The Pattern of Follicular Growth and Atresia in the Ovine Ovary

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Abstract
The ovaries of 21 2-yr-old Merino ewes at various stages of the estrous cycle were serially sectioned and all follicles > 0.5 mm in diameter were counted and measured. The mean number of normal follicles > 0.5 mm in diameter was 19.9/ewe, of which 8.7 were > 1 mm in diameter. The mean numbers of follicles > 0.5 mm in diameter in early, advanced and late stages of atresia were 4.6, 9.0 and 12.3, respectively. Early atresia was rarely seen in follicles < 1 mm in diameter; the greatest incidence was in follicles of diameter 1.5-2.5 mm. Volumes of the granulosa, theca interna and antrum were calculated for all follicle sizes and the mitotic index of the granulosa and theca determined. Follicles were classified according to granulosa volume, each class having twice the volume of the preceding class. Estimates of the doubling time of the granulosa were made from the mitotic index and an estimate of mitotic time (0.43 h) obtained by use of colchicine on 2 other ewes. In all ewes, irrespective of day of estrous cycle, the rate of growth was slow in follicles < 0.4 mm in diameter; above this size it accelerated to a maximum (doubling time 26-30 h) in follicles 0.7-2 mm in diameter. It was estimated that, on the average, a follicle takes about 5 days to grow from 0.5-2.2 mm diameter, and a further 4 days to reach 4.5 mm diameter. The number of follicles entering the rapid growth phase was estimated to be 3-4/day. The present data do not support any particular wave theory of follicular development, but indicate that follicles are growing or regressing asynchronously at any given time during the luteal stage of the estrous cycle. At 48 h after treatment of 2 ewes with PMS [pregnant mare''s serum] gonadotropin (PMSG), the number of follicles > 0.5 mm in diameter was increased, the greatest proportional increase being in those > 3.5 mm in diameter. Relatively few follicles were undergoing early atresia. At 18 h after HCG [human chorionic gonadotropin] treatment (given 48 h after PMSG to 2 ewes) all follicles > 3.5 mm in diameter were undergoing luteinization and two-thirds of the follicles between 1 and 3.5 mm in diameter were undergoing early atresia.