Impact of the corpus luteum on survival of the developing embryo and early pregnancy in mares.

It has been more than a hundred years that studies aiming to elucidate the processes involved in cyclicity and pregnancy pointed out the requirement of ovaries and corpora lutea for embryo survival and pregnancy establishment. For horses, luteal progesterone is essential for pregnancy only during the first trimester. This progestational support is complex among domestic animals as ovarian luteal function is further enhanced by the LH-action role of equine chorionic gonadotropin (eCG) starting ∼ on Day 35 of pregnancy. Increased eCG secretion leads to the formation of supplementary corpora lutea resulting from follicles that luteinize (accessory corpora lutea) or ovulate (secondary corpora lutea), thus increasing concentrations of blood progesterone. Physiological details of progesterone-driven embryo-maternal interactions continue to be elucidated. In recent years, researchers studying the transcriptomes and secretomes of uterine tubes, endometrium and early embryo provided insight into the composition of molecular and cellular events that enable embryo survival and remodeling of the endometrium before a functional placenta is formed. Aluteal pregnancy models have also shown that while fertilization and early embryo development until the early blastocyst stage can occur under a progesterone-deprived environment, dysregulation of important pregnancy-related genes occur; embryo development is compromised unless progestin supplementation is provided once the embryo arrives into the uterus. As the body of knowledge on embryo-maternal interactions in the horse continues to grow, a fact remains true: luteal support is essential for embryo survival mainly at the uterine stage, driving directly or indirectly gene expression that promotes adequate embryo-maternal physiological interactions until a full competent placenta is formed, resulting in optimal chances of delivering a live foal at term.