Fetal and postnatal ontogenesis of the rat cochlea, from the 16th gest
ational day (16DG) until 3 months post partum, were studied using scan
ning electron microscopy with emphasis on the stereocilia during the e
arliest stages of development. The epithelium of the cochlear duct in
16DG rat consisted of polygonal cells topped with numerous microvilli
and one central kinocilium, which form the Inner hair cells (IHCs) app
eared at 18DG in the basal cochlea. They were characterized by tufts o
f cilia of the same height and with a kinocilium. The first outer hair
cells (OHCs) can be seen at 20DG. The earliest stages of ciliary diff
erentiation, at 18DG for IHCs and 20DG for OHCs, were similar on both
types of cells and were characterized by the presence of round bundles
of cilia arising from the surrounding microvilli. A three-dimensional
V-shaped organization for OHCs and the linear arrangement for IHCs ap
peared by the end of the first postnatal week, accompanied by the disa
ppearance of transient cilia on the modiolar side of the hair cell and
the kinocilium on the external side. The apical pole of OHCs reached
adult-like morphology before that of IHCs. Various links between stere
ocilia were detected already at birth. Morphometric analysis showed th
at auditory cells from the base of the cochlea reached adult size by t
he end of the first postnatal week while those from the apex increased
their size later. A review of the literature including comparative ob
servations across species on the ontogenesis of the stereocilia shows
that hair cells of the state-acoustic system may present the same earl
y ontogenesis.