Enhanced Binding of Zona Pellucida Proteins to the Acrosomal Region of Intact Boar Spermatozoa in Response to Fertilizing Conditions: A Flow Cytometric Study1

In this investigation we sought to determine whether sperm capacitation in vitro is accompanied by changes in the functional presence of zona binding sites on the plasma membrane of boar spermatozoa. During sperm incubation at 39 degrees C in various modifications of a Tyrode's-based in vitro fertilization medium, the zona binding ability of individual spermatozoa was assessed with fluorescein-conjugated solubilized zona pellucida proteins, using a flow cytometer. Propidium iodide was routinely included to allow simultaneous assessment of membrane integrity; rhodamine-conjugated peanut agglutinin was used to assess acrosomal status. During incubation in the fertilization medium, a subpopulation of live acrosome-intact spermatozoa developed enhanced binding of the fluorescein-conjugated solubilized zona proteins. Microscopy revealed that the increase in cytometrically detected zona binding was paralleled by an increase in the area on the sperm head to which zona proteins bound, from the apical region to the whole of the acrosomal region. The changes were accelerated by phosphodiesterase inhibitors, were attenuated by omission of bicarbonate, and were completely inhibited by addition of EGTA. In the fertilization medium, numbers of sperm showing enhanced zona binding maximized after 60-90 min. This time course is somewhat similar to that reported by others for development of egg-penetrating ability in vitro. We suggest that the observed changes in zona binding ability bring about optimal sperm-egg attachment; they may also relate to induction of the acrosome reaction by zona pellucida components. In consequence, the zona binding changes may be an important part of the process by which the sperm acquires fertilizing ability as a result of capacitation.