FIG. 3.

Chronic inflammation and sensory hair cell, spiral ganglion neuron and inner hair cell synaptic count, and type IV fibrocytes in the cochleae of 12-month-old Fus1 KO mice. (A) Hematoxylin and eosin-stained cochlear sections showing predominance of activated myeloid progenitor cells (arrows) in the bone marrow of the otic capsule in aging Fus1 KO mice. In contrast, young KO mice and young/old WT mice have compact, undifferentiated myeloid cells (arrowheads). (B) Staining with Iba-1, a marker of microglia/activated macrophages, confirmed this finding in the bone marrow (arrows), and also revealed an increased infiltration of activated macrophages (arrowheads) in the inferior region and along the lateral edge (adjacent to the otic capsule) of the spiral ligament. (C) The images show DAPI-stained IHCs and OHCs from the middle turn of an 11-month WT and Fus1 KO cochlea. The nuclei (blue) are all uniform in size, shape, and staining intensity, which is characteristic of healthy inner and outer hair cells. The graph illustrates the percentage survival of OHCs in the apical, middle, and basal cochlear turns of 11-month-old WT and KO mice. There was no significant loss of OHCs in all three cochlear turns in both WT and KO mice. (D) The representative images and graph illustrate the average number of spiral ganglion neurons in the cochleae of 11-month-old WT and KO mice. There was a marginal but significant decrease in the number of spiral ganglion neurons in the Fus1 KO cochlea compared with the WT cochlea. (E) Confocal image of IHCs from a 2-month WT cochlea with the nuclei stained with DAPI (blue) and presynaptic ribbons stained with CtBP2 (red), showing a normal number (14–18) of synapses per IHC. A single IHC is outlined with dotted lines. The graph illustrates the average number of CtBP2-positive IHC presynaptic ribbons on the IHCs from the apical, middle, and basal cochlear turns of 11-month-old WT and Fus1 KO mice. Fus1 KO mice had significantly less synaptic ribbons compared to WT mice in the apical and middle turns. (F) Light microscopic images showing substantial loss of type IV fibrocytes in the inferior region of the spiral ligament (circled) in 12-month-old Fus1 KO mice compared with age-matched WT mice. Data presented as mean ± SEM. *p < 0.05 (Student's t-test, unpaired). Magnification × 400. IHC, inner hair cell; OHC, outer hail cell. To see this illustration in color, the reader is referred to the online version of this article at www.liebertpub.com/ars