Clarksdale, MS--the golden buckle on the cotton belt
Originally Posted by equusaround
I can't open the article and don't subscribe to the journal. :>(
Here is what it says:
A unique feature of equine reproduction is the production of structures called endometrial cups during pregnancy. Endometrial cups form from placental cells that invade the uterine lining of the equine, beginning at about 35 days of pregnancy.
The invading cells are from the developing fetus and not from the mare. The cells form a series of discrete masses in a ring pattern at the base of one uterine horn of the mare. Endometrial cups produce large quantities of a hormone called equine choronic gonadotrophin, or eCG, beginning at the onset of cup formation and continuing until the cups disappear at about 120 to 150 days of pregnancy.
The hormone stimulates the production of additional corpora lutea, and the corpora lutea produce progestrone, which causes the body to go through changes to support pregnancy. So the endometrial cups are nature's way of giving additional support to early pregnancy in the mare.
Once formed endometrial cups will typically persist for three to four months, whether or not the mare stays pregnant.
Because the mare's body thinks it's still pregnant, thanks to the extra progestrone, the cups make rebreeding more difficult in the case of a lost pregnancy. She won't go back into heat.
Abnormal persistence of endometrial cups occurs in mares with early abortion and in mares with a normal pregnancy. Persistent endometrial cups have been associated with failure to show behavioral estrus, ovarian inactivity, sporadic follicular growth, and luteinization of partly developed follicles.
"I'm a lumberjack, and I'm okay." Thread killer Extraordinaire
Here's a few additional points taken from our course which may or may not have been in Dr. McCue's article (which I do not have access to) and which may prove of interest/use:
While the formation of the endometrial cups is consistently in the region of 35 days post ovulation, the regression - and the cessation of secretion of eCG - is not as uniform.
Because of this lack of uniform cessation, Progesterone levels may drop to a base line any time from about 100 days to 210 days of pregnancy, either of which is perfectly normal. Other Progestins take over the function of progesterone from about 100 days onwards, and these will not assay as progesterone, so a "low" progesterone level after 100 days is perfectly normal and not an indication of a need for supplementation.
As one of the early blood-pregnancy tests is an evaluation for eCG presence (e.g. the "Pregnamare" test), this test can only test positive after the secretion of eCG has commenced, and prior to the cessation of secretion. The most reliable time frame for this test is therefore between about 40 and 90 days post fertilizing-ovulation.
It must be remembered that as eCG secretion will be likely to continue following a pregnancy loss that occurs after 35 days, that the mare will still test "positive" for pregnancy when using an eCG assay in that situation even though she is not.
A correlation between eCG levels and twins has not been seen, so an assay for eCG presence is not a suitable sole-diagnostic for pregnancy (besides the false-positive possibility).
Research has suggested that eCG may be responsible for luteal resurgence of the primary CL, and in some mares, secondary CLs may be absent and yet the pregnancy safely maintained. (Daels P.F., Albrechta B.A., and Mohammeda H.O. (1998). Equine Chorionic Gonadotropin Regulates Luteal Steroidogenesis in Pregnant Mares. Biology of Reproduction 59, 1062-1068)
Research suggests that a small percentage of mares may retain the endometrial cups beyond the normal period of ~120 days of pregnancy, and continue secretion of eCG (Steiner J, Antczak DF, Wolfsdorf K, Saville K, Brooks S, Miller D, Bailey E, Zent W. (2006) Persistent endometrial cups. Animal Repro. Sci. 94:274-275). Persistence ranged from 6 to 30 months in the researched animals.