The pattern of ciliary development in fetal mouse vestibular receptors. A qualitative and quantitative SEM study.

The development of vestibular receptors in the mouse was studied by scanning electron microscopy between the 13th gestation day to birth. On the 13th gestation day, the utricle was entirely covered with microvilli, which were often grouped around small kinocilia at the center of the macula. The vertical cristae were not clearly differentiated at this stage. On the 15th gestation day, the opposite orientation of ciliary tufts in the utricle indicated the beginnings of the striola. During the whole period studied, gradients in differentiation of ciliary tufts were observed between the center and the periphery of the utricle, and the top and base of the cristae. The auxiliary structures (otolithic membrane and cupula) began to appear at the same time as the first ciliary tufts differentiated. Otoliths, still immature, were only observed as from the 16th gestation day. Differentiation of ciliary tufts on the utricle appeared to be progressive during the fetal period. However, between the 16th and 17th gestation days, a pause in the differentiation of ciliary tufts was registered. A day later, there was a pause in the increase of the utricular sensory surface, which coincided with a temporary stabilization of the decrease in the thickness of the sensory epithelium.