Control of early ovarian follicular development.

Early follicular growth refers to the development of an ovarian follicle from the primordial to early antral phase. In sheep and cows these phases of growth can be classified by the configuration of granulosal cells in the largest cross-section of the follicle as types 1 (primordial), 1a (transitory) 2 (primary), 3 and 4 (preantral) and 5 (early antral). Follicles classified as type 1 may be highly variable within each species with respect to number of granulosal cells and diameter of oocyte. Much of the variation in granulosal cell composition of type 1 follicles may occur at formation and this may account for the variability in granulosal cell composition throughout subsequent stages of growth. There appear to be important differences among species (for example sheep and cattle) in the number and function of granulosal cells relative to the diameter of the oocyte during the initiation of follicular growth. There is evidence that most, if not all, of the growth phases from types 1 to 5 are gonadotrophin-independent and that follicles develop in a hierarchical manner. In sheep, cows and pigs, numerous growth factor, growth factor receptor and gonadotrophin receptor mRNAs and peptides (for example c-kit, stem cell factor, GDF-9, beta B and beta A activin/inhibin subunit, alpha inhibin subunit, follistatin, FGF-2, EGF, EGF-R, TGF beta 1,2 and 3 FSH-R and LH-R) are expressed in a phase of growth (for example types 1-5)-specific and cell-specific manner. However, the roles of many of these factors remain to be determined.