Development of apical-surface structures of mouse otic placode.

In 8.0 gestation-day embryos, a central cilium and microvilli were present on the apical surface of the cells of the presumptive otic placode and other ectoderm regions as well. The cilium at this stage lacked a central pair of fibres and was regarded as a primitive cilium with 9+0 composition. The microvilli gained in length and density of distribution as soon as the otic placode began to invaginate in 8.5-d. embryos. With further development in 9.5-11.0-d. embryos, they took on a regular distribution pattern. Narrowing and concomitant bulging out appeared in the cells along the neck of the otic invagination in 8.5-d. embryos. Marked regional differences also occurred in the apical-surface structures of the cells, particularly in the relatively more advanced form of the embryos. Kinocilia of the adult vestibular sensory cells were found to lack a continuous central pair of fibres, resembling the apical cilia of the otic placode. This finding seems to suggest that a vestibular kinocilium represents a direct descendant from the primordial cilium, which generally emerges from the primordial ectodermal cells and is entirely lost from the keratinizing ectoderm in the later development.