Facial myxomatous chondroma in a neon tetra (Paracheirodon innesi)

Neoplasms of mesenchymal origin are generally rare in fish, with the only exception of fibromas and fibrosarcomas. Spontaneous cartilage tumors have been reported occasionally; examples are a chondroma of the gill arch in a brown trout, an osteochondroma of the operculum in a jewel fish, a cranial chondrosarcoma in a paddlefish and ocular chondrosarcomas in mangrove rivulus. Bone neoplasms are also uncommon, and reports include a branchial osteosarcoma in a barbel, a cutaneous osteosarcoma in a channel catfish, a branchial osteochondroma in a gilthead sea bream and multiple osteomas in thinspine sea catfishes. Furthermore, osteosarcomas, chondromas, rhabdomiosarcomas and leyomiosarcomas have been reported in laboratory fishes exposed to N-methyl-N’-nitro-N-nitrosoguanidine (MNNG). Mouth, lips, and jaws are frequent site of neoplasia in fish, presenting as proliferative, nodular exophytic growths, generally resembling papillomas; they have often been histologically classified as fibromas or fibrosarcomas. Advanced, enlarged lesions may eventually result in death of the fish due to compromised feeding and respiration. A three-cm neon fish (Paracheirodon innesi) presented a single, well-circumscribed, nodular, roughly spherical, perlaceous, firm to hard, 4 mm in diameter mass in the right facial region; due to a difficulty in feeding it was euthanized with lethal dose of anaesthetic. The whole fish was fixed in formalin, embedded in paraffin-, and 4-μm sections were cut. From the right maxillar region a nodular, well circumscribed mass, covered by an epithelial layer and formed by abundant, highly cellular cartilaginous tissue and loose myxomatous matrix, was detected. The cartilaginous tissue contained nests of chondrocytes lying in elliptic lacunae, consistent with multicellular isogenous cell groups of mature hyaline cartilage. Beside these lacunae, large cells (50-80 μm in diameter), with distinct cell borders and single large, clear vacuoles (physalipherous cells) were present; nuclei were small, round to oval shaped, hyperchromatic and often eccentrically located. In both these types of cells, nucleoli were only occasionally discernible and mitotic figures were rare. Poorly cellular myxomatous tissue was also present. A diagnosis of facial myxomatous chondroma was made. Histological sections were stained with Alcian-PAS and Masson’s trichrome, in order to demonstrate the presence of proteoglycans and collagen within the neoplastic tissue. As defects in the pathways of cell cycle control have been reported in fish tumors, particularly in the zebrafish, immunohistochemistry was also performed using antibodies against Bcl-2 protein, telomerase reverse transcriptase catalytic subunit (h-TERT) and p53 protein. Indeed, h-TERT may suppress apoptosis interacting with Bcl-2 and p53 proteins. The antibodies tested did not show the immunohistochemical overexpression of any of these markers, which suggests that the breakdown of other mechanisms could be involved in the oncogenesis of this myxomatous chondroma