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Progression of preantral follicle development is essential to further follicle maturation and ovulation, but there are few models for studying the regulation of preantral follicle survival and growth. We have evaluated preantral follicle survival in vivo and in vitro, and have developed a serum-free rat follicle culture system that can be used to characterize the regulation of preantral follicle growth and differentiation. Analysis of ovarian cell DNA fragmentation during the first wave of follicle growth in the infantile rat indicated negligible apoptosis up to day 16 of age. However, a major increase in apoptosis was found by day 18, a time point associated with the appearance of large antral follicles. In situ analysis confirmed that apoptotic DNA fragments were limited to antral follicles. Culture of individual preantral follicles mechanically dissected from ovaries of 12- or 14-day-old rats in serum-free conditions led to major …