Myocardial microvasculature in fetuses and neonates of rat. Stereological study

The myocardial development is anchored on the development of the blood vessels. We studied 5 fetuses in each 18.5 and 20.5 day of gestation and 5 neonates in each 2 and 7 day postnatal. Only one animal randomly selected per litter was used. Fragments of the ventricular myocardium were embedded in paraffin with random orientation, sectioned with 5 microns thickness and stained with routine methods. Myocardial capillaries were counted into the test-area and test-points were considered when falling over capillaries since these did not cut the forbidden line. The length and volume densities of the myocardial capillaries were determined (Lv[cap] and Vv[cap], respectively). The absolute length and volume of the myocardial capillaries (L[cap] and V[cap], respectively) were estimated by the product of the Lv[cap] and Vv[cap] by the volume of the heart (V[heart]). The Lv[cap] decreased greatly after birth (from 1,752.4 to 684.2/mm2). These changes were significant only between the two age groups of fetuses (days 18.5 and 20.5) and neonates of age day + 7. The Vv[cap], equal to Lv[cap], was practically unchangeable during fetal period but decreased greatly after birth. The Vv[cap] was different between early fetuses and early and late neonates. Difference was significant also between late fetuses and late neonates when the Vv[cap] decreased from 8.2 to 2.1%. Both L[cap] and V[cap] presented a tendency of increase during the fetal and neonatal periods. These absolute parameters had a significant grow-up between fetuses of age 18.5 and 20.5 days, decrease immediately after birth but improved its magnitude in late neonates. These results suggest that the relative extension and volume of the capillary bed are greater in fetal period than in early postnatal period. However, the continuous growth of the cardiac myocardium improves the absolute length and volume of the capillaries.