Structural modification of the hamster sperm acrosome during posttesticular development in the epididymis

The acrosome of the mature spermatozoon functions as a regulated secretory vesicle which performs several critical functions in mammalian fertilization. Acrosome assembly occurs throughout spermiogenesis and continues during posttesticular sperm maturation in the epididymis, resulting in a structurally polarized membrane-bounded organelle that contains an assortment of hydrolases and a stable infrastructure termed the acrosomal matrix. The role of stable acrosomal matrix assemblies in acrosomal biogenesis and function are poorly understood. This article presents ultrastructural, immunocytochemical, and biochemical data on the remodeling of the hamster acrosomal matrix during spermiogenesis and posttesticular sperm maturation in the epididymis. Specific posttranslational modifications of the major acrosomal matrix protein are evident in late, step 16, spermatids and matrix protein processing continues within specific acrosomal subdomains of caput epididymal spermatozoa. At the completion of sperm maturation, the acrosomal matrix consists of two structurally distinct domains which are adherent to the outer acrosomal membrane and exhibit a localized distribution pattern. Coincident with acrosomal matrix differentiation, a paracrystalline cytoskeletal complex is assembled onto the outer acrosomal membrane of epididymal spermatozoa. This cytoskeletal network appears to establish transmembrane structural interactions with the acrosomal matrix and may maintain attachment of the acrosomal cap to the sperm head during the early steps of the acrosome reaction.