Two cell populations participate in clearance of damaged hair cells from the sensory epithelia of the inner ear

- Cochlear mononuclear phagocytes have a regular rate of turnover rate and are replaced by bone marrow precursors.
- Damage to the inner ear induces migration of macrophages from the circulation into the mammalian cochlea.
- Vestibular organs also possess a population of resident macrophages.
- Active phagocytosis of dying and damaged hair cells by macrophages are observed.
- Supporting cells play a role in debris clearance through extrusion, filling the spaces that hair cells vacate.

The cochlea and the vestibular organs are populated by resident macrophages, but their role in inner ear maintenance and pathology is not entirely clear. Resident macrophages in other organs are responsible for phagocytosis of injured or infected cells, and it is likely that macrophages in the inner ear serve a similar role. Hair cell injury causes macrophages to accumulate within proximity of damaged regions of the inner ear, either by exiting the vasculature and entering the labyrinth or by the resident macrophages reorganizing themselves through local movement to the areas of injury. Direct evidence for macrophage engulfment of apoptotic hair cells has been observed in several conditions. Here, we review evidence for phagocytosis of damaged hair cells in the sensory epithelium by tissue macrophages in the published literature and in some new experiments that are presented here as original work. Several studies also suggest that macrophages are not the only phaogocytic cells in the inner ear, but that supporting cells of the sensory epithelium also play an important role in debris clearance. We describe the various ways in which the sensory epithelia of the inner ear are adapted to eliminate damaged and dying cells. A collaborative effort between resident and migratory macrophages as well as neighboring supporting cells results in the rapid and efficient clearance of cellular debris, even in cases where hair cell loss is rapid and complete.