Submitted to: Book Chapter
Publication Type: Book / Chapter
Publication Acceptance Date: December 22, 2004
Publication Date: January 1, 2005
Citation: Echternkamp, S.E. 2005. Fertilization. In: Pond, W., Bell, A., editors. Encyclopedia of Animal Science. New York, NY:Marcel Dekker. p. 411-413. Technical Abstract: Fertilization is a complex process by which mammalian male (spermatozoon) and female (oocyte or ovum) haploid gametes unite to produce a totipotent diploid zygote that develops into a genetically distinct individual. Descriptions of the molecular and physiological mechanisms for most fertilization events are incomplete and limited primarily to laboratory animals, and for some processes (e.g., intercellular fusion of gametes) are primarily hypothetical. The fertilization process is composed of a series of sequential steps: 1) sperm capacitation; 2) binding of capacitated sperm to the zona pellucida (ZP); 3) acrosome reaction; 4) sperm penetration of ZP; 5) fusion of spermatozoon and ovum (egg); 6) egg activation; and 7) establishment of the embryonic genome. Freshly ejaculated mammalian sperm are incapable of fertilizing ova but acquire functional competence through capacitation in the female reproductive tract. Capacitation is a poorly defined maturational process that includes a series of intracellular and membranal changes in the plasma membrane surface and enables sperm to penetrate the cumulus oophorus, bind to the ZP and undergo the acrosome reaction. Ovulated oocytes/ova are surrounded by the ZP, a viscoelastic, spherical, extracellular matrix composed of three glycoproteins (ZP1, ZP2, and ZP3), which provide both structural form and species-specific receptor-ligand binding between the egg extracellular matrix and sperm. Binding of sperm to the ZP initiates the acrosome reaction and exocytosis of its content, which enables the spermatozoon to penetrate the ZP. Upon entering the perivitelline space, the spermatozoon inner plasma membrane and equatorial zone attach to the ovum plasma membrane, the membranes of the two gametes fuse by a phagocytic process, and the maternally and paternally derived chromosomes intermix to form a diploid cell and initiate embryogenesis.