Merkel cells are large oval cells found in the skin and some of parts of the mucosa (stratum germinativum) of all vertebrates. They are associated with the sense of touch, and are responsible for the highly malignant skin tumor known as Markel cell carcinoma. In mammalian skin they are clear cells found in the stratum basale of the epidermis, and measure 10 – 15 µm across. Most often they are associated with sensory nerve endings, when they are known as Merkel nerve endings.

CD56

• a useful marker for diagnosing Merkel cell carcinoma. PMID: 12727026
• a sensitive marker for MCC as well as for NK-cell lymphoma, but is not specific. Importantly, CD56 positivity in crushed or inflamed biopsies of MCC may lead to an erroneous impression of NK lymphoma. Awareness of this potential pitfall will prevent misdiagnosis. PMID: 16115052

Chromogranin A (CGA)

• well-established marker peptides of mammalian Merkel cells. PMID: 9673797, PMID: 8432919, PMID: 1795880
• a regular constituent of Merkel cell secretory granules but probably not exclusively responsible for their electron opacity. PMID: 2243601
• Light and electron microscopic immunohistochemistry of mammalian skin revealed that Merkel cells are exclusively CGA-immunoreactive (ir) and that the immunoreaction is localized in the secretory granules. PMID: 2677156
• CGA-immunoreaction is localized in the secretory granules which, again, supports the view that the Merkel cell is a paraneuron, i.e., neurosecretory in function. PMID: 2510797

Cytokeratin 20

• a sensitive and specific marker for Merkel cell carcinoma and is helpful in distinguishing between Merkel cell carcinoma and other malignant and benign neoplasms. PMID: 10027519
• a general marker of cutaneous Merkel cells while certain neuronal proteins are absent. PMID: 7769257
• Anti-cytokeratin 20 staining of Merkel cells helps differentiate basaloid proliferations overlying dermatofibromas from basal cell carcinoma. PMID: 16008693
• the greatest sensitivity and specificity with anti-CK-20 antibody in identifying micrometastatic Merkel cell carcinoma (MCC) in sentinel lymph nodes. PMID: 12004304
• Immunostaining for TTF-1, especially when combined with immunostaining for CK20, can aid in the distinction between Merkel cell carcinoma and small cell carcinoma (both pulmonary and extrapulmonary). PMID: 11175640

Fli-1 and CD99

• expression is commonly found in Merkel cell carcinoma (MCC) by immunohistochemistry may avoid misinterpretation in the differential diagnosis of MCC with other small round cell tumours. PMID: 15910593

Go alpha (alpha subunit of guanine nucleotide-binding protein Go)

• In all cases of Merkel cell carcinoma, Go alpha was consistently detected on the plasma membrane and cytoplasm of the tumor cells. Nerve fibers in the skin were also positive for Go alpha, but other epidermal or dermal components such as keratinocytes, melanocytes, fibroblasts, or lymphoid cells were negative. The present study indicates that Go alpha may be a useful immunohistochemical marker of Merkel cell carcinoma. PMID: 7560347

Keratin 20

• immunohistochemical marker for gastrointestinal, urothelial, and Merkel cell carcinomas. PMID: 7567935

NSE (neuron-specific enolase)

• immunostaining is a simple and reliable method for the specific light-microscopic staining of Merkel cells and provides further evidence for NSE as a marker for the diffuse neuroendocrine system. PMID: 6789681
• neuron-specific enolase were found to be the most reliable marker for Merkel cells (MCs) identification. PMID: 8608344

TROMA-1

• a monoclonal antibody specific for adult Merkel cells. PMID: 7798428, PMID: 7523320, PMID: 1722110, PMID: 6207057

Villin

• an excellent marker of Merkel cells and their microvilli. PMID: 9774632