ABSTRACT. A 6-year-old male red squirrel (Sciurus vulgaris orientis) developed bilateral tumors of upper and lower eyelids. The tumors in the left lid recurred despite surgical removal. Necropsy revealed metastasis to the lung. The neoplastic cells were epithelioid and highly pleomorphic, and only a few cells contained melanin granules. Occasionally melanoma cells were immunoreactive for S100, neuron-specific enolase and vimentin, and a small number of cells for cytokeratin. Ultrastructurally, the presence of premelanosomes was confirmed in the cytoplasm. Possible presence of cytokeratin-positive neoplastic melanocytes should be taken into account when differentiating a nonpigmented epithelioid melanoma from other tumors such as anaplastic carcinomas.KEY WORDS: cytokeratin, malignant melanoma, squirrel.J. Vet. Med. Sci. 64(3): 261-264, 2002 Malignant melanoma of the eyelid has been reported in dogs [8], cats [16] and man [11]. Dutta et al. [2] reported an amelanotic malignant melanoma on the cervical skin of a ground squirrel. Although considered to be absent in canine and feline melanomas [10,16], cytokeratin has been observed in human malignant melanomas [4]. Our report describes a case of malignant melanoma of the eyelid skin in a red squirrel, in which cytokeratin immunoreactivity was detected.A 6-year-old, male red squirrel (Sciurus vulgaris orientis) had a 1-year history of right palpebral tumor masses and a 4-month history of left ones. Both upper and lower lids were involved, and the tumor of the upper left lid was excised for histological examination. The other three tumors were removed 12 days later. Because of corneal ulceration, the right eye was enucleated 8 days later. Recurrent tumors on the upper and lower lids of the left eye were excised 22 days after eye extirpation, but a recurrence was detected on the left lower lid and surgery was performed 14 days later. The left eye was removed owing to corneal ulceration 19 days after surgery, and the animal died the next day. The excised tumors were white and homogeneous, and ranged from 1.5 to 1.0 cm in diameter. At necropsy, a 5 mm diameter white lesion was detected in the lung.Surgically resected eyelid tumors and necropsied tissues were fixed in 10% buffered formalin and processed according to routine histological techniques. Paraffin sections were stained with hematoxylin and eosin (HE) and MassonFontana stains. Selected paraffin sections were stained by immunoperoxidase techniques using polyclonal antibodies to S100 protein (Nichirei, Tokyo, Japan), neuron-specific enolase (NSE) (Lipshaw, Detroit, U.S.A.) and cytokeratin (BioGenex Laboratories, San Ramon, U.S.A.) and a monoclonal antibody to vimentin (Dako, Carpinteria, U.S.A.). A universal kit was used in the subsequent processes. Small pieces taken from formalin-fixed tissues were post-fixed in osmium tetroxide and embedded in epoxy resin. Thin sections were mounted on copper grids, stained with uranyl acetate and lead citrate, and examined with a transmission electron microscope.The tumors e...
