Cardiac rhythms in avian embryos and hatchlings

Abstract

Avian embryos develop within the confines of an eggshell independent of maternal physiological functions. The eggshell provides not only an airway for diffusive gas exchange of the embryo, but also a protective barrier between the embryo and the surroundings. The wide space outside the eggshell is available for measurements of embryonic physiological functions. Taking advantage of the hard eggshell, several methods have been developed to measure cardiogenic signals while maintaining an adequate gas exchange through the eggshell and cardiac rhythms of developing embryos have been determined. This review introduces the measuring methods for cardiogenic signals of embryos and shows daily changes (developmental pattern) of some cardiovascular variables. Daily determinations of heart rate (HR) from the cardiogenic signals exhibit developmental patterns of mean HR (MHR) in embryos of various species of birds. An allometric relationship is found between the fresh egg mass and embryonic MHR in 34 species of birds investigated so far. The developmental patterns of MHR are also determined by continuous measurements of HR to investigate a presence or absence of infradian, circadian and ultradian rhythms in developing embryos and also hatchlings. For investigation into beat-to-beat rhythms, instantaneous heart rate (IHR) is determined in prepipped and pipped embryos and hatchlings. In chickens, IHR baseline begins to fluctuate with appearance of transient decelerations with subsequent accelerations during the middle of incubation and distinctive patterns of IHR fluctuations develop in embryos and hatchlings. The patterns and baseline of IHR respond to exposure to altered environments and measurements of their responses have a possibility to elucidate development of some physiological functions.