Number of germ cells and meiotic prophase stages in fetal rat ovaries cultured in vitro.

Ovaries from 13.5-day-old rat fetuses were cultured in vitro in a hormone-free medium for up to 8 days. The number of germ cells increased during the first 4 days and then sharply decreased. The initial decrease was concomitant with the first leptotene stages. All stages of meiotic prophase progressively appeared in the remaining germ cells.