Understanding Sperm Heterogeneity: Biological and Practical Implications

Abstract

Contents Sperm are the most diverse cell type known. This diversity is thought to reflect adaptation to conditions under which sperm function as a way to ensure the survival of sperm in fertilization environments and to maximize fertilizing capacity thereof. The existence of morphological diversity among species is widely assumed, although this diversity seems less clear as we go deeper (between males, between ejaculates from the same male and even within the same ejaculate), with different theories addressing this heterogeneity. Moreover, the development of assisted reproductive techniques (ART) has led to changes in the physiological conditions in which sperm fertilize, which could lead, ultimately, to a selection towards more favourable sperm design. Regardless of the origin of this diversity, when studying the relationship between shape and function of sperm, it is advisable to assess the degree of heterogeneity of sperm and takes into account to be more likely to identify those morphological characteristics determining the fertile ability of sperm. Otherwise, these relationships could be hidden as a result of considering an average shape not representative of morphological characteristics of sperm. In addition, the knowledge of this morphological diversity in terms of changes arising from modifications in the sperm environment and mechanisms that generate these changes could be useful for understanding the reproductive capacity of males but also in enhancing their fertile ability.

Females discriminate against heterospecific sperm in a natural hybrid zone

“…Studies on sperm motility often average velocity values across sperm cells, but that practice is undesirable because it ignores variability in velocity within the ejaculate (Amann and Waberski 2014) and differences among males in the number of sperm cells tracked. Failure to account for within-ejaculate variability in sperm characteristics can result in failure to detect significant patterns (Ramón et al 2014). Particularly since different subpopulations of sperm cells can react differently to the female environment (e.g., in domestic pigs Sus domesticus, Satake et al 2006), it is important to account for within-ejaculate variability in sperm behavior.…”

“…Failure to account for within‐ejaculate variability in sperm characteristics can result in failure to detect significant patterns (Ramón et al. ). Particularly since different subpopulations of sperm cells can react differently to the female environment (e.g., in domestic pigs Sus domesticus , Satake et al.…”

Current status and potential of morphometric sperm analysis

“…However, the development of assisted reproductive techniques (ARTs) has introduced a new – artificial – source of variation in sperm morphology 55. The in vitro fertilization process includes stages outside the male and female reproductive tracts, during which spermatozoa are subjected to procedures aimed at maximizing reproductive success.…”

“…Regarding the accuracy of the evaluation of the sperm shape, CASA-Morph systems only provide an approximation of head shape on the basis of sperm head linear dimensions 26557392. Elliptic Fourier analysis is one method, based on the use of successive points located by a coordinate system that fits the cell perimeter to a Fourier function, and that can identify more features of morphological variation in spermatozoa than manual methods 140141.…”

Foreword to Sperm morphometrics today and tomorrow special issue in Asian Journal of Andrology

“…These questions may well be answered by looking into the study of sperm heterogeneity both within and between ejaculates and species. With the information we have now, it is no longer valid to consider the entire seminal sperm population as one population, represented in calculations by central tendency statistics; the problem requires analysis of the complex subpopulation structure 406061. Although reviews on the use of CASA for morphological analysis have been published,386263 this special issue of the Asian Journal of Andrology is devoted to sperm morphometry.…”