FIG. 11. Boar sperm acquire the ability to bind zona pellucida proteins during epididymal maturation. Sperm from the caput, corpus, and cauda regions of the boar epididymis were capacitated and incubated with Alexa-zona proteins and propidium iodide. (A) Most caput sperm do not bind zona proteins. Those that do display a very faint signal. The intensely labeled sperm is labeled with propidium iodide. In contrast, most corpus and cauda sperm exhibit intense zona protein binding. (B) Quantitation of live, acrosome-intact sperm displaying zona fluorescence. Approximately 93% of cauda sperm and 75% of corpus sperm bound zona proteins. In contrast, only about 6.4% of caput sperm bind zona proteins after extensive washing. Ninety-five percent of cauda sperm incubated for 4 hours in caput fluid (+cf) retained the ability to bind zona proteins. [Adapted with permission from Burkin HR, Miller DJ 2000 Zona pellucida protein binding ability of porcine sperm during epididymal maturation and the acrosome reaction. Dev Biol 222:99–109. Copyright Academic Press.]