Is Semen Analysis Useful to Predict the Odds that the Sperm will Meet the Egg?

Abstract

Any mammalian spermatozoon that achieves successful in vivo fertilization has to perform almost perfectly in many disparate functions and overcome a series of obstacles imposed by the female reproductive tract. This implies that during formation in the testis and epididymis, the spermatozoa did not incur any morphological, metabolic, immunological or genetic abnormalities. Given that the spermatozoa are such highly differentiated cells, this means that every cellular compartment must not only be intact but must also respond appropriately to intracellular and extracellular signals. Assuming that a spermatozoon possesses this level of perfection, and is able to reach and penetrate an oocyte, it can only be regarded as 'fertile' if the DNA it carries is intact and able to sustain embryonic development. Although the proportion of inseminated spermatozoa that meet these criteria is vanishingly small, the female reproductive tract applies stringent selection criteria during sperm transport and, as a result, the probability of conception around the time of ovulation is very high. If laboratory tests of semen quality could approach the efficacy of the female reproductive tract, it would be possible to predict the odds of spermatozoa meeting the egg; however, this is not possible at present. In this article, I suggest a simple model to illustrate how a battery of laboratory tests could eventually be used to make these predictions.

Resolving variation in the reproductive tradeoff between sperm size and number

“…Our rationale is as follows. Vertebrate systems with internal fertilization typically have low sperm densities around the egg during fertilization, owing to reproductive tracts that are large relative to sperm size (26)(27)(28). Consequently, competitive advantages of increased sperm size in the region of the ESS are likely to be relatively weak across wide ranges of natural sperm densities and related mainly to effects of size on sperm motility and survival.…”

Competition drives cooperation among closely related sperm of deer mice

“…Sperm cooperation may benefit monogamous males if the increased swimming velocity of aggregated sperm allows them to migrate faster through a potentially hostile female tract3 or manoeuvre around obstacles while travelling to the fertilization site26. Consistent with these theories, in the wood mouse, Apodemus sylvaticus, >95% of sperm found in the uterine lumen following natural matings were aggregates, not single cells, in over half of the females tested1.…”

Haploid selection within a single ejaculate increases offspring fitness

“…In addition, our findings provide insights that are crucial for clinical and agricultural assistedfertilization techniques such as IVF and intracellular sperm injection (ICSI). These techniques omit many if not all naturally occurring steps of within-ejaculate sperm selection, and the consequences of such omission need to be understood (1,37). Future research therefore should focus on the consequences of gametic selection in a broad variety of taxa with both external and internal fertilization.…”