Degradation of the homogeneous substance in the endolymphatic sac

The accumulation and degradation of a homogeneous precipitate in the lumen of the endolymphatic sac (ES) was studied in mice. Filling of the endolymphatic sac was induced by surgical labyrinthectomy and the sacs were studied 1-8 weeks postoperatively. The initial phase (1-2 weeks postlabyrinthectomy) was characterized by filling of the ES with the homogeneous precipitate. The number of freely floating cells in the lumen was increased after two weeks. Three weeks postoperatively the ES lumen was generally clear, with apparently no stainable material. Ultrastructural analysis of the ES showed that this clearance of the endolymphatic space resulted from degradational activity in the epithelial cells initiated in the proximal portion of the sac. Breakdown of the homogeneous substance seemed to result from cellular ingestion with concomitant lysosomal digestion. Four weeks postoperatively cell clusters were observed subepithelially and were filled with densely staining precipitate, indicating that these cells or macrophages were involved in the turnover of the homogeneous substance in the ES. The functional significance of a degradational system of this substance in the ES is discussed.