Spermatogenesis is a complex process that begins in the seminiferous tubules of the testes and continues throughout the male reproductive tract. However, sperm are not able to successfully fertilize an ovum until a short time before actually reaching it inside the uterine tubes of the female. Also, spermatogenesis and testicular function are closely regulated by hormones in the body, and producing viable sperm depends on the proper functioning of the three cells found within the testes.
In the hypothalamus, a hormone called gonadotropin releasing hormone (GnRH) is produced. As the name implies, the release of this hormone induces the release of gonadotropins from the adenohypophysis called follicle stimulating hormone (FSH) and luteinizing hormone (LH). These hormones are released into the bloodstream where they can then act upon their intended target cells. Specifically, the LH stimulates the production of the male sex hormone, testosterone, in the Leydig cells in the testes. The FSH stimulates spermatogenesis by the Sertoli cells in the testes. Under normal physiological conditions, testosterone and its analogues (estrogen, progesterone, dihidrotestosterone, etc.) negatively feedback on the adenohypophysis and hypothalamus to control the levels of GnRH, LH and FSH they produce, which subsequently controls the levels of hormones in the system.
Once sperm have been ejaculated into the female reproductive tract, they undergo a process to prepare them for fertilization. It is believed that they have inhibitory glycoproteins on their plasma membranes that are removed in the vaginal environment. In addition to this preparatory process, sperm have some very important structural and chemical attributes necessary to both reach and penetrate an ovum in the female. The outermost membrane, the plasma membrane, contains an enzyme called hyaluronidase, which is necessary to penetrate the layer of the ovum called the cumulus oophorous. The layer deep to the cumulous oophorous in the ovum, called the corona radiata, is penetrated by another enzyme in the sperm’s plasma membrane aptly called corona radiata penetrating enzyme. Once this layer of the ovum is broken down, the next deepest layer, called the zona pelucida, has on it a ZP3 receptor. The sperm has an interactive ligand on its plasma membrane that binds to the ZP3 receptor, which induces it to release two enzymes (acrosin and neuramidase) that penetrate the zona pelucida. The final layer of the ovum to be penetrated is the vitelline membrane. It has ZP2 receptors that the sperm interacts with, which induces a fenestration to appear through which the sperm can finally pass its genetic material to combine with that of the ovum.
Male contraception can be accomplished in a number of ways by taking advantage of one or more of the many processes and structures that are necessary for a sperm to be viable.