Abstract.A factor responsible for progression to pregnancy establishment in the mare has not been definitively characterized. To identify factors possibly involved in the establishment of equine pregnancy, the endometrium was collected from day 13 (day 0=day of ovulation) cyclic and day 13, 19 and 25 pregnant animals. From initial subtractive hybridization studies, a calcium regulating factor, Stanniocalcin-1 (STC1) mRNA, was found as a candidate molecule expressed uniquely in the pregnant endometrium. Endometrial expression of STC1 mRNA was noted on day 19 and was markedly increased in the day 25 gravid endometrium. STC1 protein was found in the extracts of day 25 gravid endometrium and immunochemically localized in the uterine glands. In addition, STC1 protein was detected in uterine flushing media collected from day 25 pregnant mares. High concentrations of estradiol-17β (E2) were detected in day 25 conceptuses. E2 levels were much higher in the gravid endometrium than in other regions, whereas progesterone levels did not differ among the samples from different endometrial regions. Expression of STC1 mRNA, however, was not significantly upregulated in cultured endometrial explants treated with various concentrations of E2 (0.01-100 ng/ml) with or without 10 ng/ml progesterone. These results indicate that an increase in STC1 expression appears to coincide with capsule disappearance in the conceptus, and suggest that STC1 from the uterine glands likely plays a role in conceptus development during the pregnancy establishment period in the mare. Key words: Endometrium, Equine, Estradiol-17β, Stanniocalcin-1 (STC1), Uterine secretion (J. Reprod. Dev. 57: [203][204][205][206][207][208][209][210][211] 2011) he establishment of equine pregnancy is a unique and long process, during which a series of physical and possibly biochemical interactions are required between the conceptus and uterus. The equine conceptus first enters into the uterus on days 6-8 of pregnancy (day 0=day of ovulation) [1] and then begins to transmigrate throughout the entire uterine body and horns until approximately day 16 of pregnancy [2][3][4]. The frequency at which a conceptus moves from one uterine horn to another increases up to 10-20 times per day [5]. The conceptus ceases migration and lodges at the caudal portion of one of the uterine horns, known as "fixation", on approximately days 16-18 of pregnancy [2]. Between days 11 and 16 of pregnancy, the conceptus rapidly expands its volume, but keeps its spherical shape surrounded by a unique, smooth structure called a "capsule" [6,7] that emerges beneath the zona pellucida on days 6-8 of pregnancy. The capsule is an acellular mucin-like glycoprotein [8] that is structurally strong and is considered essential for the survival of the conceptus and continuation of pregnancy [9,10]. Together with peristaltic myometrial contractions, the capsule is likely to give motility to the conceptus, although the precise stimulus for conceptus migration is unknown. Thereafter, the capsule disappears by d...
