Ciliary inter-microtubule bridges

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Abstract
Electron micrographs of both negatively contrasted and thin-sectioned lamellibranch gill cilia reveal several new features of ciliary fine structure, particularly in regard to those structures forming intermittent or permanent crossbridges between microtubules. Negativecontrasting reveals the presence of a 14 5-nm repeating bridge between the central microtubules. Frontal views of negatively contrasted dynein arm rows along subfibre A show that the arms (23-nm repeat) in the outer row are displaced in a left-handed manner by 3–4 nm with respect to those in the inner row. This displacement is probably a direct reflexion of the helical tubulin subunit lattice of the subfibre. Interdoublet (nexin) links are seen connecting adjacent A and B subfibres at intervals of 86 nm along the doublet. Negative-contrasting shows thin, highly elastic connexions holding the doublets together. When seen in longitudinal thin sections, the interdoublet links are often tilted to considerable angles, indicating they may have an elastic response to interdoublet sliding.