Sperm function in different human sperm subpopulations

Abstract

Human semen contains a heterogenous population of spermatozoa of which only a certain fraction is physiologically suitable for fertilization. Functional characteristics of these subpopulations and sperm separation techniques are not well understood. Furthermore, the questionable reliability of basic semen analyses has led to more emphasis being placed upon the assessment of detailed sperm functional parameters in determining male factor infertility as well as media supplements that could possibly enhance male fertility. Our study was designed to investigate and compare the functional characteristics of two sperm motility subpopulations and to determine which group of semen parameters provide predictive value into the presence of a high motility sperm subpopulation. In addition, the study aimed to investigate the effects of biological components present in follicular fluid on sperm functionality of different sperm motility subpopulations. Furthermore, it was aimed to determine whether the use of a new flagellar analysis and sperm tracking program (FAST) may provide more information on sperm motility functions as compared to computer-aided sperm analysis (CASA). Semen was separated into a high motile (HM) and low motile (LM) sperm subpopulation via double density gradient centrifugation. Subpopulations in human tubal fluid (HTF) were assessed for functional characteristics namely sperm viability, motility and kinematics, hyperactivation, acrosome reaction, reactive oxygen species and mitochondrial membrane potential, chromatin intactness and maturity. In the second phase, subpopulations were exposed to various media namely, HTF, capacitating medium (CAP), HD capacitating medium (HD-C), progesterone, myo-inositol, dopamine and prolactin, and subsequently subjected to the same functional tests as mentioned above. In the final phase of the study, subpopulations were exposed to selected concentrations of the media and individual spermatozoa further analysed with CASA and FAST for comparison of kinematic and flagellar characteristics. Two distinct sperm motility subpopulations were observed, with HM sperm subpopulations displaying significantly improved functionality compared to LM sperm subpopulations. Semen morphology abnormalities correlated with grouped motility parameters of the LM subpopulation. On the other hand, a combined group of semen total motility, progressive motility, viscosity and mucus penetration correlated with HM subpopulations’ grouped motility parameters. Media did improve sperm functionality, however progesterone and myo-inositol had the greatest effects on HM subpopulations, whereas dopamine and prolactin were more favourable to LM subpopulations. Furthermore, variable effects were observed for media on functional parameters. Collectively, total and progressive motility, viscosity and mucus penetration present a reliable group of semen characteristics for predicting the quality of a HM sperm subpopulation. Separating the same donor semen samples into two significantly varying motility sperm subpopulations can be a new approach to mimic the qualities of fertile and subfertile males’ sperm populations. Although motility subpopulations respond differently to various media, the study concludes that treatment of subfertile semen samples with biological substances present in follicular fluid could assist to develop new strategies for IVF treatment. Finally, FAST and SCA present with similar kinematic parameters, however flagellar parameters provide predictive value into rapid and medium progressive speed groups of individual spermatozoa.