections of the rotundoectostriatal pathway in chicks (Gallus gallus) during embryogenesis: A retrograde transport study

“…All these findings support the notion that the development of axonal projections of the second-and higher-order levels of the retinotectofugal system at early stages is not dependent on a sequential influence of retinal innervations. This parallel pattern of development provides us with a model system to investigate the synaptogenesis of the three system components during embryogenesis and, moreover, peripheral influences upon the formation of their arbor terminals, as shown in the mammalian somatosensory (Killackey & Fleming, 1985;Killackey et al, 1994) and auditory (Altman & Bayer, 1981;Webster et al, 1986;Ryan & Woolf, 1988) systems.…”