Lack of physiological plasticity in the early chicken embryo exposed to acute hypoxia.

By exposing chicken embryos to hypoxia (10%) acutely (2, 4, and 6 hr) during early development (2, 3, and 4 days) we tested the hypothesis that hypoxia has an impact on embryonic growth and impairs cardiac development at the time cardiac morphogenesis is taking place. After the hypoxic perturbation, the embryos were allowed to develop until day 9, when embryo mass, heart mass, and rate of oxygen consumption were recorded. Four-day-old embryos exposed to 6 hr of hypoxia showed an increased mortality (38.9% versus 18% for controls), indicating the immediate effect of hypoxia on survivability. While only 8% of the controls displayed morphological abnormalities, 3- and 4-day-old embryos exposed for 6 hr showed more frequent developmental abnormalities (25% and 30% respectively). No significant differences in embryo or heart mass were found except in 4-day-old embryos exposed for 2 hr. Mass-specific oxygen consumption was not different between controls and embryos exposed to hypoxia at 2 or 3 days of development, but it was increased in 4-day-old embryos exposed for 4 hr (P < 0.05). These results suggest that an acute hypoxic episode does not have an impact when occurring very early in development (days 2 or 3). However, when the hypoxic episode occurs on day 4, survivability is largely decreased. Considering the lack of permanent effects on the surviving embryos, we suggest that the early embryo resorts to a simple strategy of death or survival, and the individual capacity for survival must be based on interindividual differences rather than the existence of compensatory mechanisms. J. Exp. Zool. 286:450-456, 2000. Copyright 2000 Wiley-Liss, Inc.