HISTOLOGIC, IMMUNOHISTOCHEMICAL, AND ELECTRON MICROSCOPIC CHARACTERIZATION OF A MALIGNANT IRIDOPHOROMA IN A DWARF BEARDED DRAGON (POGONA HENRYLAWSONI).

A dwarf bearded dragon (Pogona henrylawsoni) was presented with a white subcutaneous mandibular mass and multiple nodules in the oral mucosa, heart, liver, kidney, intestine, and visceral fat. Histologically, the tumor consisted of densely packed spindle-shaped cells with brow intracytoplasmic pigment that exhibited white-blue birefringence with polarized light. Immunohistochemical staining was negative for S-100 and weakly positive with melan A. Electron microscopic examination revealed cytoplasmic irregular and oblong empty spaces, laminated and often arranged into short stacks, compatible with reflecting platelet profiles typically seen in iridophores. However, in unstained ultrathin sections, electron-dense crystalline material was present, which filled the empty spaces described for stained sections before. Based on histology, immunohistochemistry, and biologic behavior, a malignant iridophoroma was diagnosed. To the authors' knowledge, iridophoromas in lizards have rarely been characterized by using electronic microscopy. Moreover, this is the first description of an iridophoroma in a dwarf bearded dragon.