The ovary of the newborn hamster is characterized by numerous oogonia which are undergoing mitotic divisions. By day 8, the germ cells have developed into oocytes (dictyate stage). The oocytes and granulosa cells are surrounded by an undeveloped network of fibrous stromal cells until day 14. Between days 14 and 21, the stroma is transformed by hypertrophy and hyperplasia into a primary interstitium consisting of large epithelial cells.
The critical period of follicular development is from days 21 to 28, with antral follicles first appearing on day 26. The earliest spontaneous ovulations occur on day 29. This differs from the temporal relationship in the rat and mouse in which antral follicles not only develop several weeks before puberty but can also be induced to ovulate prematurely with exogenous gonadotropins. In contrast, in the hamster the maximal ovulatory response to pregnant mare's serum (PMS) develops rapidly between days 27 to 30. Over this period, induced ovulations increase from an average of 10.5 ova to 55 eggs. The altered responsiveness to PMS does not correlate with any change in the diameter of follicles or number of secondary or tertiary follicles. It is therefore concluded that progressively smaller follicles become competent to respond to exogenous gonadotropin between days 27 and 30.