Serial section reconstruction of the guinea pig outer hair cells as studied with a high-voltage electron microscope...

The fine structural arrangement of the nerve endings on the guinea pig outer hair cells (OHCs) was studied by a serial sectioning technique by means of a high-voltage electron microscope and computer-graphic three-dimensional reconstruction. Sixty-nine series of consecutive sections were observed. Fifteen OHCs were located about 0.5 mm from the basal end (hook portion of basal turn) and 4 OHCs were seen at 6.0 mm, 20 OHCs at 8.0 mm (basal turn), 13 at 12.8 mm (second turn), 8 at 14.8 mm, 7 OHCs at 15.9 mm (third turn), and 2 OHCs at 17.5 mm (apical turn). Of 69 OHCs, 31 cells were from the first row, 22 from the second and 16 from the third row. The total number of nerve terminals on each OHC varied between 5 and 36. The OHCs with the least number of terminals were found in the hook portion, while the OHCs with the most numerous innervation were observed in the third row of the upper third turn. The afferent/efferent ratio (A/E ratio) of each OHC was evaluated and the OHCs were divided into two groups: Type A and Type B, depending upon the range of the A/E ratio. Thirty-five OHCs (50.7%) out of 69 were identified as being type A with an A/E ratio range of 0.5-1.5, while the remaining 34 OHCs (49.3%) were Type B cells with a greater A/E ratio than Type A cells. The detailed distribution of both types of cells was also studied along the entire length of the basilar membrane.