Figure 4.

A, B, Immunostaining for a cholinergic marker (VAT; green) and a neural marker (NF 200; red) reveals normal afferent and efferent innervation in an adult (6 weeks old) M₂/M₄ knock-out (A) compared with an age- and place-matched control (B). Each panel shows the x–y and y–z projections from a confocal z-stack through the organ of Corti in the 22–32 kHz region, from outer hair cells (OHCs) to the inner spiral bundle (ISB). In the z-dimension, each stack spans the region in which efferent terminals are found. Filled arrowheads (A) point to the efferent terminal clusters under each of the three rows of outer hair cells: the approximate outlines of the basal poles of the outer hair cells are indicated by the dashed lines. Open arrowheads (B) point to small VAT-positive puncta among the supporting cells under outer hair cells, indicating medial olivocochlear synapses with type II afferent fibers. Scale bar in A applies to both panels.