Components and development of the centriolar complex during and beyond spermiogenesis in a passeridan bird, the Masked weaver (Ploceus velatus)

Abstract

The fate of the proximal centriole in passeridan birds is an area of controversy and relative lack of knowledge in avian spermatogenesis and spermatology. This study examines, for the first time, spatiotemporal changes in the centriolar complex in various phases of spermiogenesis in a passerine bird, the Masked weaver (Ploceus velatus). It also describes the configuration of the centriolar complex and the relationship between it and the granular body in both intra- and extra-testicular spermatozoa. It is shown that the proximal centriole is retained and attaches, at its free end, to the granular body of spermatids in every step of spermiogenesis, as well as in mature intra-testicular and post-testicular spermatozoa, including those in the lumen of the seminal glomus. As the centriolar complex, along with its attached granular body, approaches the nucleus in the early spermatid, the proximal centriole articulates with the distal centriole at an acute angle of about 45°, and thereafter, both centrioles, still maintaining this conformation, implant, by means of their articulating proximal ends, at the implantation fossa of the nucleus. In the mature spermatid and spermatozoon, the granular body winds itself helically around the centriolar complex in the neck/midpiece region of the cell, and, thus, becomes the granular helix. The significance of this observation must await future studies, including possible phylogenetic re-evaluation and classification of birds.