Absorption of horseradish peroxidase in the endolymphatic sac: ultrastructural cytochemistry using a new electrophoretic technique.

Using a newly developed injection technique, the absorption of horseradish peroxidase (HRP) in the endolymphatic sac (ES) of the guinea pig was examined by light and electron microscopy. HRP (molecular weight: 40,000; molecular diameter: about 5-nm) was directly injected into the lumen of the ES by electrophoresis after the recording of a direct current potential in the ES lumen. Both the macrophages floating in the ES lumen and the epithelial cells in the intermediate portion of the ES absorbed intraluminal HRP. The macrophages internalized the intraluminal HRP at a higher rate than the epithelial cells, suggesting that macrophages play a major role in macromolecular absorption in the ES. It was considered that the macrophages took up intraluminal HRP by phagocytosis, while the epithelial cells of the intermediate portion took it up by pinocytosis. In contrast, the epithelial cells in the proximal portion of the ES absorbed little HRP. No penetration through the junctional complexes between epithelial cells was observed in either the intermediate or the proximal portion at any interval after the injection of HRP. This finding indicates that these junctional complexes are impermeable to intraluminal HRP.