Sperm Maturation
Sperm Maturation
Deposited sperm in the female reproductive system are initially unable to fertilize the oocyte. They have to undergo various maturation phases that enable them to fertilize the oocyte. Sperm undergo motility changes and physiological changes as maturation phases. As ovulation approaches, a proportion of the ‘stored’ sperm is continuously distributed towards the ampulla and individually activated leading to step-wise capacitation and the attainment of hyperactivated motility. Hyperactivation and Capacitation are important sperm maturation phases that are essential for sperm transport to the correct site of fertilization.
Types of Sperm Motility
Like other mammalians, human sperm display two types of motility; activated motility as seen in freshly ejaculated sperm and hyperactivated motility as seen in mature, capacitated and ready to fertilize sperm. The flagellum of an activated generates a symmetrical, lower amplitude wave motion that drives sperm in a linear direction. Activated motility aids sperm transport in the female genital tract. Most sperms that lack activated motility fail to reach the uterotubal junction and therefore incapable of in vivo fertilization.
Hyperactivated Motility
In hyperactivated motility, the pattern of flagella beat changes to that of a more asymmetrical and vigorously higher amplitude. Sperm acquire hyperactivated motility when the reach the oviductal reservoir. Unlike activated motility, hyperactivated sperm swim with vigorous and ‘whip like’ motions. This provides the sperm with the force needed to penetrate the protective coat of the oocyte membrane.
Capacitation
Sperm originating from the epididymis and the seminal plasma are coated with substances when they enter the oviduct. Capacitation is a series of transformations that sperm undergo in the female reproductive tract were they loose their protective coat enabling them to travel to the correct site of fertilization, bind to the outer membrane of the oocyte, undergo the acrosome reaction and fertilize the oocyte. Sperm capacitation is a gradual, event and prerequisite for fertilization. Capacited sperm acquire hyperactivated motility. Once sperm loose teir protective coats during capacitation, hyperactivation of sperm enhances the detachment of sperm from epithelial lining of the oviduct and enables sperm to progress to the site of fertilization. Thus capacitation and hyperactivation are complementary processes.
