Avian embryos in hypoxic environments

Avian embryos at high altitude do not benefit of the maternal protection against hypoxia as in mammals. Nevertheless, avian embryos are known to hatch successfully at altitudes between 4000 and 6500 m. This review considers some of the processes that bring about the outstanding modifications in the pressure differences between the environment and mitochondria of avian embryos in hypoxic environments. Among species, some maintain normal levels of oxygen consumption (V̇_O2) have a high oxygen carrying capacity, lower the air cell-arterial pressure difference (P_AO2-P_aO2) with a constant pH. Other species decrease V̇_O2, increase only slightly the oxygen carrying capacity, have a higher P_AO2-P_aO2 difference than sea-level embryos and lower the P_CO2 and pH. High altitude embryos, and those exposed to hypoxia have an accelerated decline of erythrocyte ATP levels during development and an earlier stimulation of 2,3-BPG synthesis. A higher Bohr effect may ensure high tissue P_O2 in the presence of the high-affinity hemoglobin. Independently of the strategy used, they serve together to promote suitable rates of development and successful hatching of high altitude birds in hypoxic environments.