Figure 7.

Basolateral processes contact one another. All panels show ultrathin sections through utricles from normal CBA/CaJ mice between 6 and 9 weeks of age. A. A transverse section of two type II hair cells (HC_II)(already shown in Fig. 3F) to illustrate the position of process-to-process contacts. The two type II hair cells are tinted green and indicated with arrows (at their tops); they surround a type I hair cell (HC_I). The boxed area shows a site of contact between two basolateral processes that is shown at higher magnification in panel B. B. Two processes, P1 and P2, have a point of contact (arrowhead). See panel A for position of this contact within the sensory epithelium. C. A horizontal section through the region near the supporting cell nuclei. The arrows point to ribbon synapses in type II hair cell processes (P1, tinted blue, and P2, tinted green) whose plasma membranes are closely apposed (arrowheads). Each ribbon synapse is shown at higher magnification in an inset. D. The contact site (small arrowheads) between two type II processes (P1 and P2) is shown. The large arrowhead points to a site of increased electron density in P1, next to its site of contact with P2. The arrow points to a ribbon synapse in P1. The higher magnification in panels B–D provides evidence that the cytoplasm of the processes is full of polyribosomes, a hallmark of hair cells. An example of a cluster of polyribosomes is shown in D, to the right of the red asterisk in P2 and around the red asterisk in P1. SC = supporting cell nucleus; eff = efferent fiber; aff = afferent fiber. Scale bars: A = 5 μm; B, C, D = 500 nm.