A rhabdomyoblast is a cell type which is essential to the diagnosis of a rhabdomyosarcoma. A rhabdomyoblast found histologically is considered diagnostic for embryonal, alveolar, and pleomorphic rhabdomyosarcomas. Histology will show an elongated or round cell, exhibiting an embryonic morphology. Occasionally cells will exhibit cross striations by light microscopy, reflecting sarcomere formation and advancement of differentiation. This differentiated phenotype increases following chemotherapy and radiotherapy.
Elongated cells are referred to as strap cells or tadpole cells. Rhabdomyoblasts contain an eosinophilic granular cytoplasm rich in thick and thin filaments. The presence of desmin and myogenin can be detected using immunohistochemistry.