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	Stuck-on brown waxy plaque	Sudden eruption: internal cancer (Leser-Trélat)
Acanthosis nigricans	Velvety dark flexural skin	Insulin resistance; rarely gastric cancer
Skin tag	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.

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.

Add to my cards (3)

Elsewhere in DurgiPedia

Connections

← All of general pathology

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 (MedlinePlus, US National Library of Medicine, 2025).

Spotted a mistake?We are always improving for the community. Tell us what is wrong and our team will review it.Report a mistake

Representative model, for educational purposes only.