Abstract

Histological and ultrastructural studies have been undertaken on a perineal rhabdomyosarcoma from a newborn child. The spontaneous tumour has the typical feature of mesenchymoma. The recurrent tumour, however, displays some rhabdopoietic characteristics. The myosin of the recurrent tumour has been extracted and compared with human fetal myosin. These two myosins are identical in their synthetic filaments and their light-chain composition. Nevertheless, whereas the ATPase activity of fetal myosin can be stimulated considerably by increasing the ca2+ concentration, that of tumoral myosin remains very low. These results show that there are isoenzymes of myosins and there must be differences in the myosin heavy chains, particularly in the active sites. These findings are identical with those seen in experimental rhabdomyosarcoma.