Apoptosis in the developing human heart resembles apoptosis in epithelial tissues.

It is widely accepted that apoptosis plays an important role in the development of the heart as well as in different heart diseases. Despite extensive research efforts, many issues regarding apoptosis in the heart remain unsolved, including the detection of apoptotic cardiomyocytes, their morphological features, the mechanisms of their removal and the clinical significance of apoptosis in the heart. It has been suggested that fetal cardiomyocytes resemble epithelial tissues. To test this hypothesis, we analyzed the expression of an epithelial marker cytokeratin 18 (CK18) and caspase-cleaved-CK18, recognized by antibody M30, as well as the expression of cleaved caspase-3 and desmosomal and classical cadherins, major components of desmosomes and adherens junctions in fetal hearts in comparison to infant and adult human hearts. We found that, in fetal hearts, cardiomyocytes expressed CK18 and that apoptotic cardiomyocytes expressed caspase-cleaved CK18, being recognized by antibody M30. Furthermore, desmosomal and classical cadherins exhibited a membraneous reaction similar to epithelial tissues. In adults and children after the age of 6 months, cadherins were localized in the intercalated disks, cardiomyocytes lost CK18 expression and apoptotic cardiomyocytes were no longer recognized by M30. We conclude that apoptosis in the developing human heart resembles apoptosis in epithelial tissues, exhibiting different characteristics than in the adult human heart.