Considering these functions of the acrosome during the process of fertilization, the morphology and functional integrity of the acrosome in ejaculated human spermatozoa are of fundamental importance in attachment, species-specific binding and zona penetration preceding gamete fusion. Furthermore, chemical agents which bring about the reaction in vitro, such as the ionophores ionomycin or a23187, have been used to shed light on its regulatory mechanisms. In addition, a novel protein, termed saa-1, that was first detected on human spermatozoa is discussed with respect to its potential role as a regulatory protein closely involved in the initiation of the acrosome reaction. For this purpose, biochemical and immunocytochemical methods have been used to investigate acrosin activity and possible alterations of the acrosomal membrane system in different groups of sub- and infertile patients. The acrosome reaction is a crucial step during gamete interaction in all species, including man.
Acrosome human sperm. After 8 h of capacitation, a large decrease in sperm enzyme levels took place only in the case of hyaluronidase, although small decreases were also noted in total acrosin, proacrosin and inhibited acrosin. The acid phosphatase levels decreased twofold, presumably due to the loss of seminal (prostatic acid phosphatase that loosely adheres to spermatozoa. Decreases in total acrosin and proacrosin, but not in inhibited acrosin, also occurred when spermatozoa were incubated under noncapacitating conditions for 8 h, indicating that capacitation may specifically cause the release of some acrosin inhibitor from human spermatozoa. It is therefore mandatory to focus on acrosomal membrane functions and consider particularly the occurrence of acrosomal disturbances in spermatozoa from men of barren marriages. Capacitation of human spermatozoa was assessed by their ability to penetrate denuded hamster oocytes. With the exception of acid phosphatase, no alteration in enzyme activity occurred after 4 h of incubating the spermatozoa under capacitation conditions although gamete fusion took place. The effect of in vitro capacitation (events that occur before the acrosome reaction) on the acrosomal enzymes of human spermatozoa was determined.
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A number of molecules have been postulated to regulate the acrosome reaction in mammals, for example a galactosyl-transferase and a sperm protein tyrosine kinase. The acrosome of mammalian spermatozoa contains high amounts of acrosin, which is believed to be essential for gamete fusion, particularly for binding to, and penetration of, the zona pellucida.