Cochlear inner hair cells exist transiently in the fetal Bronx waltzer (bv/bv) mouse

In the cochlea of the adult Bronx waltzer (bv/bv) mouse, the majority of inner hair cells are missing or deformed. As a result, Bronx waltzer mice are severely hearing impaired or deaf. Previous studies determined that most inner hair cells in these mice are missing by the time of birth, but no studies have resolved whether the missing inner hair cells ever exist in the mutant cochlea. The present study used light and electron microscopy to locate inner hair cells in the mutant mouse before birth. Most, and possibly all, inner hair cells exist in the embryonic day (E) 17 mouse. The shapes of the cells vary from normal and elongated in the youngest animals, to round and protruding through the reticular lamina a few days later. The density of sensory cells in the inner hair cell region (inner hair cells/millimeter) decreases in the basal turn between E17 and birth, and in the apical turn between birth and the third postnatal day. The initial presence of the full complement of inner hair cells, taken together with the temporospatial pattern of degeneration, suggests that the cause of inner hair cell death in the Bronx waltzer mouse is related to a differentiation event subsequent to cell birth.