Theriogenology question of the month. Endometrial cups.

This research paper discusses a study involving a 20-year-old thoroughbred mare that developed endometrial cups, abnormal cell structures that originate from fetal cells, despite a confirmed lack of pregnancy, prompting a detailed examination of these structures and their implications on the mare’s reproductive functions.

Endometrial Cups: Definition and Function

• Endometrial cups form from a subtype of highly invasive trophoblast cells known as fetal chorionic girdle cells, budding around day 35 of a pregnancy.
• These cells infiltrate and engulf the maternal endometrial epithelium forming irregular plaques or tissue formations with the umbilical cord located centrally. This invasive process does not destroy the underlying endometrial glands.
• Endometrial cups, characterized by their secretory structures that mimic apocrine gland-like cells, produce equine chorionic gonadotropin (ECG). The production of ECG parallels detectable blood concentrations, peaking between 50 to 80 days of gestation before gradually decreasing. After 120 to 150 days, ECG becomes untraceable in blood samples.
• The cups function in maintaining the primary corpus luteum and secondary corpora lutea during gestation, essential structures for hormonal regulation and/or production during pregnancy.

The Effect of Pregnancy Loss

• When mares lose a pregnancy post the formation of endometrial cups, the cups continue functioning, preventing the resumption of regular estrous cyclicity until the cups cease function.
• Though fertility of such ovulations has not been documented, anovulatory estrus with significant follicular development may occur before endometrial cups stop functioning. Endometrial cups substantially reduce chances of a mare conceiving during the same breeding season.

Efforts to Reimpregnate

• Various methods have been tested to attempt reimpregnation during the same breeding season. For example, electrosurgical removal of endometrial cups at 50 days of gestation resulted in mares becoming pregnant within 41 days after removal.
• Cryotherapy and use of irritating solutions like 0.5% iodine or tetracycline in propylene glycol have also been attempted to decrease endometrial cup function, but their effects on future fertility have not been extensively studied.
• The paper also mentions that prostaglandin F2α does not impact the function of endometrial cups, and it is therefore challenging to induce abortion in mares during the period when the endometrial cups are functional. This is primarily attributed to the protective effect of ECG or other accompanying factors on the accessory corpora lutea of pregnancy.

Endometrial Cups: Lifecycle and Regression

• Once developed, endometrial cups stay functional for around 90 to 120 days, and their functional duration does not depend on fetal viability.
• The regression of endometrial cups is caused by a maternal leukocyte response and correlates with the development stage of endometrial cup trophoblasts. The regression of these cells is mediated by a non-MHC maternal leukocyte response via cytotoxic cells.
• Although endometrial cups are usually eradicated after around 150 days of gestation, their remnants and scars often remain visible in the placenta.