An electron microscopic study of the function of the root cells in the external spiral sulcus of the cochlea.

The ultrastructure of the area of the external spiral sulcus of the cochlea was studied in 15 hamsters. Intravital fixation with glutaraldehyde was followed by osmium citrate fixation and embedding in araldite. The endolymphatic surface of the external spiral sulcus is nearly completely covered by Claudius cells under which the root cells are situated. These epithelial cells are composed of bundles (roots), which protrude into the connective tissue of the spiral ligament in a tree-like fashion, forming primary, secondary, and tertiary branches. The basement membrane is a reliable landmark in this complex region, serving as a differentiating zone between the root cells and the mesenchymal tissue. Two different types of root cells are found. There are organelle rich "dark" cells and "light" cells which contain only a few cell organelles. All types of transitional forms between these two cell types are found, which can be interpreted as an expression of different metabolic activity of the same cell type. Between the root cells are seen extracellular clefts into which the "dark" cells, with their high synthetic and secretory activity, expel their secretions. These clefts lead to the endolymphatic space. A continuous production and secretion of part of the endolymph by the root cells is very likely. The anchoring of fibrils at the basement membrane of the root cell bundles, and their continuity with the basilar membrane leads to the assumption that this whole system may act as a mechanical pump. At high sound pressure levels, the strong vibration of the basilar membrane may lead to a locally and temporarily enhanced secretion of substances from the root cells into the cochlear duct.