Endometrial cup

Endometrial cups are unique, transient structures that form in the endometrium (the lining of the uterus) of pregnant horses. They are essential for the maintenance of pregnancy in the equine species, primarily because they produce the hormone equine chorionic gonadotropin (eCG), also known as equine chorionic gonadotrophin. This hormone plays a crucial role in stimulating the ovaries to produce progesterone, a hormone necessary for sustaining pregnancy.

Formation and Function

Endometrial cups begin to form around day 36 of pregnancy, when the embryo attaches to the uterine lining. They are derived from the trophoblast cells of the chorionic girdle, a structure that encircles the early equine embryo. By day 38 to 40, these trophoblast cells invade the endometrium, differentiating into endometrial cup cells that secrete eCG.

The primary function of eCG is to stimulate the development of secondary corpora lutea on the ovaries. These corpora lutea, in turn, produce progesterone, which maintains the uterine environment in a state conducive to the development of the fetus. The presence of eCG in the bloodstream also makes it a reliable marker for confirming pregnancy in horses.

Degradation

Endometrial cups have a finite lifespan and begin to degenerate after approximately 120 to 140 days of pregnancy. Their regression is thought to be due to an immune response by the mare, as the cups are partially of fetal origin and thus genetically distinct from the mare. Despite their degradation, the pregnancy can continue because by this stage, the placenta has developed sufficiently to take over the role of progesterone production.

Clinical Significance

The unique nature of endometrial cups and their role in equine pregnancy have implications for veterinary medicine and horse breeding. For example, abnormalities in the formation or function of the endometrial cups can lead to early pregnancy loss. Additionally, the measurement of eCG levels can be used for early pregnancy diagnosis and monitoring.

Understanding the biology of endometrial cups also has implications for reproductive technologies and interventions in horses, such as the use of eCG in controlled breeding programs to enhance fertility.

See Also

• Equine reproduction
• Hormones of pregnancy
• Trophoblast
• Corpus luteum

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