Activin effects on follicular growth in in vitro preantral follicle culture

As the follicular environment transits from being activin dominant to inhibin dominant during folliculogenesis, it is assumed that activin plays an important role in the early stage of follicular growth. We examined the effects of activin on morphological, biochemical and molecular changes in isolated preantral follicles. Preantral follicles were mechanically isolated from 14-day old female C57BL/6 mice. Each follicle was cultured and observed for 14 days using an in vitro follicle culture system containing FSH, FSH + activin A and FSH + inhibin in the culture medium. We subsequently examined FSH receptor (FSH-R) mRNA expression in isolated follicle cultures with or without activin on days 0 and 2. Activin was observed to significantly stimulate follicle enlargement on days 2, 4, 6 and 8, accelerate morphological changes and increase estradiollevels in culture medium on days 4, 12 and 14. In contrast, inhibin did not alter follicular growth. Additionally, activin stimulated the expression of FSH-R mRNA in isolated granulosa cells. It was demonstrated that activin stimulated the growth of preantral follicles, mainly during the early stage of folliculogenesis, by inducing FSH-R expression, in an isolated follicle culture system.