[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/) › Benign epidermal growths

# Benign epidermal growths

- Lesion

Many common skin bumps are harmless growths of the epidermis. A few, like sudden dark velvety skin, can be a sign of something inside.

Updated Nov 12, 2026

## Key points

- Seborrheic keratosis: stuck-on, waxy, brown plaque; basaloid cell proliferation with horn cysts and pseudo-horn cysts; activating FGFR3 or PIK3CA.
- Leser-Trélat sign: sudden eruption of many seborrheic keratoses as a paraneoplastic sign (gastric adenocarcinoma among others).
- Acanthosis nigricans benign form: obesity, insulin resistance, type 2 diabetes, PCOS, inherited FGFR3 forms; insulin acts through IGF-1 receptors on keratinocytes.
- Acanthosis nigricans malignant form: older adults, abrupt, extensive; gastric adenocarcinoma is the classic cause.
- Fibroepithelial polyp: soft pedunculated tag of fibrovascular stroma covered by epidermis; associated with obesity, pregnancy, insulin resistance.
- Epidermal (infundibular) inclusion cyst: lined by epidermis with a granular layer, filled with laminated keratin; trichilemmal (pilar) cyst on the scalp lacks a granular layer.

## What it is

Seborrheic keratoses are very common benign epidermal tumours of middle and old age. Acanthosis nigricans is a velvety thickening of flexural skin that signals insulin resistance or, rarely, internal cancer. Fibroepithelial polyps (skin tags) and epithelial inclusion cysts are frequent minor lesions. All are benign, but each has associations worth knowing.

## Structure

- Seborrheic keratosis: exophytic, sharply demarcated sheets of small basaloid keratinocytes above the level of the surrounding skin, with keratin horn cysts.
- Acanthosis nigricans: hyperkeratosis and papillomatosis, mild acanthosis, basal hyperpigmentation; little inflammation.
- Skin tag: papillary dermis-like loose stroma with vessels, thin epidermis.
- Inclusion cyst: wall of stratified squamous epithelium, contents of flaky keratin; rupture provokes a foreign-body giant cell reaction.

## Benign growths and their signals

| Lesion | Look | What it can signal |
| --- | --- | --- |
| [Seborrheic keratosis](https://durgical.com/en/conditions/seborrheic-keratosis/) | Stuck-on brown waxy plaque | Sudden eruption: internal cancer (Leser-Trélat) |
| [Acanthosis nigricans](https://durgical.com/en/conditions/acanthosis-nigricans/) | Velvety dark flexural skin | Insulin resistance; rarely gastric cancer |
| [Skin tag](https://durgical.com/en/conditions/obesity/) | Soft pedunculated papule | Obesity, insulin resistance |
| Inclusion cyst | Firm dermal nodule with a pore | Gardner syndrome when many and early |

## Clinical notes

- Irritated seborrheic keratosis can mimic squamous cell carcinoma or melanoma clinically.
- Acanthosis nigricans in children and adolescents is a useful screening sign for insulin resistance.

## Memory aid

A seborrheic keratosis is a wax stamp stuck on the skin; acanthosis nigricans is velvet that says «check the insulin».

## Test yourself

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

## What are horn cysts?

Small keratin-filled cysts within the basaloid proliferation of a seborrheic keratosis.

## How does insulin resistance cause acanthosis nigricans?

High insulin binds IGF-1 receptors on keratinocytes and fibroblasts, stimulating their growth.

## What is the Leser-Trélat sign?

Sudden appearance of many seborrheic keratoses as a paraneoplastic sign of internal malignancy.

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## Elsewhere in DurgiPedia

- [Paraneoplastic syndromes: the table to know](https://durgical.com/en/general-pathology/paraneoplastic-syndromes-the-table-to-know/)
- [Hyperplasia: physiologic and pathologic](https://durgical.com/en/general-pathology/hyperplasia-physiologic-and-pathologic/)

## Connections

## Related 4

- [Skin pathology: terms and reaction patterns](https://durgical.com/en/general-pathology/skin-pathology-terms-and-reaction-patterns/)
- [Type 2 diabetes](https://durgical.com/en/general-pathology/type-2-diabetes/)
- [Paraneoplastic syndromes: the table to know](https://durgical.com/en/general-pathology/paraneoplastic-syndromes-the-table-to-know/)
- [Adnexal tumours of the skin](https://durgical.com/en/general-pathology/adnexal-tumours-of-the-skin/)

## Related conditions 5

- [Seborrheic keratosis](https://durgical.com/en/conditions/seborrheic-keratosis/)
- [Acanthosis nigricans](https://durgical.com/en/conditions/acanthosis-nigricans/)
- [Type 2 diabetes mellitus](https://durgical.com/en/conditions/type-2-diabetes-mellitus/)
- [Obesity](https://durgical.com/en/conditions/obesity/)
- [Polycystic ovary syndrome](https://durgical.com/en/conditions/polycystic-ovary-syndrome/)

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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: [Skin Conditions](https://medlineplus.gov/skinconditions.html) (MedlinePlus, US National Library of Medicine, 2025).

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Representative model, for educational purposes only.

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