Studies on the eel sperm flagellum. I. The structure of the inner dynein arm complex.

The highly motile, 9 + 0 sperm axoneme of Anguilla has inner dynein arms (IDAs) but not outer dynein arms. The in situ morphology of these IDAs is shown here to be essentially identical to the IDA morphology already seen in the axonemes of Chlamydomonas, Tetrahymena and Beroë, and in the sperm tails of echinoderms and several vertebrate species. In addition, this study demonstrates: (1) that the nexin (circumferential) links are present in Anguilla and are typical; (2) that IDA1 incorporates an archway, supported by a pillar-structure; (3) that images from thin sections and whole mounts are consistent with those from replicas of rapidly-frozen specimens; and (4) that the IDA and nexin link morphology is apparently unaffected by whether the axoneme is depleted of ATP, relaxed with ATP and vanadate, or inhibited by high ATP. An attempt has been made to reconcile the emergent morphology of the IDA complex with all earlier descriptions in the literature. From a detailed comparison of the results with published information on Chlamydomanas mutants, it is concluded that the nexin (circumferential) link is a major part of the 'dynein regulatory complex'.