Mixed Müllerian tumors of the uterus. Ultrastructural studies on the differentiation of rhabdomyoblasts.

In the present study the differentiation of rhabdomyoblasts of two carcinosarcomas of the uterus is analysed electronmicroscopically. During the development of rhabdomyoblasts three cell types can be distinguished: 1. the undifferentiated mesenchymal cell with abundant ribosomes but few other cell organelles. Usually these cells are already associated to each other in strands of 2-4 cells. Occasionally cytoplasmic areas with numerous nonspecific filaments can be observed. 2. the light rhabdomyoblast which is characterized by its conspicuous nonspecific cytoplasmic filaments and its reduction in ribosomes. Specific myofilaments can be visualized only occasionally in this cell type. 3. more differentiated rhabdomyoblasts. These cells can be identified clearly by the presence of large numbers of myofibrils and myofibrillar fragments. Higly organized myofibrils as seen in skeletal muscle are seldom found. Transitional forms between these cell types are also observed. The functions of Z-bodies and of the sarcotubular system in the process of myofibrillar differentiation are discussed.