Structures attached to doublet microtubules of cilia: computer modeling of thin-section and negative-stain stereo images.

With a single set of positional coordinates for longitudinal and transverse attachment of the inner and outer rows of dynein arms with respect to the doublet microtubules of Tetrahymena ciliary axonemes, a computer model has been constructed at 4-nm resolution that reconciles negative-stain en face stereo images of arm and spoke positions to traditional images of tannic acid/glutaraldehyde-fixed sections. In this model, inner and outer arms correspond in substructure; both repeat with a 24-nm periodicity without stagger between rows, and a pair of arms is in exact alignment with the first spoke (S1) in each doublet spoke group. The model and the supporting micrographs suggest that each arm cycles in three dimensions and that, during cycling, the inner and outer arms move in opposite directions with respect to the center of subfiber A of the doublet (N). Attachment is off-center with respect to subfiber B of the adjacent doublet (N + 1), causing the sliding doublets to skew with respect to one another.