Aerobic scope in chicken embryos

We investigated the aerobic scope of chicken embryos, that is, the margin of increase of oxygen consumption (V̇O2) above its normal value. V̇O2 was measured by an open-flow methodology at embryonic ages E3, E7, E11, E15, E19 and at E20 at the internal (IP) and external pipping (EP) phases, at the normal incubation temperature (Ta = 38 °C), in hypothermia (Ta = 30 °C) and in hyperthermia (Ta = 41 and 44 °C). In the cold, Q10 averaged ~ 2 at all ages, except in IP and EP when lower values (~ 1.5) indicated some degree of thermogenesis. In hyperthermia (38-44 °C) Q10 was between 1 and 1.4. Hyperthermia had no significant effects on V̇O2 whether the results combined all ages or considered individual age groups, except in IP (in which V̇O2 increased 8% with 44 °C) and EP embryos (+ 13%). After opening the air cell, which exposed the embryo to a higher O2 pressure, hyperthermic V̇O2 was significantly higher than in normothermia in E19 (+ 13%), IP (+ 22%) and EP embryos (+ 22%). We conclude that in chicken embryos throughout most of incubation neither heat nor oxygen availability limits the normal (normoxic-normothermic) values of V̇O2. Only close to hatching O2-diffusion represents a limiting factor to the embryo's V̇O2. Hence, embryos differ from postnatal animals for a nearly absent aerobic scope, presumably because their major sources of energy expenditure (growth and tissue maintenance) are constantly maximized.