Communication requested: Boar semen transport through the uterus and possible consequences for insemination.

ABSTRACT

Recent insemination techniques bypass the interactions between sperm and the uterine wall because the semen is deposited deep into the tip of uterine horn or directly into the oviduct. Such techniques allow high dilution of the ejaculates. After normal mating, semen entering the uterus communicates with the uterine milieu. Intact sperm of high mitochondrial membrane potential bind to uterine epithelial cells, whereas most of the unbound sperm in the uterine lumen have damaged membranes. Lectins are the most likely factors to mediate these sperm-uterine interactions. The lectin wheat germ agglutinin is known to induce the strongest binding of sperm, whereas binding is impaired when sialic acid receptors are blocked by wheat germ agglutinin. This suggests that sialic acid is involved in porcine sperm-endometrium interactions, and it is hypothesized that the use of a semen extender supplemented with sialidase would allow insemination with reduced sperm numbers. A lack of contact of sperm and seminal plasma with the uterine wall, as a result of deep insemination, may adversely affect (1) events during ovulation, (2) induction of immunologic tolerance against paternal antigens, (3) preparation of the endometrium for implantation and placentation, and (4) immunologic support required for the fetus during pregnancy. Seminal plasma is known to signal post-insemination changes in the uterine endometrium involving the redistribution of leukocytes. This may involve migration of leukocytes from the uterine wall to the ovary, as seminal plasma particularly increases the appearance of the major histocompatibility complex class II-positive cells. Uterine epithelial cells respond to sperm binding by the production of pro- or anti-inflammatory cytokines. These cytokines may include synchronizing substances, transferred through a counter-current pathway to the ipsilateral ovary, thereby accelerating the final maturation of preovulatory follicles and advancing time of ovulation. In several species, an ovulation-inducing factor exists in seminal plasma, first identified as ß-nerve growth factor in camelid semen, indicating another pathway that influences the hypothalamic-pituitary-gonadal axis. In summary, low-dose inseminations may not necessarily require semen deposition deep into the uterine horn, as binding inhibitors can circumvent the binding of sperm to the uterine wall. However, subsequent immune-relevant events that control ovulation and prepare the uterine milieu for the developing embryo should be taken into account.