Membrane remodelling during sperm maturation in the epididymis.
Identification of molecules or factors on spermatozoa that are involved in the recognition and binding to the zona pellucida of the egg is one of the central problems in current research on mammalian fertilization. This information is important, not just from the standpoint of scientific interest, but also for treatment of male infertility and design of future contraceptive technology. One of the most informative approaches to solving this puzzle has been to study the maturation of spermatozoa in the epididymis. The gradual development of motility and zona-binding capacity by spermatozoa after they leave the testis affords an opportunity to investigate what distinguishes a fertile from an infertile spermatozoon and what mechanisms are involved in the transformation from one state to the other. This chapter has reviewed some of the current information on sperm maturation in the epididymis and attempted to correlate it with concepts emerging from in vitro fertilization experiments. Emphasis has been placed on mechanistic aspects of maturation changes to the sperm plasma membrane, not only because in many species it is the surface membrane that encounters the zona and it is therefore likely to be the site of potential recognition molecules, but also because the plasma membrane influences many intracellular events and has a bearing on the maturation of the nucleus and axoneme as well. An example of the latter is shown by the induction of motility in demembranated testicular sperm by the addition of ATP and Mg2+ (Mohri and Yanagimachi 1980; White and Voglmayr 1986). This suggests that the motor apparatus for motility is essentially present in immature spermatozoa and that one of the requirements for it to become fully operative are subtle alterations to the permeability properties of the plasma membrane. A similar mechanism may also be true of the maturing sperm nucleus to which only passing reference has been made here. Very little is known about this aspect of sperm maturation beyond the increasing stabilization of the nucleoprotein by formation of disulphide bonds (Bedford 1975). The fact that isolated heads from immature spermatozoa fail to form pronuclei if injected directly into the egg (Uehara and Yanagimachi 1977) may explain, in part, the high embryonic mortality observed when not fully mature spermatozoa are induced to fertilize eggs (Orgebin-Crist et al. 1975). Thus maturation events within the nucleus are just as important as those on the surface membrane.(ABSTRACT TRUNCATED AT 400 WORDS)
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