Developmental Potential of Ovarian Follicles in Mammals: Involvement in Assisted Reproductive Techniques

Ovarian follicles develop through several distinct phases during fetal and postnatal periods and they release their matured ova upon puberty. A finite number of primordial follicles form in the fetal ovary from primordial germ cells (PG) during the first stage of fetal development. The primordial follicles consist of oocytes surrounded by a single layer of pregranulosa follicular cells and they remain dormant in the meiotic prophase I stage. Primordial pregranulosa follicular cells initiate activation of primordial follicle and govern the development of dormant oocytes. The primordial follicles take about 6 months to grow and develop to ovulatory graafian follicles in cattle and humans. Growth of preantral follicles is gonadotropin-independent whereas growth of antral follicles is gonadotropin-dependent. Changes occur during these stages in mammalian ovarian follicles to prepare the oocyte for successful maturation, fertilization and further embryonic development. The changes enable the zygotes to overcome maternal zygotic transition stage and follow their developmental competence to fetus. The changes were affected by in vivo and in vitro molecules and factors of the organisms and the surrounding conditions, respectively. Because of the importance of follicular changes during growth and development stages, which reflected in the developmental competence of oocytes, an attempt was made in this review to collect and combine the current knowledge on growth, development and maturation of ovarian follicles and resulting oocytes and their applications in assisted reproductive techniques.