[Foetal Rhabdomyoma. Case report and ultrastructural study].

Benign voluntary muscle tumours are exceedingly rare. Only 9 fetal and 35 adult type rhabdomyomata have been reported in the literature, former usually in children, the latter in adults. Both types affect the musculature of the head and neck almost exclusively. In this ultrastructural study of a fetal rhabdomyoma in a girl aged eleven years, we have found that the tumour is composed of cells showing myoblastic or myotubular differentiation although the myofilaments are poorly arranged and the myofibrils irregularly distributed. Motor-end plates have not been observed. These observations, together with the light microscopic findings and a review of previously described cellular characteristics, suggest that fetal and adult rhabdomyoma are different stages in the life history of the same tumour. Whether the lesion should be considered to be a hamartoma or a neoplasm is uncertain.