THE EPIDIDYMIS AND SPERM MATURATION - A PERSPECTIVE

In common mammals, sperm leaving the testis are incapable of fertilizi ng a female gamete. Sperm have limited biosynthetic capability and nee d to minimize demand for ATP. Hence, modification of sperm to achieve their maturation requires pre-programmed cleavage of integral molecule s (planned self-modification) and remodelling by action of molecules f ound in the suspending fluids. Most of these biocatalysts are secreted by a series of specialized regions in the epididymal epithelium, but some are provided in seminal plasma. The role of the epididymis in spe rm maturation is postulated to be 'setting a series of triggers' each capable of initiating cellular changes either at emission or near or i n the oocyte, and 'setting a safety' for each trigger to prevent prema ture occurrence of the event. The attributes required in a spermatozoo n for in vitro fertilization and natural mating are different, and the ir expression is dependent on the site of sperm sampling. Some attribu tes needed for fertility are probably like an on-off switch, whereas o thers probably allow a gradually reduced probability of success before going to the off position (analogous to a conventional light switch a nd a dimmer-type light switch). All essential attributes of a spermato zoon must be expressed in a 'combined effective amount' for that cell to be fertile. Because of mixing, in any segment of the epididymal duc t the population of sperm is heterogeneous in age and biological statu s. Thus, when assessing sperm maturation it is necessary to establish the proportion of sperm that has completed and retained all steps of m aturation necessary to achieve fertilization of oocytes under the cond itions imposed. In a normal animal, most sperm leaving the epididymis have a 'combined effective amount' of attributes, and the population h as a high fertilizing potential.