Follicular development in the adult guinea-pig and responses to human chorionic gonadotrophin.

The development of antral follicles and of atretic follicles throughout the cycle of adult guinea-pigs is a continuous process, but there are two stages when atresia is most pronounced: immediately after oestrus and in the late luteal phase. New atretic antral follicles were not found in the ovaries until around day 10 of the cycle, when an injection of HCG caused atresia of the medium-sized antral follicles within the ovary and luteinization of the largest follicles but spared the smallest antral follicles. Following the induced atresia, cycle lengths were prolonged, but the population of antral follicles could be restored to normal within 10 days of gonadotrophin treatment. It is suggested that the growth rate of antral follicles is flexible and proceeds most rapidly at the end of the luteal phase. It is not clear whether ovarian steroid play an integral part in regulating follicular growth and atresia.