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Cardiac rhabdomyoma is the most common tumour of the heart in infancy and childhood. The clinical presentation, diagnosis and histopathological characteristics have been extensively studied; however, reports on the ultrastructure and histogenesis of this lesion are scanty and inconclusive. The case to be discussed is that of a 10-year-old male who presented with a cardiac rhabdomyoma occupying almost the entire ventricular apex. Ultrastructurally, the rhabdomyoma cells have a central, deeply-indented nucleus surrounded by an admixture of mitochondria and sarcomeres. The remainder of the cytoplasm is occupied by pools of glycogen granules, randomly-orientated myofibrils and small mitochondria. Intercellular junctions are numerous and consist of alternating zonula occludens and macula adherens. Typical satellite cells, sharing a common basement lamina are seen apposed to the rhabdomyoma cells. It is tempting to postulate that the proliferation of the rhabdomyoma cells is accomplished by differentiation of satellite cells, a process known to occur in skeletal muscle. Ultrastructurally, the rhabdomyoma cells are indistinguishable from Purkinje cells. The presence of Purkinje-like cells in ectopic locations within the heart and their association with satellite cells is likely a form of embryological atavism.

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