Embryo Culture and pH

LETTER TO THE EDITOR Embryo Culture and pH To the Editor: In their article ‘‘Differential pH in Embryo Culture’’ (1), Hentemann et al. attempted to demonstrate differential requirements of media pH during stages of mouse embryo development. Unfortunately, it is not clear that any conclusion regarding pH can be drawn from the publication. In fact, results should be interpreted with caution. The authors varied sodium bicarbonate levels of their commercial media to adjust pH, similar to the approach used previously by Quinn and Cooke (2). Using this method, it is impossible to attribute any subsequent effect on embryo development to pH alone. The title ‘‘Differential Bicarbonate Levels in Embryo Culture’’ may have been just as appropriate. Bicarbonate concentration may affect embryo development independently of pH, because it is used by a variety of cellular transport mechanisms and could affect metabolism. This same limitation of confounding variables exists with earlier attempts at determining an optimal culture media pH by adjusting CO2 levels (3). Carbon from CO2 is fixed differentially by embryos for incorporation into various molecules (4) and may affect development independently of pH. The cause for concern lies in the fact that readers of the current article who may wish to adopt a similar differential pH paradigm

for embryo culture would presumably accomplish this by altering incubator CO2, because bicarbonate levels are set by the media manufacturers. However, doing so may result in entirely different outcomes than observed in this study. Furthermore, the apparent lack of statistics or statistical differences in the article precludes any definite conclusions regarding the potential effect of bicarbonate/pH on the embryo. What is known is that the authors have begun to optimize their own commercial culture medium formulation and culture environment for mouse embryos. This is a necessary step in improving culture conditions and embryo development in vitro, and the authors should be commended. However, similar results may not be apparent when using other media, because differing media composition, such as lactate or pyruvate concentration or amino acid composition, can differentially affect internal pH of the embryo (5, 6). Therefore, to determine an optimal medium pH for embryo development, it is advisable for each laboratory to do their own direct comparisons in their respective culture systems. Jason Swain, Ph.D. University of Michigan, Ann Arbor, Michigan March 28, 2011 doi:10.1016/j.fertnstert.2011.04.024

REFERENCES 1. Hentemann M, Mousavi K, Bertheussen K. Differential pH in embryo culture. Fertil Steril 2011;95: 1291–4. 2. Quinn P, Cooke S. Equivalency of culture media for human in vitro fertilization formulated to have the same pH under an atmosphere containing 5% or 6% carbon dioxide. Fertil Steril 2004;81:1502–6.

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3. Carney EW, Bavister BD. Regulation of hamster embryo development in vitro by carbon dioxide. Biol Reprod 1987;36:1155–63. 4. Quinn P, Wales RG. Fixation of carbon dioxide by preimplantation mouse embryos in vitro and the activities of enzymes involved in the process. Aust J Biol Sci 1971;24:1277–90.

5. Edwards LJ, Williams DA, Gardner DK. Intracellular pH of the preimplantation mouse embryo: effects of extracellular pH and weak acids. Mol Reprod Dev 1998;50:434–42. 6. Edwards LJ, Williams DA, Gardner DK. Intracellular pH of the mouse preimplantation embryo: amino acids act as buffers of intracellular pH. Hum Reprod 1998;13:3441–8.

Fertility and Sterility
Vol. 95, No. 8, June 30, 2011 Copyright ª2011 American Society for Reproductive Medicine, Published by Elsevier Inc.

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