In vitro analysis of ovarian differentiation and the initiation of meiosis in the rat.

Intact, cranial, and caudal portions of fetal rat gonads on days 13, 14, 15, 16 and 17 of gestation were cultured singly on Nucleopore filters in Eagle's MEM and McCoy's 5A media (both supplemented with fetal calf serum and glutamine) to evaluate the role of the mesonephros in mammalian gonadogenesis. After 2--7 days in culture, in vitro and in vivo control tissues were examined by light and electron microscopy to determine the extent of morphological differentiation and gonium maturation. The onset of morphological differentiation and meiotic activity, as determined by synaptonemal complex formation, was found to be independent of the mesonephric epithelium. Ovaries and testes were also cultured as heterologous pairs on opposite sides of collagen-coated Nucleopore filters for 4--6 days in McCoy's 5A media. In the absence of transfilter cellular migration and establishment of cell-to-cell contact, the in vivo pattern of sexual dimorphic onset of meiosis was observed. This finding lends support to the concept that local somatic-germ cell associations as opposed to extragonadal factors, are capable of providing the necessary environment for regulation of meiosis in the fetal rat.