The human sperm acrosome reaction: physiology and regulatory mechanisms. An update

Abstract

The acrosome reaction is a crucial step during gamete interaction in all species, including man. It allows spermatozoa to penetrate the zona pellucida and fuse with the oocyte membrane. Spermatozoa unable to undergo the acrosome reaction will not fertilize intact oocytes. This article concentrates on the characteristics and regulatory mechanisms of the acrosome reaction in human spermatozoa. During recent years, various entities found in the vicinity of the ovulated oocyte have been identified as stimulators of the acrosome reaction, of which zona protein is considered the prime physiological inducer in vivo. The steroid hormone progesterone has been shown to evoke critical responses in sperm cells leading to the acrosome reaction. Calcium has also been shown to play a central role during the acrosome reaction. Calcium flux is induced specifically by progesterone in capacitated and uncapacitated sperm cells, whereas only capacitated spermatozoa are able to subsequently complete the acrosome reaction. Progesterone as well as zona protein has been shown to evoke crucial responses within human spermatozoa, shedding light on the cascade of intracellular signalling events leading to the completion of the acrosome reaction. 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. 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. 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.

Acrosome reaction in the cumulus oophorus revisited: involvement of a novel sperm-released factor NYD-SP8

“…AR, one of the critical events in mammal fertilization, is defined as the fusion of the outer acrosome membrane with inner spermatozoa plasma membrane in multiple sites (Yanagimachi, 1994;Brucker and Lipford, 1995;Ikawa et al, 2010). In response to appropriate stimuli, the outer acrosome membrane and the overlying sperm plasma membrane fuse and vesiculate, leading to exposure of acrosomal contents.…”

Spermatozoa Epididymal Maturation in the Mexican Big-Eared Bat (Corynorhinus Mexicanus)

“…These important physiological properties of mammalian sperm cells necessary for fertilization include: the loss of cytoplasmic droplet, the potential to move progressively [Orgebin-Crist 1987;Yanagimachi 1994], capacitation potential , the ability to undergo the acrosomal reaction [Brucker and Lipford 1995], and to interact with the zona pellucida of the egg [Bedford 1998]. Epididymal maturation in most mammalian species studied to date, including human, depends on androgen secretion by the testes and requires approximately 10-20 days [Holland et al 1992;Orgebin-Crist 1996;Moore and Akhondi 1996].…”

Cytoskeleton localization in the sperm head prior to fertilization

“…In somatic cells, actin filaments lying beneath the plasma membrane form a cortical network involved in functions such as cell-to-cell contact and adhering to extracellular matrix, cell growth and migration, and exocytosis. In spermatozoa, a number of cytoskeletal proteins are implicated in key events such as capacitation and the AR (Yanagimachi 1981, Virtanen et al 1984, Brucker & Lipford 1995, Brewis & Moore 1997, Howes et al 2001, Moore 2001, Brener et al 2003. For instance, actin may play a role in the AR (Vogl 1989, Palecek et al 1999) and inhibition of actin polymerization by cytochalasin B and D blocked induction of the human AR by zona pellucida (Liu et al 1999, Dvorakova et al 2001.…”