Endometrial cups

Keywords: equine, pregnancy, eCG, cups

Endometrial cups, eCG and infertility

At about 28 to 35 days of gestation, cells from the embryo invade the endometrium, forming endometrial cups. It is not clear why they were named as such, perhaps because they have a cupped appearance when cut in cross-section. These structures are a source of equine chorionic gonadotropin (eCG) and are shown here, adjacent to a healthy embryo at about 38 days of gestation.  The cups are indicated by green arrows.

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As shown elsewhere, endometrial cups are associated with secondary (accessory) ovulations during pregnancy but the exact relationship between endometrial cups and secondary corpora lutea remains an enigma.

Endometrial cups are rejected by the immune system of the dam at variable stages of pregnancy but usually eCG concentrations reach baseline by about 120 to 150 days of gestation. In unusual cases, they can persist throughout pregnancy.

Endometrial cup formation

Keywords: equine, mare, eCG, endometrial, cups, uterus, abortion

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Endometrial cup formation at about 45 days of gestation. The fetus has been removed from its placenta and placed on the endometrial cups as a source of reference.

A section of the endometrial cup/s is indicated here by the green arrows. In this authors opinion, the term "cup" is misleading because these structures are only cupped in cross section but otherwise bear no resemblance to cups. In fact, as is seen here, the cups are often elongated and some cups coalesce with others. They are irregular in shape and vary greatly in size within a single pregnancy.

The cups are actually of fetal origin, producing eCG, formally called pregnant mare serum gonadotropin or PMSG. Equine chorionic gonadotropin production usually starts at about 28 to 35 days of gestation and continues to about 120 days of gestation, sometimes longer and on rare occasions, throughout pregnancy.

The function of eCG is an enigma (2013) because it is associated with multiple ovulation during pregnancy yet cannot induce superovulation in non-pregnant mares. Also, as seen here, most mares have multiple ovulations, forming accessory corpora lutea while eCG is being produced. Yet, a minority have normal pregnancies with no accessory corpora lutea.

A further intriguing finding is that the production of eCG persists after abortion and in most cases usually causes prolonged anestrus. There is no easy and effective method for shortening the life of endometrial cups once they have formed therefore it is advisable to perform therapeutic abortions before day 28, when endometrial cups begin to form. However this situation requires clarification because recent reports on polo ponies challenge these assertions. In polo ponies male pregnancies are often aborted after fetal sexing at about 60 days yet many of those mares have estrous cycles after abortion.

Selected reference:

Estrade, M.J. et al. 2016 eCG Concentration and subsequent reproductive activity in mares after abortion at day 70. J.Equine Vet.Sci. 42:88-93