Subcellular differentiation arrested in Artemia embryos under anoxia: Evidence supporting a regulatory role for intracellular pH

Abstract

AbstractPrevious work has clearly documented that intracellular pH (pHi) mediates metabolic arrest during anaerobic dormancy in Artemia embryos. This study provides evidence that subcellular differentiation during preemergence development (PED) is regulated by transitions in pHi and also reports the first detailed ultrastructural description of PED in these embryos.Mitochondrial maturation, elaboration of glycogen fields, and alteration in yolk platelet structure are clearly evident during aerobic development (embryo pHi ≥ 7.9). Mitochondrial maturation involves two populations of mitochondria that are spatially segregated in embryonic cells—one in the cytoplasm and the other located inside of yolk platelets. Identification of the latter population is based in part on histochemical staining of DNA. While both mitochondrial populations mature structurally during PED, enzymatic evidence indicates that the intra‐yolk platelet population is not functional.Subcellular morphogenetic events observed during aerobic development are blocked when embryos are incubated under anoxia (pHi ⩽ 6.8) or aerobic acidosis (a quiescent state induced by artificially elevating CO2 to 60%; pHi = 6.8). Embryonic cells exposed to either of these experimental treatments for 12 h are essentially identical to cells of the undifferentiated, hour 0 embryo. Analogously, cytochrome c oxidase specific activity increases 230% over 12 h of aerobic development, but this ontogenetic increase is arrested by 12 h of anoxia or aerobic acidosis. Returning the acidified embryos to control aerobic conditions causes enzyme proliferation to resume at rates identical to aerobic, control values. A similar arrest of the proliferation of cytochrome c oxidase activity is seen under aerobic acidosis when CO2 is present at only 11%. Finally, alkalinizing the pHi of embryos under anoxia by adding ammonia to the medium results in a 37% increase in cytochrome c oxidase levels relative to those measured under anoxia. These data support pHi‐ dependent regulation of development in preemergence Artemia embryos.