Figure 3.
Macrophages appear to be recruited to and activated in regions of noise-induced demyelination. a, Low-magnification images immunostained for Iba-1 from a control cochlea (left) and a cochlea at 1 d (D1) after exposure to 106 dB noise for 2 h (right). OSL and RC regions are indicated. White arrows indicate Iba-1-positive cells. c–e, Immunostaining for Iba-1 revealed many macrophages (Mc) within the OSL (c) and RC (d, e) at D1 after noise exposure. b, No Iba-1+ cells were seen in the OSL or RC of control non-noise-exposed cochlear sections. Nuclei were counterstained with DAPI (a–d). f, Ly6G-positive cells (green, white arrows) are present in the bone marrow of a young adult control mouse. g–i, No Ly6G+ cells could be found in the ANs of control or noise-exposed mice at D1 or D7 after 106 dB SPL noise exposure (3 animals examined per experimental condition; 2 sections analyzed per animal; sections contained either apical and middle or middle and basal regions). Nuclei were counterstained with PI (f–i). j, A macrophage is present among three AN axons in a control cochlea section (left). Middle, An enlarged image of the boxed area of the left panel. Right, A macrophage is shown next to a blood vessel and some AN fibers in a control cochlea. k–p, Numerous macrophages appeared in regions with myelin abnormalities at D1 (k–n) and D14 (o, p) after noise exposure. White arrowheads indicate an empty space in a myelin sheath (k), a fused myelin sheath (n), and a type I SGN ensheathed by only one layer of myelin (o). Most macrophages contained numerous inclusion bodies with electron-dense contents resembling myelin and often were seen closely apposed to segments of myelin sheath (m). The panel to the right of m shows an enlarged image of the boxed area in m. At D14, macrophages appear to have engulfed many segments of degenerating axons (white arrowheads in p). q, At D30, macrophages with numerous inclusions containing electron-dense material are still present in regions of remaining healthy-looking AN fibers. Scale bars: a, 20 μm; b (for b–d), 10 μm; e, 8 μm; f (for g–i), 10 μm; j (left), k, m–q, 2 μm; j (right), l, 800 nm.