Abortion, eCG and infertility
Keywords: abortion, eCG, infertileAn aborting equine pregnancy.
Image size: 1200 x 835px
This old but unique image shows a pregnancy in the process of aborting at approximately 55 days of gestation, midway through the breeding season.
There is considerable physiological and practical importance attached to this situation. The primary corpus luteum (black arrow) and secondary corpora lutea (in the contralateral ovary and not visible here) had undergone luteolysis. Yet, if this mare had not died, it is unlikely that she would have begun to have estrous cycles again during that breeding season.This is because the endometrial cups (yellow arrows) persist, even after the death of the fetus. The patho-physiological reason for this is unknown and somewhat mysterious. The mystery lies in the fact that the advent of endometrial cups at about 28 to 35 days of gestation is marked by eCG production by the cups and ovulation of follicles; hardly inactive events. However, those same endometrial cups and eCG are associated with ovarian inactivity or unpredictable follicle growth and failure of ovulation if the fetus should die. In most cases therefore, if an equine fetus dies after 35 to 40 days of gestation, the mare cannot be re-bred during the same season. In a minority of mares however, ovarian activity may begin again and a few of these mares will even ovulate and even conceive.
Further mystery is engendered by publications from Argentina, where male pregnancies are aborted after sexing because females are more desirable as polo ponies. Because sexing of the equine fetus is routinely performed at about 60 days of gestation, endometrial cups should be well-formed and functional in these mares. Yet, there are reports of significant numbers returning for breeding in the same breeding season. The reasons for this discrepancy in world-wide findings is not known.
Because cells from the embryo invade the endometrium when endometrial cups are formed they are afforded support from the endometrium and are able to live independently of the fetus. It is only after immunological rejection of the cups at about 120 days of gestation, that mares would be expected to start to have estrous cycles again. However, this usually after the breeding season has ended. Interestingly, in some mares endometrial cups persist for even longer, to well over 200 days and in a few cases to beyond foaling!
With high frequency ultrasound transducers, endometrial cups can sometimes be seen by transrectal ultrasonography. If this is not possible and a mare is anestrus or cycling irregularly, an eCG assay may clarify the situation. However an eCG assay is only useful for a short period after abortion but only soon after abortion (two months at most) because eCG concentrations decrease rapidly after abortion despite the continued presence of endometrial cups.
PS: First, note the concave shape of the endometrial cups when the image is enlarged; a somewhat lame reason (in the author's opinion) for referring to them as "cups". Perhaps endometrial "plaques" would have been more fitting, especially because may are not even circular but elongated, like slug trails. Secondly, note the presence of a para-ovarian cyst in this image (encircled by the yellow ring); often thought to be a remnant of the Wolffian (mesonephric) system. However these structures are sometimes shown to be of Mullerian (paramesonephric) origin. There is even confusion in the human literature on that subject.
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
