Polarities of the centriolar structure: Morphogenetic consequences

Centrioles and basal bodies are two versions of the same conserved eukaryot ic organelle and share two remarkable properties: nine-fold symmetry of the ir microtubular shaft and capacity to generate a new organelle in a fixed g eometrical relationship to the mother organelle. it can thus be postulated that what is true for basal bodies is likely to be true also for centrioles . While the functions of centrioles are difficult to dissect, the functions of basal bodies are easier to approach. Over more than two decades, studie s on protists have led to the notion that ciliary and flagellar basal bodie s display polarities, not only a proximo-distal polarity, like in centriole s, but also a circumferential polarity accorded to the polarities of the ce ll and of its cytoskeleton. The major cytological and genetical data, mainl y on Chlamydomonas, Paramecium and Tetrahymena, which support the notion th at the microtubule triplets of basal bodies are non-equivalent, are reviewe d. The morphogenetic implications of this circumferential anisotropy, perpe tuated through the process of basal body duplication itself, are discussed. The question is raised of the possibility that centrioles also display a c ircumferential polarity, like basal bodies, and whether at least certain of their functions depend on such asymmetries. (C) 1999 Editions scientifique s et medicales Elsevier SAS.