Uterine secretions in the cow and sheep.

but essential (Beier, 1974). Culture studies in vitro with single protein fractions as supplements to the medium, or with non-fractionated uterine secretion proteins, indicate that single fractions (as uteroglobin) are not regulating blastocyst development alone, but it is evident that a regulatory system of several components is acting here (Maurer & Beier, 1976). Blastocysts of delayed-secretory uteri do not expand and shown herniations of trophoblast cells after breaking of the zona pellucida (Beier, 1974). Identical pictures are obtained when blastocysts are developing in vitro without uterine proteins in their culture environment. Presumably blastocyst development, including radial expansion and dissolution of the mucin coat, does require the action of uterine secretion proteins in combination or in a cascade-like reaction chain, similar to the event leading to fibrin formation or fibrinolysis. With regard to its role within a system that is composed of proteinase-proteinaseinhibiting activities the analysed trypsin-inhibiting activity of uteroglobin should be elucidated, to shed more light on the biological significance of this challenging protein component.