Effect of the stage of the oestrous cycle on the follicular population in pony mares.

In the middle of the breeding season, 16 pony mares (n = 4 per day) were slaughtered on four different days (days 6, 14, 17 and the preovulatory day) of the oestrous cycle, day 0 being the day of the last ovulation. All the ovaries were examined histologically; the number, size and atresia (defined by granulosa cell pyknosis) of all follicles larger than 1 mm in diameter were studied, using a Kryotome-video recorder-TV system. Follicular distribution of all the sizes studied (1-5 mm, 5-10 mm, greater than 10 mm in diameter) was very similar in the right and left ovaries. However, compared to the other ovary, the ovary with a luteal structure had fewer (P less than 0.05) healthy follicles larger than 10 mm. All follicles larger than 10 mm in diameter at day 6 were atretic. At day 14, a group of normal follicles larger than 10 mm was found in all the animals, and number of follicles larger than 10 mm was significantly increased (P less than 0.01). The largest normal follicle has increased in size at day 17 compared to day 14. However, a significant increase in the number of follicles undergoing early atresia was observed (P less than 0.05). Except for the large preovulatory follicle, all the follicles larger than 10 mm were atretic just before ovulation. There was a relationship between the follicular population and the duration of the follicular phase; the number of normal follicles 1-10 mm in diameter and the duration of the follicular phase were highly correlated (R = -0.56; P less than 0.02).