Growth allometry of the myocardium in human embryos (from stages 15 to 23)

The relative growth of the myocardium was studied in 27 staged human embryos (Carnegie stages). The volume of the myocardium was determined for each embryo according to Cavalieri's principle (by using point-counting planimetry to determine the area of the profiles of the myocardium). The volume of the myocardium (variable Y) was correlated to embryonic crown-rump length (variable X in millimeters) and age (in days). The bivariate allometric equation was used as Y = aXb. The scatterplot was discontinuous, presenting two trends during the postsomitic period. The first part was composed of embryos staged from stages 15 to 20, and the second part by embryos staged from stages 21 to 23. The breakpoint between these different trends was found at the level of stage 20 (embryo of 22 mm in crown-rump length and age nearly of 52 days). From stages 15 to 20, the growth rate of the myocardium was allometrically negative. On the other hand, from stages 21 to 23 this growth rate was moderately allometrically positive. These differences in growth of the myocardium were analyzed and, at least partially, might be due to the functional circulatory increase in the peripheral vascular bed in correlation to the cardiac hemodynamic demand required at the end of the embryonic period proper.