The Fertile Life of Spermatozoa

THE FERTILE LIFE OF SPERMATOZOA G. W. SALISBURY, R. G. HART, andJ. R. LODGE* The spermatozoon is the only mammalian cell designed by nature for a life independent of its parental tissue and capable of existence as a foreign body in another individual. This capacity reflects the pronounced intracellular division of labor of the male gamete which is unmatched by any other mammalian cell. From the motile tail, driven by the energy supplied by the metabolic machinery of the midpiece, to the acrosomal cap, each component part of the male gamete is designed to perform a precise function so as to achieve at fertilization a transfer of genetic information. The biological costs of this specialization are undoubtedly complex and multiple and converge in a manner which limits the extracorporeal life of the gamete. Its projectile-like form necessitated during spermatogenesis extrusion of a considerable part of the cell's mass. This reduced its intracellular store of molecules of high-potential energy yield and established a dependence of the cell upon its environment. The duration of spermatozoan survival solely on endogenous reserves clearly is inversely related to their rate of disappearance. Livability is further restricted by the inability of spermatozoa to replace for continuous activity the organic catalysts which become spent during metabolism. Presumably this impairment in protein anabolism is a result of an effective suppression of the haploid genome of the gamete by its own basic nucleoproteins. Either of these conditions, or for that matter a combination of them, severely limits the duration of spermatozoan viability. Moreover, spermatozoa possess few cytoplasmic enzymes, and the energy-releasing enzymes responsible for the work of the tail are located primarily in the midpiece mitochondria. Yet it is capable of a great though not a protracted burst of energy expenditure in completing its biological function , in which endeavor it is accompanied by innumerable competitors from the same source bent on the same mission. In mammals, these conditions have imposed upon spermatozoa in the female reproductive tract a fertile life which is brief and seldom exceeds 24-48 hours, though exceptions are reported for the ferret and the horse [I]. On the other hand, in some species there exist specialized anatomical regions of the *Department of Dairy Science, University of Illinois, Urbana, Illinois 61801. Perspectives in Biology and Medicine · Winter 1976 | 213 female reproductive tract possessing environments protracting sperm life [1-4]. The essential burden of the spermatozoon is of course its deoxyribonucleoprotein complex, the chromatin, in its haploid nucleus which constitutes the essential part of the nuclear bridge from one somatic generation to the next. The penetrance of the egg and the contribution of chromatin to the new zygote is the only function performed by the individual spermatozoon alone. The traverse of the excurrent ducts of the male, the metabolic response to the environmental changes during that passage, as well as spermatozoan behavior during ascent of the female reproductive tract and after arrival at the site for union with the female gamete are all spermatozoan population matters. All these the evolutionary process has perfected as selection procedures in the interest of species survival. We have come to the view that the overall strategies for optimizing species survival center not only on optimizing fertilization success but also on optimizing opportunities for generating the genetic adaptability the species must have to meet varying and changing environmental conditions . Such genetic experiments of adaptability must occur in the gene pool of the species at minimal biological cost. In a population of genes selected over time to optimize species survival, any random shift in the nature of those genes by mutation has a high probability of decreasing that optimized survival, and the change would thus be self-eliminating. For optimum biological economy these latter obligations must occur at a stage in the individual life cycle least expensive to perpetuation of the species. In our opinion gamete production and transfer is that stage, failure of fertilization, postponing pregnancy only one estrous cycle, being but a minimum cost to species survival as compared to zygote death later. For a species of long generation interval, like the cow, for example, involving several years and a gestation of about 9 months, this is...