Ultrastructural and functional features of the developing mammalian heart: a brief overview.

The heart undergoes marked ultrastructural alterations during fetal and postnatal development. Early in fetal development, cardiac myocytes contain abundant pools of glycogen, scattered mitochondria and sparse, peripheral myofibrils. Transverse tubules are absent, and sarcoplasmic reticulum and intercalated discs are poorly developed. During late fetal and early postnatal development, myofibrils extend into the myocyte interior and attain a mature appearance, and the glycogen pools are reduced in size. In addition, transverse tubules develop and the morphological appearance of the sarcoplasmic reticulum and intercalated disc becomes increasingly complex. Experimental studies in sheep, corroborated by clinical studies in humans, also point to marked functional changes during development. In the fetus, the right ventricle is the dominant pumping chamber because right ventricular output exceeds left ventricular output, while pulmonary arterial and aortic pressures are similar. This functional difference is reflected in myocardial blood flow patterns, with blood flow to the right ventricle exceeding that to the left ventricle. The ventricular outputs equalize after birth, but a functional left ventricular dominance rapidly emerges following a postnatal increase in systemic vascular resistance and a decrease in pulmonary vascular resistance. This postnatal switchover in functional dominance is accompanied by a corresponding alteration in the relative level of ventricular myocardial blood flows. Consistent with right ventricular dominance in utero, myocytes in the right ventricle of the fetal sheep are larger and contain more myofibrillar material than those in the left ventricle. Left ventricular myocytes become larger than right ventricular myocytes after birth, but this adaptation to altered postnatal haemodynamics requires some weeks to become fully established.