Culture environment modulates maturation and metabolism of human oocytes

Abstract

Background: The clinical use of oocytes matured in vitro for IVF is increasing, but little is known about the effect of culture conditions on oocyte maturation. Methods: Denuded immature oocytes identified following superovulation prior to ICSI were individually matured in one of two commercial media: tissue culture medium (TCM) 199 or modified Eagle medium with Earle's modified salts (MEME). During maturation, depletion of pyruvate and accumulation of lactate in the culture medium were non-invasively measured. Results: Maturing oocytes took up pyruvate (20-30 pmol/oocyte/h) and produced lactate (2-10 pmol/oocyte/h). Oocytes matured faster in MEME, with significantly more oocytes reaching metaphase II by 24 h after oocyte retrieval compared with TCM 199 (P = 0.03). The oocytes that matured more quickly in MEME had significantly lower lactate production than oocytes that matured more slowly (P = 0.02). In TCM 199, pyruvate uptake by rapidly maturing oocytes was lower than by slowly maturing oocytes (P = 0.05). During the second incubation, from 24 to 48 h post-oocyte retrieval, pyruvate uptake in MEME was 30% lower than in TCM 199 (P = 0.007). Pyruvate uptake and lactate production differed depending on the stage of nuclear maturation: pyruvate uptake and lactate production were greater during germinal vesicle breakdown than during polar body extrusion in MEME (P < 0.05). Conclusions: We have shown that: (i) pyruvate is a major energy source during oocyte maturation; (ii) the composition of the culture medium can affect the rate of maturation; and (iii) the culture medium and stage of nuclear maturation can affect pyruvate uptake and lactate production.