Merkel cell carcinoma

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Histological appearance of a canine merkel cell carcinoma, showing dense aggregates of neoplastic round to polyhedral cells interspersed within a proteinaceous background[1]

Merkel cell carcinoma (APUdoma; multicentric cutaneous neuroendocrine carcinoma) are a rare benign neoplasm of the canine skin.

Merkel cells are mechanireceptors comprised of follicular epidermal and dermal neuroendocrine cells found throughout the skin, oral cavity, and internal organs[2], where they aggregate around dermal nerve terminals[3]. Immunohistochemical studies have shown that these tumors behave more as neural, rather than basal, epithelial cells[4].

Although Merkel cell polyomavirus appears to initiate malignant transformation in humans[5], similar viruses have not been isolated in dogs, and the majority of canine cases are relatively benign.

In dogs, primary neuroendocrine tumors can arise in sites such as the intestines, esophagus, nasal cavity, skin, bile duct, gallbladder, liver (hepatic neuroendocrine tumor), and nasopharynx[6]. Peritoneal carcinomatosis and metastasis to regional lymph nodes occurs frequently[7].

These tumors primarily affect older dogs and appear as locally expansive firm nodular cutaneous masses on the skin around the head[8], oral cavity[9] and interdigital skin[10].

Diagnosis requires histological examination of biopsied material.

Histologically, these tumor locally effaces periadnexal and deep dermis tissue and comprise mainly clusters of membrane-bound dense-core neuroendocrine granules within the dermis[1].

A differential diagnosis would include mast cell tumor, cutaneous papilloma, squamous cell carcinoma, subungual keratoacanthoma, melanocytic nevus, malignant melanoma and hemangiopericytoma.

In the majority of cases, surgical excision is curative and reports of metastases appear uncommon.


  1. 1.0 1.1 Joiner KS et al (2010) Multicentric cutaneous neuroendocrine (Merkel cell) carcinoma in a dog. Vet Pathol 47(6):1090-1094
  2. Narisawa Y et al (1993) Epithelial skirt and bulge of human vellus hair follicles and associated Merkel cell nerve complex. Arch Dermatol Res 285:269–277
  3. Toyoshima K et al (1993) Occurrence of melanosome-containing Merkel cells in mammalian oral mucosa. Acta Anat (Basel) 147(3):145-148
  4. Gil da Costa RM et al (2010) Two canine Merkel cell tumours: immunoexpression of c-KIT, E-cadherin, beta-catenin and S100 protein. Vet Dermatol 21(2):198-201
  5. Pastrana DV et al (2009) Quantitation of human seroresponsiveness to Merkel cell polyomavirus. PLoS Pathog 5:e1000578
  6. Patnaik AK et al (2002) Neuroendocrine carcinoma of the nasopharynx in a dog. Vet Pathol 39:496–500
  7. Patnaik AK et al (1981) Canine hepatic carcinoids. Vet Pathol 18:445–453
  8. Konno A et al (1998) Immunohistochemical diagnosis of a Merkel cell tumor in a dog. Vet Pathol 35(6):538-540
  9. Whiteley LO & Leininger JR (1987) Neuroendocrine (Merkel) cell tumors of the canine oral cavity. Vet Pathol 24(6):570-572
  10. Carpenter JL et al (1991) Distinctive unclassified mesenchymal tumor of the digit of dogs. Vet Pathol 28(5):396-402