Correlation between blastocyst oxygen consumption and trophoblast cytochrome oxidase reaction at initiation of implantation of delayed mouse blastocysts

Delayed blastocysts had an oxygen consumption of 0 ·24 nl/h, while only 4h after an oestrogen injection the respiration had increased nearly two fold, remaining at this level both 8 and 18 h after activation for implantation. The mitochondria of delayed blastocysts exhibited no positive cytochrome oxidase reaction, neither in the trophoblast nor in the embryoblasts. A few mitochondria at 8 h and most of those of blastocysts activated for 18 h were positive. It is suggested that the activation of blastocysts for implantation is initiated by a surge of substrates for glycolysis into the uterine secretion causing an increased energy production by glycolysis which in turn makes possible an increase of the cytochrome oxidase activity of the mitochondria thus getting oxidative phosphorylation into action.