Physiological effects of hypoxic conditions during the plateau period on the chicken embryo

The chick embryo employs several adaptive responses to hypoxic challenges, affecting both metabolism and oxygen (O2) transport. The present study assessed the effects of hypoxic conditions (17% O2) during the plateau phase on embryonic metabolic rate, cardiovascular parameters, and development up to hatching. The study was divided into 2 experiments: (1) Control; 17% O2 for 6 h/d on E16–E18 (6H), and 17% O2 for 12 h/d on E16–E18 (12H), and (2) Control; 12H, and 17% O2 continuously for 72 h on E16–E18, (72H). Hypoxic embryos exhibited a significant increase in heart rate and an upward trend starting on E17 in hematocrit and hemoglobin levels. We observed a decrease in metabolism in 12H and 72H embryos during the plateau period; their oxygen consumption as well as yolk consumption were lower compared to Control and they hatched with a significantly lower body temperature, indicating lower heat production. There was no evidence of adaptation or long-term effects of exposure to 17% O2 for 6 h/d. Exposure to 72 h of hypoxic conditions led to significant physiological changes and had a detrimental influence on embryonic development and growth. In contrast, exposure to 12 h/d produced moderate hypoxic changes, which helped the embryo to cope with the stress without significant influences on its growth and development. The decrease in metabolism may represent a metabolic adaptation through a decrease in resting metabolic rate and lower heat production. Such alterations may affect post-hatch performance and energy allocation between maintenance and growth, especially under stress when there is increased oxygen demand.