Is the Egg Activation-Induced Intracellular pH Increase Necessary for the Embryonic Development of Xenopus Laevis (Anuran Amphibian)?

The initiation of embryonic development in the anuran amphibian, Xenopus laevis, is accompanied by an increase in intracellular pH (0.3 pH unit) which starts around 5 min after fertilization. This intracellular pH (pHi) increase has no known physiological role. In the present study, we have analyzed the consequences of maintaining the pHi of fertilized eggs at the same level as that in unfertilized eggs during the early embryonic cell divisions. Among several weak acids and drugs capable of producing changes in pHi, CO2, a weak acid, was preferred because it allowed to impose very rapid and controllable variations in pHi. Our results indicate that those embryos in which the fertilization-induced pHi increase was prevented showed a delay in development as early as the second or third division. When pHi “clamping” was maintained after the lengthening of the cell cycle was observed, this frequently resulted in a total arrest of embryonic development. On the other hand, artificial perturbation of the periodic pHi oscillations (0.05 pH unit around the plateau value, 7.7–7.8) which begin at the time of the first cell division, had no noticeable effect on embryonic development.