Culture environment modulates maturation and metabolism of human oocytes

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.

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.

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).

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.