Csf1 Signaling Regulates Maintenance of Resident Macrophages and Bone Formation in the Mouse Cochlea

In the mammalian cochlea, resident macrophages settle in the spiral ligament, spiral ganglion, and stria vascularis, even at the steady state. Resident macrophages in the cochlea are believed to maintain homeostasis in the inner ear and become active, as part of the front line defense, following inner ear damage. However, the exact roles of cochlear resident macrophages require further clarification. Colony stimulating factor-1 (Csf1) signaling regulates survival, proliferation, and differentiation of resident macrophages and appears to be essential for resident macrophages in the inner ear. To examine the roles of Csf1 signaling in auditory function, we examined the ossicles and inner ear of homozygous Csf1 mutant (Csf1^op/op) mice. The ossicles including the incus and stapes of Csf1^op/op mice macroscopically demonstrated bone thickening, and the otic capsules of the inner ear were also thick and opaque. Histological analyses demonstrated that the otic capsules in Csf1^op/op mice were thickened and showed spongy bone degeneration. Measurements of the auditory brainstem response revealed significant elevation of thresholds in 4-week old Csf1^op/op mice compared with wild-type littermates, indicating that Csf1^op/op mice demonstrate hearing loss due to, at least in part, deformity of the ossicles and bone capsule of the inner ear. Furthermore, Csf1^op/op mice are deficient in the number of resident macrophages in the spiral ligament and stria vascularis, but not in the spiral ganglion. These data provide evidence that Csf1 signaling is important not only for bone formation in the inner ear, but also for the maintenance of resident macrophages in the spiral ligament and stria vascularis in the adult mouse cochlea.