Capacitation and the acrosome reaction in equine sperm

During sexual reproduction, the sperm and oocyte must fuse before the production of a diploid zygote can proceed. In mammals such as equids, fusion depends critically on complex changes in the plasma membrane of the sperm and, not surprisingly, this membrane differs markedly from that of somatic cells. After leaving the testes, sperm cease to synthesize plasma membrane lipids or proteins, and vesicle-mediated transport stops. When the sperm reaches the female reproductive tract, it is activated by so-called capacitation factors that initiate a delicate reorientation and modification of molecules within the plasma membrane. These surface changes enable the sperm to bind to the extracellular matrix of the egg (zona pellucida ZP) and the zona then primes the sperm to initiate the acrosome reaction, an exocytotic event required for the sperm to penetrate the zona. This paper will review the processes that occur at the sperm plasma membrane before and during successful penetration of the equine ZP. It is noted that while several methods have been described for detecting changes that occur during capacitation and the acrosome reaction in bovine and porcine sperm, relatively little has been documented for equine sperm. Special attention will therefore be dedicated to recent attempts to develop and implement new assays for the detection of the capacitation status of live, acrosome-intact and motile equine sperm.