The polar ring of coccidian sporozoites: a unique microtubule-organizing centre.

The infective stages, or 'zoites, of coccidian parasites possess an organized network of spirally arranged microtubules that closely follow the helical body shape of these vermiform cells. These subpellicular microtubules are anchored anteriorly by insertion into a highly structured circular microtubule-organizing centre (MTOC) known as the polar ring. This MTOC has been examined both in situ and in isolated, critical-point-dried whole cytoskeletons. The 24 microtubules attach laterally to the MTOC through shallow depressions on the inner face of the ring: the ends do not appear to be physically capped. The polar ring has no obvious or regular substructure, although it has a faintly fibrous appearance. The polarity of the microtubules, determined by 'hook decoration', is such that the plus or fast-growing end is distal to the MTOC. The coccidian 'zoite MTOC is unique both in its highly defined structure and in the degree of organization it confers upon the developing cell in terms of the number, spacing, orientation and polarity of the subpellicular microtubules.