The fine structure of the developing otolithic organs of the rat.

The fine structure of the otolithic organs in fetal and neonatal rats were studied by light and transmission electron microscopy. The otoconia were found at the 16.5 gestational day while the otolithic membrane appeared much later. Types II and I hair cells were first observed at the 16.5 and 18.5 gestational days, respectively. Secretory granules in the supporting cells seemed to release an organic material which was destined to be incorporated into the organic matrix of the otoconia and/or the otolithic membrane. Protrusions of apical cytoplasm, cytoplasmic globules and dilated rough endoplasmic reticulum (rER) were observed in epithelial cells of both sensory and non-sensory regions, in particular, in the transitional cells. By contrast, otoconia were mainly located above the neuroepithelial area. The functional roles of protrusions and cytoplasmic globules in the genesis of otoconia remain to be clarified. In addition, the pathway for transport of calcium remains obscure.