Challoner S
J Anat. 1975 Feb;119(Pt 1):149-56.
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.
仓鼠减数分裂前期的开始发生在出生后，因此有可能获得处于相对早期分化阶段的卵巢。卵巢在体外培养，这样就可以单独研究可能影响减数分裂前期开始的因素。研究发现，如果在妊娠第15天之前取出卵巢，减数分裂前期会开始，但随后会在细线期受阻：剩余的卵原细胞和处于细线期的卵母细胞在产后第5天左右退化。到产后第7天，发现少数剩余的卵母细胞处于类似细线期的异常阶段。向培养基中添加促性腺激素或母体血清对减数分裂的启动没有影响，与完整输卵管一起培养也没有影响。相比之下，在妊娠第15天或之后切除卵巢可使减数分裂前期正常进行，8天后在外植体中观察到处于双线期的卵母细胞。在妊娠第15天之前取出的卵巢进行培养会导致卵巢基质发育异常：只有在第15天或之后取出卵巢时，卵母细胞“巢”的发育才正常。根据对雌雄间性动物和人工嵌合体的研究，讨论了内分泌对卵子发生控制的证据。这些研究强烈表明，卵原细胞在特定于该物种的时间自发进入减数分裂。
