Ultrastructural analysis of primary cilium in the embryonic nervous tissue of mouse.

In the embryonic nervous system, the morphology and spatial organization of the ciliary apparatus in the primordial cells, the neuroepithelial bipolar cells, has been analysed from frontal and horizontal ultrathin sections. Projecting into the ventricular fluid, the primary cilium is located just at the interface between the ventricular fluid and the cytoplasm of these cells. Always associated with a basal body and a centriole, their structural components are nine microtubule doublets equally spaced around the luminal cylinder without central doublet. The cilium is short with a thick base displaying a reduced ciliary necklace and a 9 + 0 pattern. In its tip, the pattern decreases from 7 + 0 to 2 + 0. The basal body differs from the basal body of the motil cilium by internal and external sheets interconnecting triplets from base to apex, the constant existence of an accessory basal foot and the scarcity of a ciliary rootlet. All these characteristics suggest that in the neuroepithelial bipolar cells the ciliary apparatus is a rigid, polarized and transient structure probably important in the possible exchange between the ventricular fluid and the developing nervous system.