Confocal Laser Scanning Analysis of an Equine Oral Mast Cell Tumor with Atypical Expression of Tyrosine Kinase Receptor C-KIT

A 20-year-old female horse showed a nodular, firm, focal ulcerated mast cell tumor at the right dorsobuccal face of the tongue. Histologically, the nonencapsulated tumor consisted of dense, infiltrating aggregates of well-differentiated, Cresyl violet-positive mast cells accompanied by numerous eosinophils. Furthermore, they exhibited a strong, diffuse, intracytoplasmatic immunohistochemical signal for tryptase and a faint membrane-associated and perinuclear signal for tyrosine kinase receptor KIT. Confocal laser scanning microscopy confirmed an aberrant spatial colocalization of KIT in the Golgi apparatus, which may be the result of a defective protein processing within the tumor cells. The tumor was not associated with a poor prognosis.