Development of hair cell stereocilia in the avian cochlea.

The establishment of an embryonic hair cell's stereociliary bundle involves the coordinated regulation of several morphogenetic events. The developing hair cell organizes the assembly of individual stereocilia, regulates the growth of the stereociliary bundle, and aligns the orientation of the bundle. During development, individual stereocilia exhibit three phases of growth: (1) an initial assembly and elongation of a small number of actin filaments; (2) the development of the stereocilia rootlet and the addition of more filaments to each stereocilium; (3) a second growth phase where elongation of the actin filaments resumes. These three phases involve different biochemical conditions for actin assembly and, thus, are temporally separated during development. Each hair cell also regulates the size of its stereociliary bundle so that it fits into the precise basal-to-apical gradient in stereocilia length and width seen in the mature cochlea. Orientation of the stereociliary bundles also changes during development. Very young hair cells exhibit a non-uniform orientation. Early in development, neighboring groups of cells rapidly acquire a uniform orientation. A more gradual shift in orientation continues throughout development, so that by maturity most of the hair cells are oriented toward the abneural edge of the sensory epithelium.