Asymmetrical microtubule capping structures in frog palate cilia.

Capping structures at the distal tips of frog palate cilia are attached to the A- and central pair microtubules by electron-dense plug structures similar to those found in protozoan cilia and flagella and in epithelial cilia from invertebrates and vertebrates (W.L. Dentler, 1980, J. Cell Sci. 42, 207-220; W.L. Dentler and E.L. LeCluyse, 1982, Cell Motil. 2, 549-573). The caps in frog palate cilia are composed of a proximal shelf, to which doublets Nos. 1-3, 8, 9 and the central microtubules are attached and a larger distal cap to which doublets Nos. 4-7 are bound. The smaller proximal shelf is positioned to one side of the cilium and gives the cap an asymmetrical appearance. Striated ciliary rootlets attached to the basal bodies are also described. The smaller cap is placed on the same side of all cilia on the palate relative to both the direction of the effective stroke and the position of the rootlets. These results confirm that capping structures are common to most, if not all, cilia and provide direct evidence that they are precisely positioned on specific microtubules.