Dynamics of the mammalian sperm plasma membrane in the process of fertilization

Sexual reproduction requires the fusion of sperm cell and oocyte during fer tilization to produce the diploid zygote. In mammals complex changes in the plasma membrane of the sperm cell are involved in this process. Sperm cell s have unusual membranes compared to those of somatic cells. After leaving the testes, sperm cells cease plasma membrane lipid and protein synthesis, and vesicle mediated transport. Biophysical studies reveal that lipids and proteins are organized into lateral regions of the sperm head surface. A de licate reorientation and modification of plasma membrane molecules take pla ce in the female tract when sperm cells are activated by so-called capacita tion factors. These surface changes enable the sperm cell to bind to the ex tra cellular matrix of the egg (zona pellucida, ZP). The ZP primes the sper m cell to initiate the acrosome reaction, which is an exocytotic process th at makes available the enzymatic machinery required for sperm penetration t hrough the ZP. After complete penetration the sperm cell meets the plasma m embrane of the egg cell (oolemma). A specific set of molecules is involved in a disintegrin-integrin type of anchoring of the two gametes which is com pleted by fusion of the two gamete plasma membranes. The fertilized egg is activated and zygote formation preludes the development of a new living org anism. In this review we focus on the involvement of processes that occur a t the sperm plasma membrane in the sequence of events that lead to successf ul fertilization. For this purpose, dynamics in adhesive and fusion propert ies, molecular composition and architecture of the sperm plasma membrane, a s well as membrane derived signalling are reviewed. (C) 2000 Elsevier Scien ce B.V. All rights reserved.