Oxygen supply of early chick embryo in normoxia and hypoxia

The oxygen partial pressure (PO2) was measured in the large vessels of the yolk sac of 4- and 6-day old chick embryos using recessed-tip oxygen microelectrodes. The eggs were either incubated in air or in a hypoxic gas mixture (13.5% O2/86.5% N2). For the measurements, a part of the eggshell and the shell membranes were removed. Between 4 and 6 days of development, the mean PO2 in the anterior and posterior vitelline veins, which is approximately equal to the PO2 of the blood leaving the gas exchange vessels, was about 12 kPa (92 torr) in the normoxic group and about 9.3 kPa (70 torr) in the hypoxic group, suggesting that oxygen transport is diffusion-limited. The mean PO2 in the vitelline arteries, which are supplied by the dorsal aorta, was found to be in the range of 6-6.5 kPa (45-49 torr) in both normoxia and hypoxia. Taking into account that the eggshell and the shell membranes form an additional diffusion resistance, we have evaluated the PO2 and oxygen saturation of the blood of intact normoxic and hypoxic embryos.