Metabolic Activities of the Endolymphatic Sac:An Enzyme Histochemical and Autoradiographic Study

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Abstract
The distribution of diphosphopyridine nucleotide diaphorase, triphos-phopyridine nucleotide diaphorase, lactic dehydrogenase, malic dehydrogenase, succinic dehydrogenase, isocitric dehydrogenase, glucose-6-phosphate dehydrogenase, and β-hydroxybutyric dehydrogenase, cytochrome oxidase, alkaline and acid phosphatase, leucine- and alanine-aminopeptidase activity was demonstrated histochemically in the endolymphatic sac and in the perisaccular connective tissue. Enzyme activity was stronger in the epithelial cells lining the intermediate portion and in the cells floating free in the endolymph than in other parts of the sac, except for aminopeptidase activity which was marked in the perisaccular connective tissue, too. Foreign protein (peroxidase) was injected into the cochlear duct and after two days was found phagocytized in cells floating free in the endolymphatic sac but not in the lining cells themselves or elsewhere in the membranous labyrinth. Peroxidase was also observed in fibroblasts of the perisaccular connective tissue four days after injection. Radioactive carbon labeled foreign protein was directly injected into the endolymphatic sac and visualized autoradiographically. Five minutes after injection radioactivity was seen only on the surface of the epithelial lining, but after 85 minutes survival there was radioactivity in the free-floating cells as well as in the epithelial lining cells and in the perisaccular tissue. These observations are in agreement with the concept that endolymph flows from the cochlear duct to the endolymphatic sac, where proteins are phagocytized and/or transported via the lining epithelial cells into the perisaccular connective tissue. The endolymphatic sac is a metabolically active structure that functions as a filter for the membranous labyrinth.