The in situ organization of the inner dynein arms of Chlamydomonas axonemes was investigated by analyzing a number of tilt-series thin-section electron micrographs taken with improved fixation and staining protocols. The use of mutant axonemes lacking the outer dynein arms and the radial spokes enabled us to detect a novel feature: inner-arm projections arranged in two rows. The arrangement of projections within each row was deduced by making a model that fitted both tilt-series longitudinal thin-section images and cross-section images. The model suggested that, within a repeating unit of 96 nm, one of the two rows, positioned more centrifugally, consists of four discrete domains and the other, positioned more centripetally, consists of three pairs of globular structures. Whereas the centripetal row resembles the inner-arm arrangement reported previously, the centrifugal row appears to have been identified for the first time in this study. These findings make necessary the re-examination of the arrangement of various inner-arm subspecies within the axoneme.
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Muto et al. (1991) studied this question.