Development of the auditory hair cell surface in human fetuses. A scanning electron microscopy study.

Thirteen human fetal cochleas were investigated by scanning electron microscopy. Our observations concentrated on the hair cell surface. Ciliogenesis appeared to start during the 11th week of gestation on the inner hair cells (IHCs) and one week later on the outer hair cells (OHCs). The earliest stages of stereociliary development were similar on both types of cell and were characterized by the presence of round bundles of cilia arising from the surrounding microvilli. A three-dimensional V-shaped arrangement suddenly appeared, accompanied by the disappearance of short cilia on the internal side. Between the 20th and the 22nd weeks of gestation, both types of hair cell had an adult stereociliary pattern, i.e. a rectilinear arrangement on IHCs and W-shaped on OHCs. However, there were signs of immaturity, such as a disarray of OHCs and the presence of the kinocilium, suggesting that the surface of the auditory hair cells achieves its maturation during the last trimester of pregnancy.