Intracellular pH regulation in the human oocyte.

We have used fluorescence techniques to study the regulation of intracellular pH during fertilization and preimplantation embryo development in human oocytes. The intracellular pH of human oocytes during maturation and fertilization was always 7.4, suggesting that these processes do not involve long-term changes in intracellular pH. The recovery of intracellular pH of human oocytes and embryos after extracellular acid or alkaline shock was investigated. Fresh metaphase II oocytes and preimplantation embryos showed similar rates of recovery following alkaline shock. However, aged and immature oocytes had a significantly slower rate of recovery to physiological pH. Early embryos up to the morula stage were unable to recover following acidosis, whereas blastocysts could control both acidosis and alkalosis. We assessed the sensitivity of fertilization and early development in the human to extracellular pH. Our results show that insemination in the human is pH-sensitive, whereas intracytoplasmic injection (ICSI) activated oocytes at all pH tested. We suggest that this is due to the pH-sensitivity of the sperm-zona pellucida interaction, which is bypassed during the ICSI procedure. Further development in human embryos is more sensitive to alkalinity than acidity. We discuss these results in terms of the extracellular pH in vivo in the female reproductive tract.