Polarities of the centriolar structure: Morphogenetic consequences

Centrioles and basal bodies are two versions of the same conserved eukaryotic organelle and share two remarkable properties: nine-fold symmetry of their microtubular shaft and capacity to generate a new organelle in a fixed geometrical 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, studies on protists have led to the notion that ciliary and flagellar basal bodies display polarities, not only a proximo-distal polarity, like in centrioles, but also a circumferential polarity accorded to the polarities of the cell and of its cytoskeleton. The major cytological and genetical data, mainly of Chlamydomonas, Paramecium and Tetrahymena, which support the notion that the microtubule triplets of basal bodies are non-equivalent, are reviewed. The morphogenetic implications of this circumferential anisotropy, perpetuated through the process of basal body duplication itself, are discussed. The question is raised of the possibility that centrioles also display a circumferential polarity, like basal bodies, and whether at least certain of their functions depend on such asymmetries.