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disadvantage of requiring a surgical procedure, although the degree of invasion is minimal (especially with nonscalpel vasectomy [NSV] with chemical or other methods of vas occlusion); however, reversal requires a more elaborate surgical procedure, with return of fertility dependent upon numerous unknown factors. Furthermore, vasectomy provides no protection against transmission of sexually transmitted infections.
Variations on the theme of vasectomy have been suggested over the years, for example, valves that regulate vasal fluid flow (Kuckuck et al. 1975), and interruption of control of the vas musculature (Amobi and Smith 1995). A promising approach has been developed in China where percutaneous intravasal injection into the vas lumen of a quick-curing polymer results in a plug that effectively blocks seminal flow (Zhao et al. 1992). Still, this approach also requires a period of alternative contraceptive use until azoospermia is achieved (as much as 12 months [Chen et al. 1992]), although reversal is a simple and apparently effective procedure of merely removing the plug and does not require elaborate microsurgery.
These mechanical methods rely on restricting movement of sperm into the female reproductive tract. An equally effective strategy could be to render sperm inactive prior to ejaculation or to produce azoospermia with drugs. In the following sections we provide an overview of research in male contraception and suggest strategies that might be employed to develop effective contraceptives for men. In order to provide some rationale for different strategies, we begin with a brief excursion into the structure and regulation of function in the male reproductive tract.
Overview of the Male Reproductive Tract
In developing contraceptives for men, it is obviously essential to take into account the dynamics of the male reproductive system, particularly spermatogenesis. Spermatogenesis is a precisely timed process whose regulation is a poorly understood, complex set of interactions among many cell types. Post-testicular sperm maturation is also highly complex, regulated as it is by circulating factors from the testis and from other organs, as well as by factors derived from intraluminal secretions of the testis itself. The intricacy of these processes is well recognized, but most of its details are unknown. The contraceptive strategy employed, therefore, will depend upon where in the male reproductive tract intervention occurs.
Sperm are produced in the seminiferous tubule of the testis; are then released into the lumen of the tubule as immature cells, unable to fertilize eggs; and are carried by bulk flow through the rete testis and efferent ductule into the single,