The functions of uterine secretions.

The likely functions of uterine secretions, often termed histotroph, in the nurture of the early conceptus are reviewed. Particular emphasis has been placed on the pig in which the uterus synthesizes and secretes large amounts of protein in response to progesterone. In this species, which possesses a non-invasive, diffuse type of epitheliochorial placentation, the secretions provide a sustained embryotrophic environment which is distinct from that of serum. A group of basic proteins dominates these uterine secretions after Day 11 of pregnancy and its best characterized member is uteroferrin, an iron-containing acid phosphatase with a deep purple colour. Evidence has accumulated to suggest that uteroferrin, rather than functioning as an acid phosphatase, is involved in transporting iron to the conceptus. Three basic polypeptides which are found noncovalently associated with uteroferrin have been shown to be antigenically closely related to one another and to have arisen by post-translational processing from a common precursor molecule. Their function is unknown. A group of basic protease inhibitors has been identified which bear considerable sequence homology to bovine pancreatic trypsin inhibitor (aprotinin) and may control intrauterine proteolytic events initiated by the conceptuses. The last basic protein so far characterized is lysozyme which is presumed to have an antibacterial role. Finally, two low molecular weight (Mr approximately 18,000) acidic polypeptides have been purified and have sequence homology to a plasma retinol binding protein. Like uteroferrin, these proteins may be responsible for transport of an essential nutrient to the conceptus.