Morphological, immunohistochemical, and ultrastructural findings in an embryonal rhabdomyosarcoma of a newborn thoroughbred foal

Abstract

This study reports the rare case of a rhabdomyosarcoma in the shoulder of a newborn Thoroughbred foal. Histomorphologic, immunohistochemical, and ultrastructural investigations revealed an embryonal rhabdomyosarcoma with various differences: small round undifferentiated cells, rhabdomyoblast-like neoplastic cells, and spindle-shaped cells. Desmin expression appears in 30% of small round cells and in as many as 90% of spindle-shaped cells. Vimentin co-expression occurs in 90% of small round and spindle-shaped neoplastic cells but only in few (30%) rhabdomyoblast-like cells. Myoglobin was detected in 50% of the rhabdomyoblast-like cells and in 30% of spindle-shaped cells but not in small round neoplastic cells. This is the first time that rhabdomyosarcoma markers Myf-4 and Myo D1 were found in horses. Myf-4 and Myo D1 were expressed in about half of the neoplastic cells in immediately fixated specimens. Ultrastructural sarcoma-like packets and formations of primitive Z-bands, as well as a discontinuous basal lamina, were found and served to confirm our diagnosis.