Spermatid differentiation in the blowfly Sarcophaga bullata with particular reference to flagellar morphogenesis

Abstract

Young spermatids of Sarcophaga possess a single centriole apposed to the nuclear envelope and surrounded by a dense centriole adjunct. Concomitant with nuclear condensation and flagellar elongation, the centriole adjunct disappears and the centriole dedifferentiates from its normal triplet microtubule grouping. The triplet organization is retained only in the most proximal 500 Å of the centriole while subfiber C of each group becomes centrifugally displaced and is continuous with the 9 accessory tubules that extend the length of the mature axoneme. Flagellar axoneme morphogenesis is first apparent as a ring of 9 single microtubules, or the A subfibers of the presumptive doublets. This is followed by the appearance of the radial linkage along each A subfiber, the B subfiber of each doublet from a projection off subfiber A, the paired arms along subfiber A, the central tubules and sheath, the accessory tubules from a projection off the B subfibers, and finally the outer coarse fibers. Occurring at the same time as axoneme assembly is the elongation of the 2 adjacent mitochondrial derivatives and the formation of their crystalloids. In addition, the acrosome of mature spermatids shows a paracrystalline organization as revealed by negative staining.