Response of egg temperature, heart rate and blood pressure in the chick embryo to hypothermal stress
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- Tazawa, H. & Nakagawa, S. J Comp Physiol B (1985) 155: 195. doi:10.1007/BF00685213
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Chicken eggs incubated for 12–18 days were catheterized via the allantoic artery and temperature was monitored simultaneously using a probe positioned in the allantoic fluid adjacent to the embryo. Fluid temperature (referred to as egg temperature), arterial pressure and heart rate were measured following abrupt exposure to a lower environmental temperature (ca., 26∼28°C). Egg temperature and heart rate diminished exponentially: The rate of decline of egg temperature approximated Newton's law of cooling, the rate coefficient being 0.022∼0.025°C/min·°C throughout the incubation period from 12 to 18 days. The half time of temperature response of the egg was 27∼28 min. The response was much slower than that of fertile unincubated eggs (Kaplan et al. 1978), suggesting that the extraembryonic fluids act as a thermal buffer in embryonated eggs. The heart rate response in older embryos (17∼18 days) changed in the same manner as egg temperature, while in younger embryos (12∼16 days) the heart rate diminished more quickly than the change in egg temperature. During development the cardiac pacing of the embryo suggests that it becomes resistive to mild cold stress. The systolic pressure remained almost unchanged or even increased during one hour of exposure as the embryo developed, while the diastolic pressure decreased steadily after exposure irrespective of development. The 18-day-old embryos retained the systolic pressure unaltered during 3-hour exposure. In embryos 3∼4 days prior to hatching the functional capacity of the heart apparently allows continued pumping even after prolonged exposure to low environmental temperature.