Growth of capillaries and myocardial cells in the normal rat heart.

The normal growth process of myocardial cells and capillaries in the late postnatal period (after weaning) was studied in 33 normal male Wistar rats (group I: 5 weeks, n = 9; group II: 7 weeks, n = 7; group III: 13 weeks, n = 8; group IV: 52 weeks, n = 9). The rats were fixed by retrograde vascular perfusion via the abdominal aorta. Two transverse and 2 longitudinal sections per animal were selected at random from the left ventricular papillary muscles for light and electron microscopic stereological investigation. Length density and surface density of myocardial cells and capillaries were estimated with correction for partial anisotropy and curvature by means of the mathematical model of a Dimroth-Watson orientation distribution. The results were analyzed by allometric techniques. Relative left and right ventricular weight decreased continuously throughout the growth process; in terms of allometry, ventricular weight was proportional to (body weight). The capillaries showed predominantly longitudinal growth with nearly constant cross-sectional area, whereas the myocardial cells grew by nearly harmonic enlargement into all three directions of space. The relation between total capillary length and left ventricular weight could be described in close approximation by the allometric model (r = 0.98, P less than 0.001). Corresponding to the result that total capillary length was proportional to (left ventricular weight)0.71, the myocardial capillarization (length, surface area, and volume of capillaries per unit tissue volume) decreased with increasing heart size. As allometry makes possible the quantitation of the intensity of growth processes by a single dimensionless number (the allometric exponent), this method permits a convenient interlaboratory comparison of growth studies, and it allows a comparison of capillary reactions during maturation and in experimental cardiac hypertrophy.