ENDOCYTOSIS AND TRANSEPITHELIAL TRANSPORT OF ENDOLYMPH IN THE ENDOLYMPHATIC SAC

The fate of cationized ferritin (CF) injected into the endolymphatic s pace of the endolymphatic sac was observed by transmission electron mi croscopy. At 10 min after the injection, CF particles bound to the api cal plasma membrane of epithelial cells of the sac and were then endoc ytosed with coated pits. However, they never passed through the juncti onal complexes between the epithelial cells. At 30 min after the injec tion, the CF particles were transferred to endosomes and lysosomes by small vesicles of 100-150 mm in diameter. CF particles were also found in small vesicles close to Golgi cisternae and in multivesicular bodi es. Acid phosphatase positive lysosomes were found close to endosomes containing CF particles. In addition, a small fraction of the small ve sicles containing CF particles became inserted into the basolateral pl asma membrane. At 60 min after the injection, many CF particles were f ound in acid phosphatase positive secondary lysosomes. These observati ons suggest that endocytosis of endolymph is actively performed by the epithelial cells of the sac, and transepithelial vesicular transport across the epithelial cells occurs.