The earliest stages of folliculogenesis in vitro

In recent years several follicle culture systems have been pioneered in different mammalian species for studying ovarian folliculogenesis and culturing immature oocytes. Applications of these in vitro techniques include fertility preservation for humans, conservation of rare animals and development of oocyte banks for research purposes. Immature female gametes in the ovarian cortex can be cryopreserved for later use if culture techniques are available afterwards to promote growth and maturation. This review focuses on biochemical and biophysical factors related to oocyte culture in mice, the only animal in which live offspring have been produced after folliculogenesis in vitro. The advantage of using mice for these studies is that, in parallel to development of follicle culture systems, essential knowledge on folliculogenesis can be obtained from knockout mouse models. Recent experiments in mice stressed the principal role of the oocyte in follicle development and the strict timing of the biological processes underlying oogenesis in vitro. In large domestic animals and humans, study of oocyte culture is confounded by the constitutively prolonged nature of ovarian follicle development. In humans, only some aspects of follicle development have been studied because of the limited availability of suitable material for experimentation, technical difficulties related to manipulation of very small structures and lack of knowledge on physiological regulation of the early stages of follicle growth. Only a few reports describe ovarian follicular growth in vitro. In this review, relevant information on hormonal and growth factor regulation of the earliest stages of follicle growth in mammals is reviewed. Techniques are becoming available for the precise isolation of distinct classes of follicle and powerful molecular biology techniques can be used in studies of ovarian tissue culture.