Human fetal heart development after mid-term: morphometry and ultrastructural study.

A total of 44 normally developed human fetal hearts aged from 17 to 40 weeks gestation were provided for routine ultrastructural and morphometric studies. For morphometric analysis, the maximal thicknesses of the anterior, lateral and posterior wall of both ventricles and that of interventricular septum were measured and the left-to-right thickness ratios calculated. The cross-sectional areas of both ventricles in the plane of the greatest heart diameter were measured with computerized image analysis system. Data were analyzed using linear regression and one-way analysis of variance. Myofibril formation occurred by attachment of thin filaments into amorphous Z materials which were presented in sarcolemmal plaques, sarcoplasmic condensations, desmosomes and in Z lines. From these Z centers, myofibrils radiated many directions and branched and anastomosed with further development. This pattern of myofibrillar development continued throughout the whole fetal period. A transverse tubule system was clearly evident in later fetal development. It occurred by invagination of sarcolemma into myocardial cells and formation of subsarcolemmal caveolae. Mitochondria, well-developed Golgi complexes, glycogen granules and well-developed microvessels were found throughout the whole fetal period. Binucleated myocytes appeared by 32 weeks gestation and this suggests that myocyte hyperplasia may cease before birth in humans. The growth of both ventricular walls, the interventricular septum and that of both ventricular cross-sectional areas showed linear regression, and the left-to-right wall thickness ratios were nearly constant. Also, there were no differences in morphometric data between the left and right ventricles. In conclusion, development of the myocyte is an ongoing process which may be continued in the post-natal period in humans, and our statistical results do not support the theory of the right ventricular dominance during the fetal period.