Growth differentiation factor-9 mediates follicle-stimulating hormone-thyroid hormone interaction in the regulation of rat preantral follicular development.

FSH regulates follicular growth in a stage-development fashion. Although preantral follicle stage is gonadotropin responsive, FSH is not required for preantral follicular growth. With the antrum, the follicles continue growing under the influence of FSH and become gonadotropin dependent. Although thyroid hormone is important for normal female reproductive function, its role and interaction with FSH in the regulation of preantral ovarian follicular growth is yet to be defined. In the present study, we have examined the action and interaction of FSH and T(3) in the regulation of the growth of preantral follicles, especially in their transition from preantral to early antral stage, using an established follicle culture system and evaluated the involvement of growth differentiation factor-9 (GDF-9) in this process in vitro. We have demonstrated that although T(3) alone had no effect on follicular development, it markedly enhanced FSH-induced preantral follicular growth. Although FSH alone significantly down-regulated FSH receptor (FSHR) mRNA abundance in the preantral follicles and T(3) alone was ineffective, expression of the message was significantly increased in the presence of both hormones. In addition, intra-oocyte injection of GDF-9 antisense oligonucleotides (GDF-9 morpholino) induced follicular cell apoptosis and suppressed follicular growth induced by FSH and T(3). These responses were attenuated by exogenous GDF-9. Our findings support the concept that thyroid hormone regulates ovarian follicular development through its direct action on the ovary and that promotes FSH-induced preantral follicular growth through up-regulation of FSHR, a mechanism dependent on the expression and action of oocyte-derived GDF-9.