Ovarian stem cell niche and follicular renewal in mammals.

Stem cell niche consists of perivascular compartment, which connects the stem cells to the immune and vascular systems. During embryonic period, extragonadal primordial germ cells colonize coelomic epithelium of developing gonads. Subsequently, ovarian stem cells (OSC) produce secondary germ cells under the influence of OSC niche, including immune system-related cells and hormonal signaling. The OSC in fetal and adult human ovaries serve as a source of germ and granulosa cells. Lack of either granulosa or germ cell niche will result in premature ovarian failure in spite of the presence of OSC. During perinatal period, the OSC transdifferentiate into fibroblast-like cells forming the ovarian tunica albuginea resistant to environmental threats. They represent mesenchymal precursors of epithelial OSC during adulthood. The follicular renewal during the prime reproductive period (PRP) ensures that there are fresh eggs available for a healthy progeny. End of PRP is followed by exponentially growing fetal genetic abnormalities. The OSC are present in adult, aging, and postmenopausal ovaries, and differentiate in vitro into new oocytes. During in vitro development of large isolated oocytes reaching 200 μm in diameter, an ancestral mechanism of premeiotic nurse cells, which operates during oogenesis in developing ovaries from invertebrates to mammalian species, is utilized. In vitro developed eggs could be used for autologous IVF treatment of premature ovarian failure. Such eggs are also capable to produce parthenogenetic embryos like some cultured follicular oocytes. The parthenotes produce embryonic stem cells derived from inner cell mass, and these cells can serve as autologous pluripotent stem cells.