Reproductive patterns were studied in the majority of 46 mares before obtaining the ovaries for examination at slaughter. Steroids identified and determined were progesterone and 20∝-hydroxypregn-4-en-3-one in the corpora lutea and oestradiol- 17 β in the follicular fluid. A study of ovarian morphology revealed no significant cyclical variations of the ovarian weight and number of follicles, but the total volume of follicular fluid increased in the latter half of the cycle. Consistent and marked follicular growth was present at about the 25th day of gestation. Mares 16 to 28 days pregnant had significantly more follicles than cycling mares, and the ovarian weight and volume of follicular fluid were also increased at this time. Peak total oestradiol values at 25 days and maximal progesterone concentrations at 27 days preceded implantation on the 28th day.
Oestradiol was present before puberty and the amount in anoestrous animals seems to increase gradually as the breeding season approaches. In cycling mares, total follicular fluid oestradiol values are variable for up to five days after ovulation, but oestradiol is virtually absent during the mid-luteal period. From the 12th day there is a rapid increase to maximal values immediately prior to oestrus. Limited evidence suggests that ovarian oestradiol decreases from the commencement of oestrus. Pregnant animals exhibit an indistinguishable pro-oestrual rise, but drop suddenly when oestrus would be expected on the 17th day. When more than one corpus luteum was present and after two months gestation, no oestradiol was detected.
Waves of follicular growth and oestradiol synthesis commence at 2, 12, 22 and 32 days after ovulation, suggesting a cyclical release of pituitary gonadotrophin every 10 days, irrespective of whether an active corpus luteum is present or not. Maximal follicular growth during pregnancy was encountered at 25 days, before the appearance of PMSG. Actual ovulation was only encountered soon after the– expected appearance of PMSG, suggesting a synergistic effect between PMSG and pituitary gonadotrophin. When steroidal inhibition of pituitary gonadotrophin increased and PMSG levels were maximal, the ovaries were relatively quiescent. Many active luteinized follicles were encountered as PMSG levels waned.
Luteal progesterone concentrations rose linearly for 14 days; in cycling mares, regression took place on the 14th or 15th day. In pregnant animals the concentration continued to rise, reaching a peak at 27 days. Secondary corpora lutea, resulting from ovulations during the second month of gestation, had an initial concentration resembling cycling animals, but marked increases were found during the third month of gestation. The primary corpus luteum does not regress with the occurrence of secondary ovulations as is generally believed, but remains active for at least three months.
20∝-Hydroxypregn-4- en- 3- one was always present in the corpora lutea of cycling animals and increases were associated with luteal regression. This steroid was never detected in animals more than 18 days pregnant, including fresh secondary corpora lutea.
The ovaries of mules contained similar amounts of steroids as found in horses. However, the occurrence of ovulation in the presence of functional corpora lutea suggests failure of the luteal regression mechanism, or alternatively an inability of the hypothalamus to regulate gonadotrophin secretion in the usual manner. The theoretical implications of these findings on sterility work are discussed.