A new procedure for rapidly scoring acrosome reactions of human sperm

Abstract Because the acrosome of human sperm is too small to be directly visualized by phase‐contrast microscopy, acrosome reactions (that is loss of the acrosome) are generally not evaluated in studies of human sperm capacitation and fertilization. Nevertheless, it would be useful in such studies to have a technique for easily identifying and quantitating acrosome‐reacted sperm. In this paper, we describe a method for labeling the human sperm acrosome with fluorescein‐conjugated Ricinus communis agglutinin‐60 (FITC‐RCA); we show that in sperm without acrosomal caps, FITC‐RCA labeling occurs either not at all or only in the equatorial segment of the acrosome. To determine if the absence of FITC‐RCA labeling in the acrosomal cap region gives a reliable estimate of acrosome reactions, washed sperm or sperm incubated in a capacitating medium (BWW) were divided into two groups, which were then fixed for FITC‐RCA labeling or transmission electron microscopy. Counts of acrosome reactions made by each method were similar, and we observed an increase in the percentage of reactions following incubation in BWW. We conclude that the FITC‐TCA labeling technique is a reliable method for accurately scoring the percentage of acrosome‐reacted human sperm.