Embryonic control of heart rate: examining developmental patterns and temperature and oxygenation influences using embryonic avian models.

Long-term measurements (days and weeks) of heart rate (HR) have elucidated infradian rhythms in chicken embryos and circadian rhythms in chicken hatchlings. However, such rhythms are lacking in emu embryos and only rarely observed in emu hatchlings. Parasympathetic control of HR (instantaneous heart rate (IHR) decelerations) occurs at ∼60% of incubation in both precocial and altricial avian embryos, with sympathetic control (IHR accelerations) becoming more prevalent close to hatching. A large increase in avian embryonic HR occurs during hatching (presumably an energetically expensive process, i.e. increased oxygen consumption M(O) ₂), beginning during pipping when a physical barrier to O(2) conductance is removed. Alterations in ambient O(2) have little effect on early embryonic HR, likely due to the low rate of M(O)₂ of early embryos and the fact that adequate O(2) delivery can occur via diffusion. As M(O)₂ increases in advanced embryos and circulatory convection becomes important for O(2) delivery, alterations in ambient O(2) have more profound effects on embryonic HR. Early embryos demonstrate a wide ambient temperature (T(a)) tolerance range compared with older embryos. In response to a rapid decrease in T(a), embryonic HR decreases (stroke volume and blood flow are preserved) in an exponential fashion to a steady state (from which it can potentially recover if re-warmed). A more severe decrease in T(a) results in complete cessation of HR; however, depending on developmental age, embryos are able to survive severe cold exposure and cessation of HR for up to 24h in some instances. The development of endothermy can be tracked by measuring baseline HR during T(a) changes. HR patterns change from thermo-conformity to thermoregulation (reverse to T(a) changes). Further, IHR low frequency oscillations mediated by the autonomic nervous system are augmented at low T(a)s in hatchlings. Transitions of baseline HR during endothermic development are unique to individual avian species (e.g. chickens, ducks and emu), reflecting differences in life history.