Observations on intracellular pH during cleavage of eggs of Xenopus laevis.

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Abstract
The X. laevis embryo is a good system for a study of the physiology of mitosis and cytokinesis. Direct measurement of intracellular pH was made with recessed-tip pH microelectrodes in fertilized eggs of the frog, X. laevis, from .apprx. 1 h after fertilization to mid-blastula. The intracellular pH just before 1st cleavage was 7.65 .+-. 0.04 (SD; n [number] = 9). By stage 5 to the middle of stage 6, average intracellular pH was 7.70 .+-. 0.06 (n = 16). A statistically significant alkalization of 0.18 .+-. 0.03 pH unit (n = 5) was observed beginning in early blastula. A cycle of .ltoreq. 0.05 pH unit was occasionally observed during the pre-blastula period, but its significance is unknown. By exposing the early cleavage embryo to saline buffered with sodium propionate, pH 4.7-5.0, it was possible to lower intracellular pH with some degree of control. Apparently, normal cleavage continued to occur when intracellular pH was forced as much as 0.3 unit below normal. This implies no specific involvement of intracellular pH in mitosis and cytokinesis. If intracellular pH was lowered further, cell division ceased at .apprx. pH 7.2, and furrow regression began at .apprx. pH 7.0. Once furrow regression occurred, subsequent development was usually arrested or abnormal when the embryo was transferred back to normal saline.