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Regulation of intracellular pH during oocyte growth and maturation in mammals

Institute for Women's Health, University College London, 86-96 Chenies Mews, London WC1E 6HX, UK¹ Ottawa Health Research Institute and Departments of Obstetrics and Gynecology (Division of Reproductive Medicine) and Cellular and Molecular Medicine, University of Ottawa, Ottawa, Ontario K1Y 4E9, Canada

Correspondence should be addressed to G FitzHarris; Email: g.fitzharris{at}ucl.ac.uk J M Baltz; Email: jmbaltz{at}ohri.ca

Regulation of intracellular pH (pH_i) is a fundamental homeostatic process essential for the survival and proliferation of virtually all cell types. The mammalian preimplantation embryo, for example, possesses Na⁺/H⁺ and HCO₃^–/Cl^– exchangers that robustly regulate against acidosis and alkalosis respectively. Inhibition of these transporters prevents pH corrections and, perhaps unsurprisingly, leads to impaired embryogenesis. However, recent studies have revealed that the role and regulation of pH_i is somewhat more complex in the case of the developing and maturing oocyte. Small meiotically incompetent growing oocytes are apparently incapable of regulating their own pH_i, and instead rely upon the surrounding granulosa cells to correct ooplasmic pH, until such a time that the oocyte has developed the capacity to regulate its own pH_i. Later, during meiotic maturation, pH_i-regulating activities that were developed during growth are inactivated, apparently under the control of MAPK signalling, until the oocyte is successfully fertilized. Here, we will discuss pH homeostasis in early mammalian development, focussing on recent developments highlighting the unusual and unexpected scenario of pH regulation during oocyte growth and maturation.