Ciliated cells are characterized by a highly organized cytoskeleton which is connected with the ciliary apparatus. The organization of microtubules, microfilaments, and cytokeratin filaments is described and the relationships of each network with the ciliary apparatus are emphasized. Possible functions of such a complex cytoskeleton are discussed.

The different steps of ciliogenesis occurring in quail oviduct were compared to the ciliogenesis pattern described in other metazoan species. Centrioles are generated according to pathways that are found within the same cell: the centriolar and the acentriolar pathways. In the acentriolar pathway, centrioles are generated in the Golgi area, without contact with the preexisting centrioles of the centrosomes, and they migrate toward the apical membrane. The control of this polarized migration was studied by means of several drugs (colchicine, nocodazol, taxol, cytochalasin D, benzodiazepines) and immunocytochemistry. It was suggested that an actin-myosin system was involved in the migration of centrioles, whereas labile microtubules were not necessary. Basal bodies must dock with plasma membrane or cytoplasmic vesicles for the initiation of axonemal microtubule polymerization. This signal is necessary even in the presence of taxol.

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Actin microfilaments were localized in quail oviduct ciliated cells using decoration with myosin subfragment S1 and immunogold labeling. These polarized epithelial cells show a well developed cytoskeleton due to the presence of numerous cilia and microvilli at their apical pole. Most S1-decorated microfilaments extend from the microvilli downward towards the upper part of the ciliary striated rootlets with which they are connected. From the microvillous roots, a few microfilaments connect the proximal part of the basal body or the basal foot associated with the basal body. Microfilament polarity is shown by S1 arrowheads pointing away from the microvillous tip to the cell body. Furthermore, short microfilaments are attached to the plasma membrane at the anchoring sites of basal bodies and run along the basal body. The polarity of these short microfilaments is directed from the basal body anchoring fibers downward to the cytoplasm. At the cell periphery, microfilaments from microvillous roots and ciliary apparatus are connected with those of the circumferential actin belt which is associated with the apical zonula adhaerens. Together with the other cytoskeletal elements, the microfilaments increase ciliary anchorage and could be involved in the coordination of ciliary beating. Moreover, microvilli surrounding the cilia probably modify ciliary beating by offering resistance to cilium bending. The presence of microvilli could explain the fact that mainly the upper part of the cilia appanars to be involved in the axonemal bending in metazoan ciliated cells.

Using filipin as a probe for the presence of membrane cholesterol, the evolution of cholesterol distribution in the apical plasma membrane was studied during estrogen-induced ciliogenesis in quail oviduct and compared with the distribution of intramembrane particles (IMPs). Ciliary growth is preceded by the first step of microvillus differentiation. Microvilli emerge in membrane domains rich in IMPs and devoid of filipin-cholesterol (f-c) complexes. However growing microvillus membrane shows f-c complexes. During ciliary growth, microvilli lengthen from 0.5 to 2 microns, indicating that the microvillar membrane is not a membrane reservoir for ciliogenesis. During ciliary growth, the characteristic ciliary necklace IMP rows appear progressively at the base of cilia. The first IMP row is organized in a membrane circlet lacking of f-c complexes, whereas the new shaft membrane in the middle of the circlet exhibits numerous complexes. These two different domains of the cilia keep their specificity during ciliary growth. Only the ciliary tip shows fewer complexes than the shaft membrane. The apical membrane of differentiated ciliated cells is thus composed of various domains, the ciliary shaft full of f-c complexes and poor in IMPs, the ciliary necklace is devoid of f-c complexes and rich in IMPs, the microvilli membrane is rich in both IMPs and f-c complexes, and the interciliary membrane is poor in both f-c complexes and IMPs, whereas the undifferentiated cells exhibit an apical membrane in which f-c complexes and IMPs are distributed homogeneously.