Lisa Gordon
Animal Health Laboratory, University of Guelph, Guelph, ON.
AHL Newsletter 2025;29(2):9.
An adult Holstein bovid presented to the referring veterinarian for an ocular mass (Fig. 1). A piece of tan, multinodular tissue that was not recognizable as a globe was received for histological examination. One edge of one sample was lined by a non-keratinizing stratified squamous epithelium (consistent with conjunctiva or anterior corneal epithelium). Between 90 – 100% of the tissue was effaced and replaced by sheets, streams, interlacing bundles, and packets of monomorphic, large, usually spindle and less commonly round cells embedded between dense trabeculae of stroma that included numerous melanomacrophages (Fig. 2). The cells had variably distinct cell borders, scant eosinophilic cytoplasm and an elongate, oval, or round nucleus with coarsely stippled chromatin and 0-2 large, prominent nucleoli. There is scattered fine brown/black pigment scattered within the stroma, and possibly within the cytoplasm of the neoplastic cells. There was 2-fold anisocytosis and anisokaryosis, and 4 mitotic figures were counted in 2.37 mm^2 (10 HPFs). A spindle cell neoplasm was diagnosed, and differentials included a melanocytic tumour and soft tissue tumour (soft tissue sarcoma).
The most frequent ophthalmic neoplasms in cattle are squamous cell carcinoma (SCC) and lymphoma, but the submitted neoplasm was not consistent with either entity. There are also rare case reports of post-traumatic ocular sarcomas in cattle.
Based on the differential list, immunohistochemistry was performed for S-100 (a cytoplasmic calcium-binding protein) and MelanA/PNL2 (melanocyte-specific antigens). S-100 has been demonstrated to positively label both melanocytic tumours and nerve sheath tumours (schwannoma, neurofibroma, and occasionally, malignant nerve sheath tumours) in cattle. A few individual case reports and case series have demonstrated positive labelling of well-differentiated melanocytic tumours with MelanA in cattle. It is unclear whether PNL2 is useful in this species. In this case, there was strong cytoplasmic labelling of the neoplastic cells with S100, and no labelling of cells with MelanA/PNL2 was detected.
Based on the overall histologic pattern and immunolabelling, the top differential for the neoplasm is a poorly differentiated melanocytic tumour, with nerve sheath tumour as another possibility. Cattle develop melanocytomas infrequently, and there are rare reports of (malignant) melanoma in cattle; these are predominantly reported in the skin. A congenital intraocular melanocytoma has been reported in a calf. Due to the paucity of research in this area, the key defining features of a melanocytoma vs. melanoma in cattle are unclear. While nerve sheath tumours are commonly reported in cattle, the globe does not seem to be a predilection site. Additional immunohistochemical stains (e.g., HMB45, a premelanosome protein, positive in some bovine melanocytic tumours) could have allowed for additional subclassification of this neoplasm, but were not pursued for this case.
Figure 1. Globe, left. A tan mass occupies the left orbit.
Figure 2. Ocular mass. 2A. 40X, H&E stain. The cells in this field are mostly round to polygonal in shape. There are scattered deeply pigmented cells, interpreted as melanomacrophages (thick arrow). There is scattered melanin pigment which could be within the interstitium, or possibly within the cytoplasm of the neoplastic cells (thin arrow). 2B. 10X. IHC S110; brown chromogen. There is strong cytoplasmic labelling of the neoplastic cells with S100. 2C. 10X. IHC MelanA/PNL2; purple chromogen. No labelling of cells with MelanA/PNL2 was detected.
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