3D distribution of cochlear macrophages in the lateral wall of cleared cochlea

Objectives The presence of resident macrophages in the cochlea is well known. However, the exact patterns thereof in spiral ligaments has not been discussed in previous studies. We sought to document distributions of macrophages in intact cochlea using three-dimensional imaging. Methods Cochlea were obtained from C-X3-C Motif Chemokine Receptor 1+/GFP mice, and organ clearing was performed. Three-dimensional images of cleared intact cochlea were reconstructed using two-photon microscopy. The locations of individual macrophages were investigated using 100-μm stacked images to reduce biases. Cochlear inflammation was then induced by lipopolysaccharide (LPS) inoculation into the middle ear through the tympanic membrane. Four days after inoculation, three-dimensional images were obtained. Results Macrophages were scarce in areas adjacent to the stria vascularis, particularly the area just beneath it, even though many have suspected macrophages to be abundant in this area. This finding remained consistent upon LPS-induced cochlear inflammation, despite significant increases in macrophages, compared to non-treated cochlea. Conclusion Resident macrophages in spiral ligaments are scarce in areas adjacent to the stria vascularis.