Acrosome reaction capacitation

Fertilization is the fusion of the sperm and ovum. The sperm head binds to the plasma membrane of the egg (oolemma), and the entire spermatozoon enters the cytoplasm of the ovum. Only capacitated spermatozoa with intact acrosomes can enter and pass through the cumulus oophorus. The acrosome is a membrane-bound, cap-like structure covering the anterior portion of the sperm nucleus. The acrosomal reaction is the release of materials that lyse the glycoprotein coat (zona pellucida) surrounding the ovum. This is necessary for fertilization to take place. Before undergoing the acrosomal reaction, sperm go through a type of hypermotility called hyperactivation. The... 

The acrosome reaction is the loss of the acrosomal and plasma membranes in the acrosome region and the release of acrosin, hyaluronidase and other enzymes that disperse the cumulus complex and allow the sperm to penetrate the zona pellucida. After capacitation and the acrosome reaction, sperm penetrate the extracellular cumulus matrix and bind with zona protein 3, a heavily glycosylated protein of the zona pellucida. The first segment of the sperm to make contact with the oolemma is usually the inner acrosomal membrane, followed by the postacrosomal region. The plasma membrane of the sperm attaches to microvilli on the oolemma. Sperm-egg fusion is apparent from reduced movement of the sperm tail (Yanagimachi, 1970,1988 Takano et al.,... 

Boatman, D.E and Robbins, R.S. (1991). Bicarbonate Carbon-dioxide regulation of sperm capacitation, hyperactivated motility, and acrosome reactions. Biol. Reprod. 44 806-813. 

Harrison, R.A.P. and Gadella, B.M. (1995). Membrane changes during capacitation with special reference to lipid architecture. In Human Sperm Acrosome Reaction. (Fenichel, P. and Parinaud, J., Eds.), Vol. 236, pp. 45-65. Colloque INSERM/John Libbey Eurotext, Montrouse, France. 

Langlais, J. and Roberts, K.D. (1985). A molecular membrane model of sperm capacitation and the acrosome reaction of mammalian spermatozoa. Gamete Res. 72 183-224. 

Lee, M.A., Trucco, G.S., Bechtol, K.B., Wummer, N., Kopf, G.S., Blasco, L and Storey, B.T. (1987). Capacitation and acrosome reactions in human spermatozoa monitored by a chlortetracycline fluorescence assay. Fert. Steril. 48 649-658. 

Mammalian fertilization is the result of a precisely regulated series of cellular interactions. This process can be divided into a number of component stages, including the following (i) the early events that precede egg fusion, consisting of the preliminary event of sperm capacitation, sperm penetration of the cumulus oophorus, zona pellucida adhesion, initiation and completion of acrosome reactions, penetration of the zona pellucida, egg plasma membrane contact and fusion and (ii) the late events that occur within the egg that consist of sperm nuclear decondensation followed by pronuclear consolidation and syngamy. 

In conclusion, it is generally believed that the extracellular matrix of the cumulus oophorus functions as a selective filter permitting only those sperm that have not completed the acrosome reaction to penetrate to the zona pellucida. This matrix may also provide a unique microenvironment that regulates the final stages of capacitation. 

Volume 5 of Advances in Developmental Biochemistry consists of seven chapters that review specific aspects of development in several different organisms including mollusks, flies, and mice. Five of the seven chapters address aspects of fertilization, including capacitation of sperm (Chapter 3), the acrosome reaction (Chapter 7), gamete adhesion (Chapters 2 and 6), and oocyte maturation and ovulation (Chapter 4). 

Maccarrone, M, Barboni, B, Paradisi, A, Bernabo, N, Gasped, V, Pistilli, MG, Fezza, F, Lucidi, P, Mattioli, M. 2005. Characterization of the endocannabinoid system in boar spermatozoa and implications for sperm capacitation and acrosome reaction. J Cell Sci in press. 

Several physiologic roles for CS have been postulated. First, it has been suggested that the presence of CS in erythrocytes (about 700 fig of CS are present in 100 ml erythrocytes [125]) stabilizes the erythrocyte membrane [130]. Thus, CS was found to reduce hemolysis up to 56% in hypotonic solutions, whereas several other steroid sulfates and cholesterol conjugates were devoid of antihemolytic activity [125]. Furthermore, the presence of CS had a critical influence on the disc shape of erythrocytes in hypotonic solution without CS, erythrocytes tended to become spherical and extend spicules [131]. A second postulated role of CS relates to spermatozoa. It has been suggested that CS stabilizes the membranes of these cells and that it may provide a structural trigger for capacitation [132,133]. Thus, CS is present in spermatozoa (15 fig/lO cells) as well as in seminal plasma, and appears to be concentrated in the acrosomal region [132]. On the other hand, sterol sulfatase activity is present in the human female reproductive tract [133]. There is no direct evidence, however, that CS contributes to the sperm membrane modification reactions that occur in association with fertilization. 

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