Histological Changes in the Developing Heart of Human Fetuses

Histological changes of human fetal hearts wereobserved in fetuses from the 3rd to 7th month ofgestation to analyze the myocardium, endocardium, and epicardium as well as Purkinje fiber cells. In the 3rd month, cardiac muscle cells were irregular in shape and arrangement. In the 7th month, they were more homogenous in shape, longer than in3rd month, and typically arranged in parallel. These changes became evident from the 6th month, and by the 7th month their histological features had become almost the same as those in adults. There were no significant regional differences in the morphology ofthe cardiomyocytes among the parts of the heart. In both the endocardium and picardium, a relatively rich underlying connective tissue layer was observed in the 3rd month, whereas it decreased significantly thereafter. Subendocardial Purkinje fibers were notclearly identified in the 3rd month, but they wereidentified from the 4th month. The present findingsshow normal morphological development of human fetal heart and suggest the corresponding functional maturation.