Phagocytosis of boar spermatozoa in vitro and in vivo.

For successful conception, fertilization-competent spermatozoa must be present at the site of fertilization in adequate numbers until ovulation has taken place. In pigs, a large volume of semen is delivered into the uterus. Most, if not all, of the inseminated liquid is voided from the vulva within a few hours after insemination and approximately 45% of the spermatozoa are lost. Large numbers of spermatozoa are also lost due to phagocytosis by polymorphonuclear leukocytes (PMNs). In pigs, the recruitment of PMNs to the uterine lumen appears to be triggered by insemination of a volume of liquid, rather than by specific components of that liquid or by spermatozoa or seminal plasma. However, persistence of large numbers of PMNs in the uterine lumen at > 12 h after insemination appears to depend on the presence of spermatozoa in the inseminate. In vitro studies have indicated that damaged, killed or capacitated spermatozoa are not phagocytosed preferentially, but that capacitation treatment strongly reduced phagocytosis of spermatozoa. Recent studies have also shown that PMN recruitment and phagocytosis of spermatozoa in vivo can be reduced by addition of caffeine plus CaCl2 to the inseminate, which appeared to have positive consequences for the longer term availability of spermatozoa at the site of fertilization.