Cellular and molecular mechanisms regulating oocyte quality and the relevance for farm animal reproductive efficiency.

The efficiency of breeding schemes is dependent on the high fecundity of the selected individuals. Reproductive technologies are constantly pushing the physiological limits, but while the male reproductive potential is almost fully exploited, female reproductive physiology is the subject of constant research. Since the number of offspring that a female can bring to term each pregnancy cannot be changed, the ideal approach is to remove the potential offspring at the beginning of development and to transfer them to recipients of lesser genetic value. The earlier the collection takes place, the higher the number of descendants that a female can generate, so that now, the number of available oocytes becomes the limiting factor. This article will describe how detailed studies on oocyte physiology are beginning to unravel the complex sequence that transforms a small primordial follicle into a large ovulatory follicle containing a mature oocyte. Progressively, the limits to oocyte manipulation have been recognised and gradually overcome with adequate hormonal treatments in vivo and with specific media supplementation in vitro. This has led to the development of highly efficient reproductive technologies and the promise of even greater advances in the future. Surprising new findings, such as ovarian stem cells that can replenish the follicle population or long term embryonic stem cell lines that can differentiate into oocytes, are rapidly changing our expectations.