The vestibular apparatus in human fetuses and adults was examined by scanning electron microscope.
1. The intermingled coexistence of immature and mature sensory cells over the sensory epithelium of the vestibular organs was seen in specimens from 14 to 18-week-old fetuses. The number of stereocilia in each sensory hair bundle was almost the same in the immature and mature cells. Thus, small fetal stereocilia seem to grow into mature long hairs.
2. Smallsensory cell groups were found apart from the maculae and cristae in one fetal and one adult vestibule. Both groups were seen in the utricular wall, one near the utricular macula and another near the posterior ampullary crista. Such crista neglect-like structures seem to occur sometimes in the human vestibule.
3. Non-sensory ciliated cell groups were found in seven fetuses aged 14 to 25 weeks. These cells were scattered in boundary areas between the three semicircular canal ampullae and the utricle. As these cells were not found in adult specimens, it seems likely that they disappear during maturation.
