Changes in the fine structure of the ovarian follicle of the toad (Bufo bufo) prior to induced ovulation

Abstract For part of the year toads can be induced to ovulate by injecting HCG, making it possible to study changes in the ovary in relation to ovulation under experimental conditions. Here the fine structure of the mature ovarian follicle of the toad has been examined at various times prior to induced ovulation; in particular, morphological changes in the ovarian follicle have been considered in relation to the endocrine activity that has been proposed for the follicle in the period before ovulation. Significant among the changes which preceded induced ovulation was a loss of stored lipid material from the follicle cells during a well defined period which extended from 16 to 10 hr before ovulation. These changes were associated with the appearance of membranous whorls (myelin figures) which eventually occupied a large part of the follicle cell cytoplasm and persisted in the postovulatory follicle. The close association of lipid droplets and membranous whorls during the period of intense follicular activity induced by gonadotropin administration, and the growth of the whorls as the lipid was lost, suggests a functional relationship of the membranes to the process of lipid mobilization. Shortly after ovulation had taken place, little lipid was seen in the follicle cells although when follicles were examined several days after ovulation, lipid droplets were once again common. However, there were no changes which signified hypertrophy of the postovulatory follicle. The morphological changes in the preovulatory follicle are discussed in relation to the functional state of the follicle. It is concluded that loss of lipid from the follicle cells reflects preovulatory secretion of a hormone, probably steroidal in nature, and the present results thus support the concept of preovulatory release of progestin from the ovary in the toad.