Control of early ovarian follicular development

Early follicular growth refers to the development of an ovarian follicle fr om the primordial to early antral phase. Ln sheep and cows these phases of growth can be classified by the configuration of granulosal cells in the la rgest cross-section of the follicle as types 1 (primordial), 1a (transitory ) 2 (primary), 3 and 4 (preantral) and 5 (early antral). Follicles classifi ed as type 1 may be highly variable within each species with respect to num ber of granulosal cells and diameter of oocyte. Much of the variation in gr anulosal cell composition of type 1 follicles may occur at formation and th is may account for the variability in granulosal cell composition throughou t subsequent stages of growth. There appear to be important differences amo ng species (for example sheep and cattle) in the number and function of gra nulosal cells relative to the diameter of the oocyte during the initiation of follicular growth. There is evidence that most, if not all, of the growt h 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 peptid es (for example c-kit, stem cell factor, GDF-9, beta(B) and beta(A) activin /inhibin subunit, a inhibin subunit, follistatin, FGF-2, EGF, EGF-R, TGF be ta(1, 2 and 3) FSH-R and LH-R) are expressed in a phase of growth (for exam ple types 1-5)-specific and cell-specific manner. However, the roles of man y of these factors remain to be determined.