Measurement of intra-embryonic pH during the early stages of development in the chick embryo

Summary Measurements have been made of the pH in the extracellular space, adjacent to the neural tube, in 73 isolated chick embryos in vitro at stages from 4–22 somites. A pH of 7.8–8.4 was observed in the segmented region, while caudally, in the segmental plate, the pH was consistently lower falling by as much as 0.5 pH units at the regressing primitive streak. Variations were noted in the pH of embryos of the same age but the regional variation in pH was a consistent finding in all of the embryos examined. The buffering capacity of the extracellular space was found to be 12.9 mequiv/pH unit/1 in the segmented region and 13.9 mequiv/pH unit/1 in the segmental plate. Thus it is unlikely that the regional variations in pH result from local variations in the buffering power of the extracellular space. Varying the K⁺, Cl^-, Mg²⁺ or HCO ₃ ^- ion concentrations in the bathing medium caused little change in the intra-embryonic pH, while reducing the concentrations of Na⁺ or Ca²⁺ caused a small acidification. This suggests that the ectoderm and endoderm form an effective barrier between the embryo and the external environment. Exposure of the embryo to KCN reduced the intra-embryonic pH suggesting that the alkaline environment is maintained by active processes.