In vivo and in vitro development of preantral follicles

The paper presents the state of the art concerning the in vitro growth and differentiation of preantral follicles, i.e. from the primordial to the antral stage, in mammals. Attention is paid to their normal structure and development, nomenclature, isolation and culture methods, effects of systemic and local compounds on their development, and viability of isolated and cultured follicules, with special reference to the bovine. In vivo, various endocrine and locally produced compounds stimulate neovascularization or innervation of small follicles, which provide nutrients, cytokines, hormones and neuropeptidergic substances, necessary of survival of follicles and initiation of follicle growth. In vitro, FSH, activin, EGF, TGFs and IVP seem important for survival and growth of cultured preantral follicles. However, many reported in vitro effects of hormonal compounds on preantral follicles, especially on those from the bovine, have to be interpreted carefully, since these studies presented little information on oocyte quality. In contrast to preantral follicles from small laboratory animals, pigs and humans, those from cows will not soon be usable for IVP, since their oocyte is relatively small and their growth rate relatively slow. Although practical application of especially small preantral for IVP of live-stock species still is far away, it should not be considered an utopia.