The organization of the cortical endoplasmic reticulum in Xenopus eggs depends on intracellular pH: artefact of fixation or not?

The cortical endoplasmic reticulum (CER), which develops and becomes organized during oocyte maturation in Xenopus laevis (anuran amphibians), has been thought to be essential for the propagation of the activating signal at the time of fertilization, possibly by regulating intracellular Ca²⁺ (Ca²⁺_i) (Charbonneau and Grey, Dev. Biol. 102, 90–97, 1984). The present paper demonstrates that changing intracellular pH (pH_i) has an influence on the structure and organization of the CER in unactivated Xenopus eggs. Acidifying the egg cytoplasm from its normal value, pH 7.5, to about 6.8, with CO₂, produced a dramatic increase in the proportion of the CER being in the form of a continuous and well-developed network. On the other hand, alkalinizing the egg cytoplasm from 7.5 to about 8.2, with NH₄Cl, induced a general disruption and vesicularization of the CER. These effects of pH_i on a Ca²⁺_i-regulating system are discussed, taking into account possible artefacts generated during fixation.