Expression and localization of osteopontin in the equine endometrium and conceptus during the cycle and peri-implantation period
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In horses early pregnancy loss between days 15 and 40 is a big problem. Equine pregnancy is characterized by an unusually long peri-implantation period. Implantation and placentation starts at 40 – 45 days after ovulation. During the peri-implantation period the conceptus is depending on uterine secretions for nutrient and gas exchange. Several researches show the great importance of the presence of osteopontin (OPN) within the uterus during the peri-implantation period to maintain successful pregnancy in sheep, pigs, rabbits and humans. The present research focuses on the expression and localization of OPN in the equine endometrium and conceptus during the peri-implantation period. The endometrial samples were obtained from non-pregnant uteri on Day 7, 14 or 21 (estrus) of the estrus cycle and from the pregnant uteri on Day 7, 14, 21 or 28 of gestation (n = 2 samples/day). The conceptus samples were obtained on Day 14, 21 or 28 of gestation (n = 2 samples/day). For the Day 14 and 21 concepti only the yolk sac was used for the present research; for Day 28 concepti both the yolk sac and the allantochorion was used. The endometrial and conceptus samples were stained by using immunohistochemistry and examined with light microscopy. The present results show that OPN is present in all glandular epithelium (GE) and luminal epithelium (LE) cells and in the glandular secretum during the estrus cycle and early pregnancy. In the pregnant equine uteri OPN increases during the gestation period from Day 21 and is maximum on Day 28. OPN is also present in the conceptus trophectoderm during the whole peri-implantation period. The most OPN is present in the allantochorion on Day 28 of gestation. On Day 21 the embryonic capsule disappears. These results suggest that possibly the disappearance of the embryonic capsule has something to do with the ability to increase the OPN expression. OPN is highly expressed in the peri-implantation period from Day 21 and therefore there is a good possibility that OPN involvement is important to successful maintenance of equine pregnancy.