Abstract
Pigment cell tumors also known as chromatophoromas are cutaneous neoplasms originating from the pigment cells (chromatophores) in the dermis of teleosts, reptiles, and amphibians. There are four major types of chromatophores, which are identified by the pigment type they contain, and therefore their color. Melanophores contain melanin, and are dark brown to black, xanthophores contain carotenoids, and are yellow, iridophores contain purines, and are colorless and refractile, and erythrophores contain both carotenoids and pteridines, and are red.1 All chromatophores are believed to be of neural crest origin.1,2 The etiology of these tumors is unknown, but is thought to potentially have genetic, age-related, and environmental components.1,3 Five separate fish from different facilities presented to the University of California Davis Veterinary Medical Teaching Hospital over a period of seven months with similar masses which were all diagnosed as pigment cell tumors/chromatophoromas. Two of the submissions were biopsy samples, and the other three were received as necropsies, with the severity of the lesions varying from moderately to severely invasive. All of the neoplastic lesions were located along the dorsum or dorsal skull/periocular region of the fish examined. Due to the distribution of the lesions, a solar-induced etiology is hypothesized. In these cases, neoplastic cells were predominantly round to spindloid with a moderate amount of eosinophilic to basophilic cytoplasm, and were commonly arranged in short interlacing bundles and streams. Nuclei were round to ovoid with finely stippled chromatin, and anisocytosis and anisokaryosis ranged from mild to moderate. Most subtypes of chromatophoromas appear similar histologically, which interferes with the ability to make a definitive diagnosis of a certain subtype through evaluation of typical H&E stained histology slides.4 A battery of immunohistochemistry (IHC) markers and a special stain were performed on formalin-fixed, paraffin-embedded samples from the various cases discussed here in attempt to better characterize the neoplastic population and confirm chromatophore origin. All neoplasms were negative for SOX-10, which is a marker used to identify cells of neural crest origin in other species. One case exhibited moderate immunoreactivity with Melan-A, which is currently considered the most specific immunohistochemical marker for melanomas in other species.5 The same case exhibited weak immunoreactivity with PNL-2, which is a marker directed against an uncertain melanocytic antigen. In addition, one of the cases included pigment granules which were highlighted with a Fontana Masson stain, which is a special stain used to highlight melanin granules. The specificity of these IHCs and special stains in fish has not yet been fully investigated, and therefore their significance is uncertain. The purpose of this case series is to present a comparative compilation of cases with presumed chromatophoromas, and to discuss the results of gross and histopathologic examination, highlighting the need for future research validating or generating specific special stains for teleosts.
Acknowledgements
The authors wish to thank Dr. Verena Affolter, Dr. Kevin Keel, Dr. Chuck Mohr, and Dr. Patricia Pesavento of the University of California Davis, Veterinary Medical Teaching Hospital Anatomic Pathology Department, Dr. Alvin Camus of the College of Veterinary Medicine, University of Georgia, and the University of California Davis, Veterinary Medical Teaching Hospital histopathology laboratory and for their contribution to the study.
*Presenting author
+Student presenter
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