Abstract
Early equine pregnancy shares many features with that of more intensively assessed domestic animals species, but there are also characteristic differences. Some of those are poorly understood. Descent of the equine conceptus into the uterine lumen occurs at day 5 to 6 after ovulation but is only possible when the embryo secretes prostaglandin E2. Although maintenance of equine pregnancy probably involves secretion of a conceptus derived anti-luteolytic factor, this agent has not been identified. Rapid growth, conceptus mobility and presence of an acellular capsule at the time of maternal recognition of pregnancy, i.e. between days 12 and 14, are prerequisites to avoid pregnancy loss. Progesterone together with 5α-pregnanes is secreted by the corpus luteum and induces the production of endometrial histotroph which is responsible for conceptus nutrition until placention. A stable contact between the outer trophoblast layer of the allantochorion and the luminal epithelium of the endometrium is not established before days 40 to 42 of pregnancy.
Highlights
In mammals, maintenance of pregnancy depends on the continuous production of progesterone
The rule is that extension of corpus luteum lifespan beyond the length of one physiological estrous cycle requires either a luteotrophic or an anti-luteolytic factor which is produced by the conceptus
In contrast to other domestic animals, some horse-specific limitations challenge the research on early pregnancy: access to experimental animals or genital organs is mostly limited in a species where meat consumption is not common in many countries and slaughterhouse material is almost not readily available
Summary
Maintenance of pregnancy depends on the continuous production of progesterone. Presumed by many authors [1,2,3,4,5] that maintenance of pregnancy in the horse will involve secretion of an anti-luteolytic factor by the conceptus, this agent has so far not been identified. Regulation of endometrial expression of COX2 is considered a key event in either induction of luteolysis or maternal recognition of pregnancy in the horse [12, 13].
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