Boar spermatozoa in the oviduct.

In the pig, a functional tubal sperm reservoir (SR) is established before ovulation to ensure availability of suitable numbers of viable spermatozoa for fertilization. The boar's large ejaculate is split: most spermatozoa are delivered in a sperm-rich fraction (SRF) followed by a post-SRF fraction containing increasing amounts of the spermadhesin PSP-I/PSP-II-rich seminal vesicle secretion. This heterodimer acts as leukocyte chemoattractant both in vitro and in vivo, contributing to the phagocytosis of those spermatozoa not reaching the SR. Sequential ejaculate deposition of marked spermatozoa and SR screening showed that most spermatozoa in the SR arose from the fortuitous PSP-poor, first portion of the SRF fraction, escaping phagocytosis and replenishing the SR within 2-3 h. The SR-sperm numbers diminish gradually in relation to ovulation, spermatozoa being continuously redistributed toward the upper isthmus. In vitro, only uncapacitated spermatozoa bind to epithelial explants, suggesting that the SR influences sperm capacitation. In vivo, most viable spermatozoa--usually harbored in the deep furrows in the pre- or peri-ovulatory SR during spontaneous standing estrus--are uncapacitated, but capacitation significantly increases after ovulation. Pre-/peri-ovulatory SR spermatozoa promptly capacitate in vitro when exposed to the effector bicarbonate, an influence that can be reversed by co-incubation with SR fluid or its component hyaluronan. Fluid collected from the ampullar segment (rich in bicarbonate) induces capacitation in vitro. In conclusion, the lack of massive sperm capacitation in the SR and the diverse individual response to capacitation shown by tubal spermatozoa would relate both to the insurance of full sperm viability before ovulation and the presence of spermatozoa at different stages of capacitation in the upper oviduct, thus maximizing the chances of normal fertilization.