Studies of oogenesis and follicular development in the golden hamster. 2. Initiation and control of meiosis in vitro.

The onset of meiotic prophase in the hamster occurs after birth and therefore it is possible to obtain ovaries which are at a relatively early stage of differentiation. Ovaries were maintained in vitro where the factors which might influence the onset of meiotic prophase could be studied in isolation. It was found that if ovaries were explanted prior to day 15 p.c. meiotic prophase began, but was subsequently blocked at the leptotene stage: the remaining oogonia and the oocytes at leptotene degenerated around day 5 p.p. By day 7 p.p. the few remaining oocytes were found at an abnormal stage which resembled leptotene. The addition of gonadotrophic hormones or maternal serum to the culture medium had no effect on the initiation of meiosis, neither had culture with the intact oviduct. By contrast, removal of the ovaries on or after the 15th day of gestation permitted meiotic prophase to proceed normally and oocytes at the diplotene stage were observed in the explants 8 days later. Culture of ovaries removed before day 15 p.c. resulted in abnormal development of the ovarian stroma: only when ovaries were explanted on day 15 or after was the development of 'nests' of oocytes normal. The evidence for endocrine control of oogenesis is discussed in the light of work on intersex animals and artificial chimaeras. These studies strongly suggest that oogonia enter meiosis spontaneously at the time which is specific for the species.