Figure 5.

Ultrastructural development of calyces. (a) Semi-thin cross-section of the macula at P6, showing the histology of the utricular sensory epithelium (SE) and connective tissue stroma. The cells with light nuclei near the lumen are hair cells (HC), and the dark nuclei lined up at the base of the sensory epithelium are supporting cells (SC). (b): Low-magnification ultra-thin cross-section of the utricular macula at P2, showing immature columnar hair cells (arrow). (c) Low-magnification ultra-thin cross-section of the utricular macula at P6, showing differentiating hair cells, including Type I hair cells with a well-developed calyx (arrows). (d—g) High-magnification cross-sections (perpendicular to the lumen). (d,e) Two presumably immature Type I hair cells with a cup-like partial calyx at the base (transparently filled in blue) at P2 (d) and P4 (e). (f) A presumed differentiating Type I hair cell with a calyx (blue fill) that fully wraps the perinuclear region at P4. (g) An example of a presumed Type II hair cell with multiple small bouton afferents (blue fill) at P4. (h–j) Horizontal sections (parallel to the lumen) through presumed differentiating Type I hair cells at P8, with calyx afferents highlighted in blue fill. (h) A hair cell with a partial calyx (blue fill). (i) Two presumed hair cells fully wrapped by a common calyx (blue fill) at P8. (J) The neck of a hair cell wrapped by a calyx (blue fill). (k) A double ribbon synapse of a presumed Type I hair cell upon a simple calyx afferent (a) at P4. (l) A ribbon synapse of a presumed Type II hair cell upon a bouton afferent (a) at P4. (m) A ribbon synapse of a presumed Type I hair cell upon a complex calyx afferent (a), like the one shown in panel I. For (k–m), arrowheads point to the presynaptic ribbon. Scale bar in e pertains to (e–j). Scale bar in (k) pertains to (k–m)