Versatile Action of Picomolar Gradients of Progesterone on Different Sperm Subpopulations

Abstract

High step concentrations of progesterone may stimulate various sperm physiological processes, such as priming and the acrosome reaction. However, approaching the egg, spermatozoa face increasing concentrations of the hormone, as it is secreted by the cumulus cells and then passively diffuses along the cumulus matrix and beyond. In this context, several questions arise: are spermatozoa sensitive to the steroid gradients as they undergo priming and the acrosome reaction? If so, what are the functional gradual concentrations of progesterone? Do spermatozoa in different physiological states respond differentially to steroid gradients? To answer these questions, spermatozoa were confronted with progesterone gradients generated by different hormone concentrations (1 pM to 100 µM). Brief exposure to a 10 pM progesterone gradient stimulated priming for the acrosome reaction in one sperm subpopulation, and simultaneously induced the acrosome reaction in a different sperm subpopulation. This effect was not observed in non-capacitated cells or when progesterone was homogeneously distributed. The results suggest a versatile role of the gradual distribution of very low doses of progesterone, which selectively stimulate the priming and the acrosome reaction in different sperm subpopulations.

Mouse sperm begin to undergo acrosomal exocytosis in the upper isthmus of the oviduct

“…However, It is well established that the sperm population is heterogeneous and, at any given time point, only a fraction of the cells are capacitated while others have completed this process. In the later case, the occurrence of exocytosis might be associated to a biological strategy to prevent them to fertilize the oocyte (Aitken and McLaughlin, 2007; Harper and Publicover, 2005; Uñates et al, 2014), possibly by phagocytosis (Oren-Benaroya et al, 2007). However, because there is currently no way to visually identify the status of sperm capacitation, this hypothesis requires further experimentation.…”

Chang's meaning of capacitation: A molecular perspective

“…Once released from the sperm reservoir, sperm still need to reach the site of fertilization, pass through a layer of cumulus cells (depending on the species), acrosome react, penetrate the zona pellucida, and fuse with the egg membrane. Chemotactic mechanisms are thought to be required during the final approach to the egg (Ralt et al, ); interestingly, only capacitated sperm appear to perform chemotaxis in vitro (Unates et al, ).…”

The CatSper channel modulates boar sperm motility during capacitation

“…Sperm capacitation is activated differently among species; i.e. via albumin and progesterone in human and mouse [10][11][12], heparin and other glycosaminoglycans in bull [13,14] or bicarbonate (HCO3 -) in pig [3,15]. Signaling in vivo is gradual and does not massively affect spermatozoa, but occurs as individually progressing spermatozoa encounter increasing concentrations of these substances in the female genital tract [4,16].…”