Microfilaments appear in boar spermatozoa during capacitation in vitro

Boar spermatozoa were incubated in a capacitation medium and examined for the presence of filamentous actin by using the fluorescent probe NBD-phallacidin. F-actin was not observed in uncapacitated sperm, but developed in most regions of the cell during the capacitation period. Fluorescent staining was most intense in the flagellum. When fresh seminal plasma was added to capacitated sperm and the sperm was further incubated, F-actin was no longer observed. In view of previous experiments which indicated that plasma membrane proteins (PMPs), including a major integral PMP, move out of the sperm head into the flagellum during capacitation and that this movement is inhibited by the microfilament poison cytochalasin D (Peterson, Saxena, Saxena, and Russell: Biol. Reprod., in press, '86), we suggest that actin-PMP interactions play a major role in capacitating boar spermatozoa.