Interruption of cardiac output does not affect short-term growth and metabolic rate in day 3 and 3 chick embryos

The heart beat of vertebrate embryos has been assumed to begin when convect ive bulk transport by blood takes over from transport by simple diffusion. To test this hypothesis, we measured eye growth, cervical flexure and rates of oxygen consumption ((V)over dot(O2)) in day 3-4 chick embryos denied ca rdiac output by ligation of the outflow tract and compared them with those of embryos with an intact cardiovascular system. Eye diameter, used as the index for embryonic growth, increased at a rate o f approximately 4.5-5% h(-1) during the observation period, There was no si gnificant difference (P > 0.1) in the rate of increase in eye diameter betw een control (egg opened), sham-ligated (ligature present but not tied) and ligated embryos. Similarly, the normal progression of cervical flexure was not significantly altered by ligation (P > 0.1). (V)over dot(O2) (ml O-2 g(-1) h(-1)) at 38 degreesC, measured by closed res pirometry, was not significantly different (P > 0.1) on day 3 in sham-ligat ed (14.5 +/- 1.9 mi O-2 g(-1) h(-1)) and ligated 17.6 +/- 1.8 ml O-2 g(-1) h(-1)) embryos. Similarly, on day 4, (V)over dot(O2), in sham-ligated and l igated embryos was statistically the same (sham-ligated 10.5 +/- 2.9 ml O-2 g(-1) h(-1); ligated 9.7 +/- 2.9ml O-2 g(-1) h(-1)). Expressed as a linear function of body mass (M), (V)over dot(O2) in sham-ligated embryos was des cribed by the equation (V)over dot(O2) = -0.48M + 24.06 (r(2) = 0.36, N = 1 8, P < 0.01), while (V)over dot(O2) in ligated embryos was described by the equation (V)over dot(O2) = -0.53M + 23.32 (r(2) = 0.38, N = 16, P < 0.01). The regression line describing the relationship between body mass and (V)o ver dot(O2) for pooled sham-ligated and ligated embryos (the two population s being statistically identical) was (V)over dot(O2) = -0.47M + 23.24. The slope of this regression line, which was significantly different from zero (r(2) = 0.30, N = 34, P < 0.01), was similar to slopes calculated from prev ious studies over the same range of body mass. Collectively, these data indicate that growth and (V)over dot(O2) are not d ependent upon cardiac output and the convective blood flow it generates. Th us, early chick embryos join those of the zebrafish, clawed frog and axolot l in developing a heart beat and blood flow hours or days before required f or convective oxygen and nutrient transport. We speculate that angiogenesis is the most likely role for the early development of a heart beat in verte brate embryos.