Mammalian sperm are not able to fertilize eggs immediately after ejaculation. They acquire fertilization capacity after residing in the female tract for a finite period of time that varies depending on the species. In 1951, Chang (1951) and Austin (1951) independently demonstrated that such a period of time in the female tract is required for the sperm to acquire their fertilizing capacity. Both authors observed that freshly obtained rabbit sperm introduced into the Fallopian tubes shortly after ovulation were not able to penetrate the eggs; instead if sperm were introduced a few hours before ovulation, the majority of the eggs were later observed to be fertilized. This observation led them to conclude that freshly ejaculated sperm are incapable of penetrating the zona pellucida immediately, and that sperm must remain within the female tract for a period before they are able to penetrate the eggs. Following these original observations, many studies confirmed that the environment of the female tract induces a series of physiological changes in the sperm; these changes are collectively called 'capacitation' Inherent to these first observations was that capacitation state became defined using fertilization as end-point. However, a variety of evidences suggest that the functional changes occurring in the sperm during capacitation are not one event, but a combination of concomitant processes; mainly, the sperm acquisition of the ability to undergo an agonist (e.g. zona pellucida, progesterone) induced acrosome reaction and the modification in the motility pattern known as hyperactivated motility (both enabling efficient zona drilling so that the sperm can reach the oolemma).
Although more than 50 years have passed since sperm capacitation was first reported and conditions for in vitro capacitation has been established in a variety of mammalian species, it is noteworthy that the molecular basis of this process is still today not well understood. Nevertheless, recent work is beginning to point to a unified model of how this event is controlled at the molecular level. To dissect the molecular mechanisms involved in sperm capacitation, most authors have used in vitro capacitation systems incubating the sperm in chemically defined buffers that mimic the glucose and electrolyte content of the oviduct and are enriched with serum albumin components. It is important though to keep in mind that capacita-tion occurs in the female tract and sooner or later, in vitro capacitation models will need to be validated in vivo taking into consideration the physiology of the female track that is under hormonal control. The purposes of this chapter are to consider some recent contributions towards our understanding of capacitation, to summarize open questions in this field, and to discuss future avenues of research. Reviews by Baldi et al. (2000), Cohen-Dayag and Eisenbach (1994), de Lamirande et al. (1997), Flesch and Gadella (2000), Florman and Babcock (1991), Harrison (1996), Visconti and Kopf (1998), Visconti et al. (1998), and Yanagimachi (1994) provide supplementary reading and complement the material of this chapter.
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