FOLLICULAR KINETICS IN THE MARE OVARY

Using histological techniques, 8 ovaries (1 per mare) of riding or pony mares were studied at different physiological states: 2 ovaries were collected during anoestrus, 4 during early pregnancy, 2 before (day 33) and 2 after (day 43) the onset of PMSG secretion, and 2 during the oestrous season. On each ovary all the follicles greater than 50 μ (one complete layer of round follicle cells) were counted, measured, checked for atresia and calculated for mitotic index. The mean total number of follicles larger than 50μ in diameter was 48 ± 27 per mare, 23±11 of which were antral (>300 μ in diameter). Atresia was rarely seen in follicles <350μ in diameter, and the mean number of atretic follicles was 14 ± 10 per mare. Thus, the follicular population in the mare was numerically much smaller than in the ewe or the cow. To study the follicular population at different physiological states, the follicles were pooled and divided into four groups: preantral (<300μ in diameter), small-antral (<1 500 μ), large-antral (< 10 000 μ) and preovulatory (> 10 000 μ). The first three size groups from ovaries collected during the anoestrous or oestrous season showed no difference in the mean number of follicles, incidence of atresia or mitotic index. The fourth group of follicles was different as there were no preovulatory follicles in anoestrous mares. The ovaries of pregnant mares submitted to PMSG for 6 days contained significantly more follicles in the small-antrum group than the control ovaries (day 33 of pregnancy). PMSG also seemed to modify antrum formation. In contrast, PMSG seemed to have no effect on follicle number or atresia in the large-follicle population. Follicular growth rates were estimated by two methods: follicles larger than 5 000 μ in diameter: cauterization of the preovulatory follicles during late oestrus showed that a follicle of less than 5 000 μ needed about a week to grow to ovulatory size; and follicles between 60 and 1 000 μ in diameter: measurement of the mitotic index indicated a follicle needed about 7 weeks to enlarge from 60 to 1 000 μ. Thus, follicular growth in the mare seemed to be much faster than in the ewe.