Reuniting and Endolymphatic Duct Macrophages: Localization and Possible Roles

<b>Background</b>: The inner ear hosts several macrophage populations. Endolymphatic sac macrophages can phagocytose otoconia, and spiral limbus macrophages express genes for fluid shear stress sensing and bone remodeling. Obstruction of endolymph flow by saccular otoconia could be linked to endolymphatic hydrops. Since macrophages are strongly affected by inflammatory status, a role for them in otolith removal could provide a link between inflammation and hydrops. However, the distribution of macrophages around the reuniting duct (RD) and endolymphatic duct (ED), which are narrow structures likely prone to blockage, remains unexplored. <b>Methods</b>: We performed tissue clearing and light-sheet imaging on rat temporal bones. Autofluorescence and immunolabeling for collagen IV, smooth muscle actin, and Iba1 were used to visualize inner ear structures, blood vessels, and macrophages. <b>Results</b>: The connective tissue layer underlying the RD extended from the cochlear spiral limbus. The RD and spiral limbus hosted a continuous microvascular network and macrophage population, comprising both ameboid and ramified cells; macrophages also surrounded the underlying vestibulocochlear artery (VCA). A separate macrophage population, continuous with that of the saccular connective tissue, was found around the endolymphatic sinus and utriculo-endolymphatic (Bast's) valve; macrophage patterns changed in the vestibular aqueduct and endolymphatic sac. <b>Conclusions</b>: Macrophages are observed in positions consistent with potential roles in sensing luminal changes and in the clearance of obstructive material from the RD and ED; functional confirmation will require targeted experiments.