Stem cells in adult human ovaries: from female fertility to ovarian cancer.

The human ovary is a complex endocrine gland, which is responsible for production of different hormones and provides mature and competent oocytes for reproduction. Additionally, it produces various substances, such as growth factors and cytokines which are involved in the complex signalling pathways of folliculogenesis or oogenesis. The abnormalities of ovarian function might lead to infertility or manifestation of aggressive cancer. Therefore, stem cells in adult human ovaries are of great interest to reproductive medicine for improved understanding of the mechanisms leading to ovarian infertility or cancer formation, yet they represent a difficult scientific subject, because it is still generally accepted that they do not exist. The persisting dogma is that the end number of follicles is set up at the time of birth, and that there is no neo-folliculogenesis or neo-oogenesis in the postnatal or adult ovaries. The main reason for persistence of the dogma lies in the fact that it is very difficult to perform studies on adult human ovaries; it is impossible to perform in vivo studies, and there is also a lack of ovarian tissue available for research. However, there is more and more evidence about the presence of putative stem cells in postnatal and adult mammalian ovaries. First promising experimental results were obtained in the mouse model, but have been followed also in humans. The aim of this review article is to elucidate the fast upcoming new knowledge of ovarian stem cells, and their potential implications for reproductive medicine and gynecological oncology in the future.