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. 2015 Sep 11;4(3):500–519. doi: 10.3390/cells4030500

Figure 2.

Figure 2

Cilia in the inner ear of mammals shortly after birth. (A,B) Cross section of a mouse cochlea at P5 (postnatal day 5) stained with toluidine blue. The cochlea is divided into three fluid-filled compartments: scala vestibuli, scala tympani and scala media. Scala vestibuli (containing perilymph) is separated from the scala media (containing endolymph) by the Reissner’s membrane, while scala media and scala tympani (containing perilymph) are separated by the basilar membrane. The basilar membrane contains the Organ of Corti with sensory hair cells responsible for auditory function. The hair cells are arranged in four rows along the entire length of the cochlea—three rows of outer and one row of inner hair cells. The tectorial membrane responsible for their direct (outer hair cells) and indirect (inner hair cells) activation covers the hair cells; (C) Schematic of a mammalian cochlear hair cell shortly after birth. Hair cells are polarized epithelial mechanosensory cells with a mechanically sensitive organelle at the apical surface, known as stereovilli (SV). Stereovilli are non-motile hair bundles consisting of dozens of specialized F-actin-filled microvilli graduated in length to form a staircase-like structure. Within their V-shaped orientation stereovilli are connected by extracellular linkages called tip links. The longest stereovilli is closest to a single genuine microtubule-based cilium, the kinocilium with a (9 × 2 + 2) microtubule configuration (which begins to regress at around P8 in the mouse). Hair cells are surrounded by non-sensory supporting cells with microvilli on their apical surface; (D–H) Transmission electron micrographs of a mouse cochlea; (D) Cross-section of a kinocilium (KC) showing the (9 × 2 + 2) structure; (E) Longitudinal section of stereovilli and the kinocilium on the apical surface of an inner hair cell; (F) Immature synaptic ribbon (SR) of an inner hair cell with the typical electron-dense sphere surrounded by synaptic vesicles; (G) V-shaped orientation of stereovilli and the kinocilium on a vestibular hair cell; (H) Cross-section of a synaptic ribbon in a vestibular hair cell. CS: centrosome. CP: cuticular plate. Scale bars: 50 µm (A,B), 100 nm (D,H), 200 nm (E,F), 500 nm (G).