Reducing environmental stress imposed upon gametes and embryos in the IVF laboratory is crucial in optimizing culture conditions and development. One environmental parameter of particular importance is external pH (pHe) of culture media. An optimal pHe has not been identified.

METHODS

Electronic searches were performed using keywords focused on pH and the embryo using PUBMED through August 2011, with no limits placed on a beginning time. Examples of keywords include CO2, bicarbonate and hydrogen ion. Relevant papers were then examined to obtain additional publications.

RESULTS

Determining optimal pHe is problematic due to difficulty in isolating pHe from other variables, such as CO2 and bicarbonate. Various commercial media companies recommend differing pHe ranges, most within the range of 7.2–7.4, with some companies recommending altering pHe based on the gamete or stage of the embryo. However, changing pHe during culture has not been experimentally shown to improve outcomes. Further complicating attempts to define an optimal pHe is that media components can impact intracellular pH (pHi). As a result, media with different concentrations of substances, such as lactate or amino acids, may have different pHi, despite being in the same pHe.

CONCLUSIONS

Due to the plasticity of embryos, a range of pHe's can support development, and defining an optimal pHe is difficult. It is unclear whether there is any benefit in changing pHe at various steps during IVF. The ideal pHe will likely vary from media to media and, until comparative studies have been performed isolating pHe, adherence to manufacturer recommendations and maintenance of a small acceptable pHe range are advisable. 


  • What is known is that the pHi of embryos is ∼7.1–7.2. 
  •  pHe should be slightly higher than pHi to help offset the acidification that occurs as a result of intracellular metabolic processes. Thus, commercial media manufacturers often recommend, and many laboratories culture their embryos, in the range of pHi 7.2–7.4, 
  • the use of a ‘high-low-high’ system, where pHe is elevated for fertilization, lowered for cleavage-stage embryo culture and again raised for blastocyst culture has been promoted by commercial media manufactures 
  •  better sperm penetration at pHe 7.6–7.8, 
  • pHe of the oviduct in species like the monkey, rabbit and bovine appears to have a more alkaline pHe (∼7.5–7.9) (Iritani et al., 1971; Maas et al., 1977; Hugentobler et al., 2004), 
  • pHe of human uterine fluid has been measured with a pH electrode and found to be ∼6.6, while another study found a pHe of ∼7.1 (Sedlis et al., 1967; Yedwab et al., 1976).
  •  a ‘high-high-low’ or ‘high-low-low’ scheme may be more suitable/physiologic for fertilization, cleavage and blastocyst development.