Structural changes of the head components of the rat spermatid during late spermiogenesis.

The transformation of the nucleus, acrosomic system, and peri-nuclear theca (perforatorium and post-acrosomal dense lamina) was analyzed during the maturation phase, i.e., steps 14 to 19 of spermiogenesis. Following partial condensation of chromatin from steps 11-14, the nucleus continues to condense during the following steps until the end of spermiogenesis. The redundant nuclear envelope which forms along the apical and ventral aspects of the nucleus and around the implantation fossa regresses during steps 17-19. The acrosomic system splits into two portions early in step 15 to give rise to: a) the main portion with its crest-like acrosome running along the dorsal aspect of the nucleus and head cap extending over the lateral surfaces of the nucleus: and b) a smaller head-cap segment which is seen in steps 15 and 16 along one side of the nucleus at its apical extremity. This separated head-cap segment reaches the apical-ventral aspect of the head during step 17 and condenses in synchrony with the rest of the acrosomic system in step 19 of spermiogenesis. The large crescentic acrosome, which in step 15 forms a large fin at the caudal extremity of the acrosomic apparatus, moves anteriorly during steps 16 and 17, while the whole acrosomic system extends farther apically beyond the tip of the nucleus. The perforatorium and post-acrosomal dense lamina form a rigid capsule (perinuclear theca) that covers tightly the sickle-shaped nucleus and binds the inner acrosomal membrane and the post-acrosomal membranes. The post-acrosomal dense lamina, which includes the ventral spur, appears early in step 15 as a dense cytoplasmic layer applied to the nuclear envelope at the caudal extremity of the nucleus except over the perifossal zone. The perforatorium forms during step 19 of spermiogenesis as a result of the condensation of a wispy cytoplasmic material which has accumulated in the subacrosomal space during steps 14-18. Thus the spermatid's head is deeply modified during the maturation phase and takes its definitive shape only at the last step of spermiogenesis.