Afferent and efferent innervation of the guinea-pig cochlea: a light microscopic and histochemical study.

The pattern of innervation of the normal and de-efferented guinea-pig cochlea has been studied from sections prepared without dehydration or decalcification procedures. This technique has given very good preservation of fine structure, thus enabling us to observe the existence of fibre routes not before described. Tissue was treated by various techniques: osmic acid fixation alone, formalin followed by either Holmes or Bielschowsky-Gros silver stains, or by Karnovsky-Roots acetylcholinesterase or a modified Maillet zinc iodide histochemical reaction. We have been able to visualize two routes which efferent fibres take to the Hensen cells, as well as connections to the efferent network under the three rows of outer hair cells. The tunnel fibres are shown to cross to three levels of a plaque on the outer hair cell-1/Deiters' cell-1 surface. The medial tunnel fibres degenerate rapidly after olivocochlear bundle section; the ‘efferent’ stained basilar fibres remain intact. ‘Efferent’ fibres are seen among the outer spiral bundle and which can take a different route from the afferent component. These newly described routes to the Deiters' and Hensen cells now pose the question whether the role of these ‘supporting’ cells is really a passive one, as has been assumed in the past.