The cross-striated connecting-piece in the neck region of the mammalian spermatozoon develops from material that seems to arise between the triplets in the wall of both proximal and distal centrioles. The precursor material extends radially from the centrioles and polymerizes around them to form the connecting piece. This is not composed of “segments” or “lamellae” as usually described but is a cross-striated fibrous protein with a major period of 665 A and a complex pattern of intraperiod bands. It is considered to be homologous with the striated rootlets of cilia.
During differentiation of this region, the proximal centriole develops at one end, a cylindrical prolongation which is similar in its structure but not identical with the centriole proper. This centriolar adjunct is a transient structure. The proximal centriole persists in the mature spermatozoon, but the distal centriole disintegrates during formation of the connecting-piece.
The outer dense fibers of the flagellum and the cross-striated columns of the connecting piece are distinct components which arise separately and secondarily become continuous. The dense fibers originate as ridge-like radial outgrowths from the wall of the corresponding doublets in the axonemal complex. As they enlarge they separate from the doublets throughout most of their length but they remain continuous with them at their distal ends.