Abstract
Macrophages are the main intrinsic immune cells in the cochlea; they can be activated and play a complicated role after cochlear injury. Many studies have shown that the number of macrophages and their morphological characteristics within the major cochlear partitions undergo significant changes under various pathological conditions including acoustic trauma, ototoxic drug treatment, age-related cochlear degeneration, selective hair cell (HC) and spiral ganglion neuron (SGN) elimination, and surgery. However, the exact role of these macrophages after cochlear injury is still unclear. Regulating the migration and activity of macrophages may be a therapeutic approach to reduce the risk or magnitude of trauma-induced hearing loss, and this review highlights the role of macrophages on the peripheral auditory structures of the cochlea and elucidate the mechanisms of macrophage injury and the strategies to reduce the injury by regulating macrophage.
Highlights
Macrophages are cellular components of the innate immune system and have the hallmarks of heterogeneity and plasticity, and they play important roles in homeostasis, repair, and pathological changes
Most of these macrophages lie in a direction parallel to the radial fibers of the tunnel, some macrophages are vertically oriented. These cells can extend their processes to the region of inner hair cell (HC) through the habenula perforata without contact with inner HCs. The function of these macrophage processes is not fully understood, their adjacency to cochlear ribbon synapses suggests a role in removing degraded sensory cells in the organ of Corti after injury and maintaining synapse homeostasis, a function that has been identified in microglia, which are the resident macrophages in the central nervous system (CNS) (Wake et al, 2009; Hirose et al, 2017)
When the cochlea suffers from injury, monocytes in the circulatory system migrate into the cochlea and transform into mature macrophages, which combine with tissue-resident macrophages to participate in immune responses
Summary
Reviewed by: Richard Salvi, University at Buffalo, United States Huib Versnel, University Medical Center Utrecht, Netherlands. Macrophages are the main intrinsic immune cells in the cochlea; they can be activated and play a complicated role after cochlear injury. Many studies have shown that the number of macrophages and their morphological characteristics within the major cochlear partitions undergo significant changes under various pathological conditions including acoustic trauma, ototoxic drug treatment, age-related cochlear degeneration, selective hair cell (HC) and spiral ganglion neuron (SGN) elimination, and surgery. The exact role of these macrophages after cochlear injury is still unclear. Regulating the migration and activity of macrophages may be a therapeutic approach to reduce the risk or magnitude of trauma-induced hearing loss, and this review highlights the role of macrophages on the peripheral auditory structures of the cochlea and elucidate the mechanisms of macrophage injury and the strategies to reduce the injury by regulating macrophage
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