The Role of the Uterus in Ovarian Control in the Mare

Abstract

The effect of hysterectomy on ovarian activity was studied in four mares. The cyclic secretion pattern of plasma progestins normally observed in the intact mare was interrupted by hysterectomy. Follicular activity was observed in all four hysterectomized mares, in spite of prolonged luteal activity, with a large number of follicles attaining ovulatory size without the occurrence of ovulation. Some ovulations were observed at irregular intervals in two out of four hysterectomized mares in spite of plasma progestin levels which ranged from 2 to 6 ng/ml. While all ovulations which occurred in the hysterectomized mares were followed by the formation of CL as determined by palpation per rectum, the rise of progestins in the peripheral plasma was variable as significant increases followed CL formation in some instances but not in others. Four mares had CL with functional life spans of 48, 74, 75 and 77 days at the time the experiment was terminated. There was no difference in the ability of the CL formed following these ovulations to convert pregnenolone to progesterone (per unit weight) as compared to CL of 9 days duration. The longest estimated CL life span in any hysterectomized mare was 137 days. Sexual receptivity did not occur in conjunction with follicular growth and ovulation in the hysterectomized mares in the presence of an active CL.

The Role of Uterine and Ovarian Hormones in Luteolysis: A Comparison among Species

“…Loeb (1923) was the first to show that luteolysis could be delayed significantly in a mammalian species (guinea-pig) by removal of the uterus (hysterectomy). Similar effects of hysterectomy have now been described in cattle (Wiltbank and Casida 1956), sheep (Wiltbank and Casida 1956), pigs (du Mesnil du Buisson 1961, Spies et al 1960, Anderson et al 1961) and horses (Ginther and First 1971, Stabenfeldt et al 1974, Squires et al 1975. To my knowledge, the effect of hysterectomy on luteal function has not been investigated in goats.…”

Effects of TN F-.ALPHA. Injection into the Ovarian Parenchyma on Luteal Blood Vessels in Rabbits.

“…It is already well known that PGF2a is involed in luteolytic processes in several species of mammal [6][7][8][9]. It has also been reported that luteal endothelial cells are damaged by the decline in luteal blood flow due to the effects of PGF2a [4].…”

“…Prostaglandin (PG) F2a is known as the Received: April 25, 1995 Accepted: June 29, 1995 Correspondence to: Dr. Koichi NARIAI, Department of Veterinary Physiology, College of Agriculture and Veterinary Medicine, Nihon University, 1866 Kameino, Fujisawa, Kanagawa 252, Japan uterine derived luteolytic factor in several species [6][7][8][9] except for primates [10][11][12][13], dogs [14,15] and ferrets [16]. PGF2a or its analog causes a rapid and dramatic decrease in luteal blood flow and deletion of the luteal vessel [3,4].…”

Long-term characteristics of idiopathic persistent corpus luteum in the mare

“…Concentration of P4 in the PCL syndrome decreases to an intermediate plateau beginning on about Day 14 [7]. An abrupt decrease in P4 concentration (luteolysis) occurs after the intermediate plateau in PCL mares, whereas a continuing gradual decrease occurs in hysterectomized mares [7,20]. After Day 20, mares with PCL appeared to have low concentration of LH [15,19], and in one mare, LH concentration increased for 9 days before luteolysis [19].…”