The role of L-carnitine during oocyte in vitro maturation: essential co-factor?

In vitro maturation (IVM) of oocytes is a promising technology for both the treatment of human infertility and in animal production as a means of improving genetic gain. However, IVM derived oocytes remain inferior to those matured in vivo with reduced developmental potential. The environment in which an oocyte matures in vitro is vastly different to in vivo where maturation takes place within the ovarian follicle. The in vitro environment differs in oxygen concentration, exposure to light, and metabolite composition of culture media vs. follicle fluid, to name a few. Human follicle fluid contains the metabolite Lcarnitine and has shown to be associated with human fertility. L-carnitine has known biological functions as an essential co-factor for beta-oxidation, regulating ATP production from lipids, and as a potent antioxidant. Importantly, it appears that cumulus cells and the oocyte lack the machinery to synthesize L-carnitine de novo. The inability for local production of L-carnitine during IVM and its importance in human fertility warrants investigation of its affects during IVM. The potential to improve oocyte quality by inclusion of L-carnitine in the culture media thus increasing the capacity for betaoxidation and/or antioxidant activity of the culture media is receiving increased attention. This review summarizes studies to date investigating the developmental importance of L-carnitine during IVM and the mechanisms by which improved developmental potential is elicited. Overall, the inclusion of L-carnitine during IVM of several species results in improved oocyte quality with increased development to blastocyst. This is likely due to the antioxidant capacity of L-carnitine and its ability to increase ATP production from intracellular lipid stores.