Page 134 - Differential Diagnosis in Small Animal Cytology, The Skin and Subcutis

Epithelial Tumours
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8.7 Merkel Cell Tumour
VetBooks.ir Neuroendocrine tumour originating from mechanoreceptors present in the epidermis and

follicular epithelium. Also known as Merkel cell carcinoma.

Clinical features

• Very rare neoplasm observed in middle- aged or older dogs and cats.
• Lesions usually appear as single, variable sized, firm, intradermal, flesh-coloured red
nodules or plaques. The overlying skin may be alopecic and ulcerated. In cats, head
and neck seem to be preferred locations.
• Usually benign in dogs. In cats, the clinical outcome may vary among cases. Tumours
with high mitotic activity are often associated with a poorer prognosis, due to strong
tendency towards local recurrence and regional lymph node and pulmonary metastases.

Cytological features
• Cellularity is variable, often high.
• Background: pale basophilic and proteinaceous and variably haemodiluted.
• Neoplastic cells may appear as bare nuclei. When intact, they are round, cuboidal to polyg-
onal and often in cohesive clusters.
• Nuclei are round to oval, central to eccentric with granular chromatin. Nucleoli are round
and variably visible.
• The cytoplasm is scant, lightly to moderately basophilic, with variably defined borders.
• Cytological features of atypia are mild to moderate; binucleated cells and mitotic figures
may be seen.

Differential diagnoses
• Metastatic neuroendocrine tumour
• Thyroid adenoma/carcinoma
• Cutaneous lymphoma (large cell type)