Seminal plasma effects on sperm handling and female fertility.

The components of ruminant seminal plasma and their influence on the fertility of spermatozoa are reviewed. Seminal plasma can both inhibit and stimulate sperm function and fertility through the multifunctional actions of organic and inorganic components. These effects are now better understood because the composition of the seminal plasma, including its protein content and that of other structures, specifically membrane vesicles, has been clarified. Spermatozoa gain motility and fertilizing capacity as they transit the epididymis under the influence of factors produced by that organ. At ejaculation, inhibitory (termed "decapacitation") factors, sourced from the accessory sex glands, bind to the sperm surface. The major proteins isolated and characterised in ram seminal plasma, whose specific functions are yet to be determined, originate from the vesicular gland and comprise a spermadhesin together with proteins with fibronectin-II domains. In vitro handling of spermatozoa in preparation for artificial insemination (AI), involving processes such as dilution, cooling, freezing, re-warming and sperm sexing by flow cytometric sorting, can remove seminal plasma and may modify the proteins bound to the sperm surface. This destabilises the membranes and may pre-capacitate the spermatozoa, shortening their fertilizing lifespan. These changes may be reversible by seminal plasma fractions but responses differ depending on the type of sperm pre-treatment. Fertility after AI of ruminant semen may be improved if the role of seminal plasma proteins and their effect, if added individually or in combination to spermatozoa at different stages of preservation, or other manipulations such as flow cytometric sorting, can be determined.