[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [General pathology](https://durgical.com/en/general-pathology/) › [Skin](https://durgical.com/en/general-pathology/areas/skin/) › Merkel cell carcinoma

# Merkel cell carcinoma

- Lesion

A rare, aggressive skin cancer of neuroendocrine cells, often caused by a virus. It looks like a fast-growing red bump on older or immunosuppressed people.

Updated Nov 12, 2026

## Key points

- Two routes: Merkel cell polyomavirus (integrated, truncated large T antigen binds RB) or UV (TP53 and RB1 mutations, high mutation burden).
- Clinical: painless, rapidly growing red-violet nodule on sun-exposed skin of older or immunosuppressed people.
- Histology: sheets of small round blue cells, scant cytoplasm, salt-and-pepper chromatin, many mitoses.
- CK20 with perinuclear dot-like staining, plus neuroendocrine markers; TTF1 negative (unlike metastatic small cell lung carcinoma).
- Early nodal spread; responds to checkpoint inhibitors.

## What it is

Merkel cell carcinoma is a primary neuroendocrine carcinoma of the skin, a small round blue cell tumour that grows rapidly and spreads early to lymph nodes. Most cases in the northern hemisphere are caused by clonal integration of Merkel cell polyomavirus, whose large T antigen inactivates RB; the rest are driven by UV with a very high mutation load. It is more common in older people and in immunosuppression.

## Structure

- Dermal nodule of sheets and trabeculae of small cells, often sparing the epidermis.
- Nuclear moulding, crush artefact and apoptotic bodies.
- Electron microscopy: dense-core neurosecretory granules.

## Markers

- CK20 (perinuclear dot)
- Synaptophysin
- Chromogranin
- INSM1
- Neurofilament
- TTF1 negative

## Clinical notes

- Differential of small blue cells in skin: Merkel cell carcinoma, metastatic small cell lung carcinoma, lymphoma, melanoma; immunostains decide.
- Immunosuppression (transplant, HIV, chronic lymphocytic leukaemia) increases risk.

## Memory aid

Merkel wears a CK20 dot like a bindi; the lung's small cell does not, it carries TTF1 instead.

## Test yourself

Think of the answer, then tap the question to see it.

## Which virus is associated with Merkel cell carcinoma?

Merkel cell polyomavirus, whose integrated large T antigen inactivates RB.

## How do you distinguish Merkel cell carcinoma from metastatic small cell lung carcinoma?

Merkel is CK20-positive in a perinuclear dot and usually TTF1-negative; small cell lung carcinoma is CK20-negative and TTF1-positive.

## Which immunotherapy class is effective in Merkel cell carcinoma?

Immune checkpoint inhibitors (PD-1 or PD-L1 blockade).

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[More Pathology questions →](https://durgical.com/en/practice/?f=pathology)

## Elsewhere in DurgiPedia

- [Keratinocytes, melanocytes, Langerhans and Merkel cells](https://durgical.com/en/histology/keratinocytes-melanocytes-langerhans-and-merkel-cells/)
- [Oncogenic viruses and H. pylori: the table to know](https://durgical.com/en/general-pathology/oncogenic-viruses-and-h-pylori-the-table-to-know/)
- [Immunohistochemistry: the principle and the markers to know](https://durgical.com/en/general-pathology/immunohistochemistry-the-principle-and-the-markers-to-know/)

## Connections

## Related 4

- [Basal cell carcinoma](https://durgical.com/en/general-pathology/basal-cell-carcinoma/)
- [Pulmonary neuroendocrine tumours](https://durgical.com/en/general-pathology/pulmonary-neuroendocrine-tumours/)
- [Oncogenic viruses and H. pylori: the table to know](https://durgical.com/en/general-pathology/oncogenic-viruses-and-h-pylori-the-table-to-know/)
- [Immunohistochemistry: the principle and the markers to know](https://durgical.com/en/general-pathology/immunohistochemistry-the-principle-and-the-markers-to-know/)

## Related conditions 2

- [Merkel cell carcinoma](https://durgical.com/en/conditions/merkel-cell-carcinoma/)
- [HIV infection and AIDS](https://durgical.com/en/conditions/hiv-infection-and-aids/)

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Educational content on the mechanisms of disease; not a diagnostic or treatment guide and no substitute for a health professional's advice. No doses are given.

Sources: [Merkel Cell Carcinoma Treatment (PDQ®)–Health Professional Version](https://www.cancer.gov/types/skin/hp/merkel-cell-treatment-pdq) (National Cancer Institute, 2025).

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Representative model, for educational purposes only.

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Source: https://durgical.com/en/general-pathology/merkel-cell-carcinoma/
License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/general-pathology/merkel-cell-carcinoma/