Diagnosis of childhood malignancies, in particular solid tumors, is an enterprise that can be laden with a variety of uncertainties, inconsistencies, and morphologic subtlety, primarily caused by their tendency to recapitulate early organogenesis This dilemma 1s manifested by the common sobriquet “small, round, blue-cell tumor,” which reflects their frequent composition by primitive, undifferentiated cells with circular nuclei, dense chromatin, and minimal amounts of discernible cytoplasm. Nevertheless, to varying degrees these tumor cells exhibit heterogeneously expressed cytodifferentiation that may be easily observed via light microscopy or require a search via transmission electron microscopy for specific subcellular organelles. Two examples are the neuroblastoma, which is usually composed of a mixture of primrtrve neuroblasts and partially differentiated ganglion cells, and the rhabdomyosarcoma, which is typically composed of a blend of primitive mesenchyme and incompletely developed rhabdomyoblasts
