Figure 5.
Transgenic cochleas show postnatal primary degeneration of auditory nerve fibers without loss of hair cells. A, B, Sections through the upper basal turn of a wild-type (A) and homozygote (B) cochlea at P30. According to the cochlear frequency map for mouse (see Materials and Methods), this cochlear region mediates threshold response to ∼18 kHz, approximately the region of maximum normal sensitivity. Most SGNs have degenerated (B, black arrow points to one of the remaining neurons) without loss of hair cells (dotted circles). Degeneration of fibrocytes in the limbus (B, open arrowhead) is also visible in the transgenic cochlea. C, D, Sections through the upper basal turn at P0 reveal no obvious decrease in density of SGNs in the ear of homozygous mice (D) compared with the wild type. E, F, Sections through the upper basal turn of a wild-type (E) and homozygote (F) cochlea at P55 show degeneration similar to that observed at P30. G, H, Sections adjacent to those in E and F stained with anti-NF200 antibodies show similar number of profiles of type II SGNs, indicating that neuronal degeneration is restricted to type I neurons. Scale bar, 100 μm.