GDF9, BMP15 and KitL interactions during oocyte and follicular development. Interactions among factors derived from oocytes and granulosa cells are regulated by FSH, and are important for oocyte development. In rodents, both GDF9 and BMP15 have been shown to promote proliferation of granulosa cells from small antral follicles, and BMP15 has been reported to inhibit FSH-stimulated progesterone production, and is an inhibitor of luteinization. Evidence from studies with gdf9-/- mice and granulosa cell cultures indicate that GDF9 suppresses expression of both KitL-1 and KitL-2. In contrast, BMP15 promotes KitL expression in monolayers of granulosa cells from rat early antral follicles and enhances the expression of both KitL-1 and KitL-2 mRNA in mouse oocyte-granulosa cell complexes grown in vitro. FSH is known to regulate bmp15 expression in a dose-dependent manner via Kit signaling, and Kit signaling promotes both oocyte growth and cell survival. Black lines indicate actions; red lines indicate effects on mRNA expression. The thick black arrow denotes the relative importance of KitL-2 in activating Kit receptors to promote oocyte growth.