Microtubules and their protofilaments in the flagellum of an insect spermatozoon.
Spermatozoa of stick insects have nine accessory tubules, which surround the nine outer microtubular doublets and the two inner microtubular singlets. When fixed in a fixative that was designed to minimize protein denaturation (glutaraldehyde and tannic acid, no osmium post-fixation but block staining with uranyl acetate in water) the accessory tubules were seen to contain 17 protofilaments of the same type as those in the 9 + 2 microtubular doublets and singlets. The protofilaments in accessory tubules and other microtubules were roughly triangular. When studied by Markham's photographic method a somewhat different tilt of the two longer sides was seen; this makes it possible to distinguish a polarity in the microtubules, i.e. to differentiate between a microtubule that is viewed from its (-)end to its (+)end from one that is viewed in the opposite direction. The dynein arms of the doublets can be used as an independent type of marker for the polarity. In a computer-aided analysis of the fine structure of the tail axoneme, the A-subtubules of the outer doublets were seen to be not quite equidistant; rather, there were somewhat widened electron-dense interspaces in the ring of protofilaments in four places. The locations of these widened interspaces coincide with the attachment sites for the spoke, the inner dynein arm, the outer dynein arm, and the intertubular material. It is tentatively concluded that proteins of these structures, and perhaps also other microtubule-associated proteins, may be anchored deep within the wall of a microtubule rather than just superficially along it.