[Light- and electronmicroscopic investigations on ultrastructure and enzyme distribution in the external sulcus of the cochlea (author's transl)].

The ultrastructure of the external sulcus of the cochlea of young healthy guinea pigs and hamsters was investigated as well as the distribution of lactate dehydrogenase (LDH) in this region. The demonstration of LDH activity was carried out as follows. The unfixed and undecalcified cochlea was cut in 80 micron thick frozen sections, which were incubated immediately in the reaction mixture for LDH and subsequently fixed with paraformaldehyde. For electron microscopy the usual procedure was applied. The epithelial cells of the external sulcus show deep reaching extensions into the vessel-rich connective tissue of the basal part of the spiral ligament. These roots are bound together into root bundles surrounded by the basement membrane and fibrills. The root cells differ in their ultrastructure. The light cells, which have been described previously, contain a number of stretched mitochondria and only few other cell organels. Beside these, dark cells can be found (Fig. 2). Their cytoplasm shows high contrast and contains many mitochondria, ribosomes, rough endoplasmatic reticulum and Golgi apparatus. In the dark cells big vasuoles can be seen. All these structures indicate a high rate of synthesis in the dark cells. The root cells are linked together by membranous foldings, which diverge in some parts, forming intercellular spaces in which small cell protrusions are found. Other intercellular gaps can also be detected building a system of small channels. It is possible that these structures serve for extracellular transport. The LDH activity, which is demonstrated by formazan deposits can be exclusively found in the root cells. This could be demonstrated in light microscopic semithin sections (Fig. 3) and with electron microscopy (Fig 4). The possible role of LDH in the root cells is discussed.