Resurgence of the primary corpus luteum during pregnancy in the mare

Abstract

The temporal association between the first detectable increase in concentrations of circulating progesterone and equine chorionic gonadotropin (eCG) was studied in pregnant mares. Blood samples were collected on days 15, 20 and 25–40 from pony mares (experiment 1; n=11) and days 11–40 from horse mares (experiment 2; n=10). Mares were ultrasonically examined daily to evaluate the primary corpus luteum (experiment 3; n=2–6) and to detect the formation of secondary corpora lutea (experiments 1 and 2). Formation of secondary corpora lutea was not detected in any mare prior to day 40 in experiment 1, but was detected in one mare on day 38 and in two mares on day 39 in experiment 2. In both experiments 1 and 2, there was a significant main effect of day for circulating concentrations of progesterone and was attributable to decreased ( P<0.05) concentrations between days 12 and 15 (experiment 2) and increased concentrations after day 32 (experiment 1) or day 34 (experiment 2). Progesterone concentrations were higher ( P<0.05) on days 38, 39, and 40 than on days 29, 31, and 32 in experiment 1 and higher ( P<0.05) on day 40 than on days 32, 33 and 34 in experiment 2. When progesterone concentrations in experiment 2 were normalized to the day that eCG was first detected (mean, day 35), mean concentrations were lower ( P<0.05) before detection of eCG than on the second day after detection. The cross-sectional area of the ultrasonic image of the primary corpus luteum (experiment 3) was greater ( P<0.05) on days 39, 40, 41, and 42 than on days 30 and 33. The cross-sectional increases in area of the primary corpus luteum in experiment 3, closely paralleled the increases in progesterone concentration in experiments 1 and 2. Combined results of the 3 experiments supported the hypothesis that resurgence of the primary corpus luteum occurs in pregnant mares between days 30 and 40 and that the resurgence is a response to release of eCG into the circulation. More specifically, the results indicated that, on the average, the resurgence of the primary corpus luteum in the mare began on day 35.