The innervation of outer hair cells: 3D reconstruction from TEM serial sections in the Japanese macaque

Transmission electron micrographs from serial sections were obtained from t he neural pole of outer hair cells (OHCs) in the Japanese macaque (Macaca f uscata) and reconstructions of nerve terminals were made using computer sof tware. Data are based on observations of six cells in the basal turn, eight in the middle turn and four in the apex. In general, the number of afferen t (type II) terminals on each OHC increased from base to apex, and for a gi ven turn, the numbers appeared unrelated to OHC row. On the other hand, the number of efferent terminals was greater in the middle turn than in other areas, and the number decreased from row 1 to row 3. Reciprocal synapses in creased in frequency from the upper basal turn apicalward. The total number of terminals synapsing on an individual OHC increased from base to apex by nearly 100%. Three-dimensional reconstructions showed that nerve fibers te rminating on basal and middle turn OHCs ascended directly from sub-OHC regi ons to synapse on the subnuclear regions of the OHC. In contrast, apical tu rn fibers ran horizontally at the level of the subnuclear region and the te rminals appeared as en passant swellings along a single fiber. Although phy siological data are wanting for the macaque, the anatomical findings sugges t that functional differences may exist along the length of the cochlea. (C ) 1999 Elsevier Science B.V. All rights reserved.