Endocytosis in the epithelial cells of the endolymphatic sac

To investigate the properties of endocytosis of the epithelial cells in the intermediate portion of the endolymphatic sac (ES), we infused cationized ferritin (CF), microperoxidase (MPO), and horseradish peroxidase (HRP), as endocytosis tracers, into the endolymphatic space of the ES. Thirty minutes after infusion, the tissues were fixed and the distributions of HRP, MOP and CF were observed by transmission electron microscopy. The results at 30 min after tracer infusion were as follows. Concerning CF, endocytosis showed different activity depending on the area of the ES. In one area, CF was hardly observed within the epithelial cells, while in another area, it was observed within the epithelial cells. The CF-loaded vesicles were mainly coated vesicles. These results suggested that infusion of the artificial endolymphatic sac endolymph with CF may not stimulate the whole signal system of the ES. Concerning HRP, it bound to the apical membrane and was observed in the vesicles and the sorting endosomes. The diameters of these vesicles were of two types: one had a smaller diameter (less than 200 nm), while the other had a larger diameter (over 200 nm) and originated from macropinocytosis. Concerning MPO, it was much less observed in the vesicles than HRP. We discuss the different activities of endocytosis among these tracers, and suggest that a large molecular weight substance, i.e. HRP, might stimulate the fluid-phase endocytosis and/or adsorptive pinocytosis.