Exposure of actin on the surface of the human sperm head during in vitro culture relates to sperm morphology, capacitation and zona binding

Abstract

The aim of this study was to determine the relationship between the proportion of motile sperm with actin exposed on the surface of the head and sperm function. Semen samples were obtained from normozoospermic men and sperm function tests were performed. Motile sperm selected by swim-up were incubated with actin monoclonal antibody (A-mAb, 1:100) for 2 h, then anti-mouse IgG Dynabeads were used to detect sperm-bound A-mAb. Sperm capacitation was increased by phorbol myristate acetate (PMA) and decreased by bicarbonate-free medium. The proportion of sperm with exposed actin increased with time for up to 2 h incubation. Bicarbonate-free medium significantly decreased the proportion of sperm with exposed actin. PMA significantly enhanced this phenomenon. Sperm bound to zona pellucida (ZP) had a significantly higher proportion with exposed actin than did sperm remaining in medium. Of the 79 samples studied, an average of 9.4% (range 1-27%) of motile sperm had exposed actin after 2 h incubation and this was significantly correlated with sperm normal morphology and ZP binding. Exposure of actin on the surface of the sperm head during in vitro culture may be related to membrane modification during sperm capacitation and hence may be a useful marker for this subpopulation of sperm.