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PA33.4 | Common Skin Tumors & Morphology — SDL Guide

Learning Objectives

Identify and describe the gross and histological features of common benign epithelial skin tumors: seborrheic keratosis, acanthosis nigricans, acrochordon, and epidermal inclusion cyst.
Describe the key histological features of selected adnexal tumors: cylindroma, syringoma, and pilomatricoma.
Classify melanocytic nevi (junctional, compound, intradermal) and describe the distinguishing features of dysplastic nevi.
Identify and distinguish the morphological features of premalignant and malignant skin lesions: actinic keratosis, Bowen disease, basal cell carcinoma, squamous cell carcinoma, and melanoma.
Recognize dermatofibroma and explain its characteristic histological findings.
Apply a systematic pattern-recognition approach to unknown skin biopsy slides in the practical examination.

INSTRUCTIONS

This SDL is your morphology practical companion. Work through each tumor group systematically — for every entity, study the clinical image first (what does the clinician see?), then the histological image (what does the pathologist see?). Build mental 'snapshots' of each pattern. Use the Master Recognition Table at the end to consolidate before your practical exam. Estimated reading time: 33 minutes.

References

Robbins & Cotran Pathologic Basis of Disease, 10th ed., Ch. 25 (textbook)

Version 2.0 | NMC CBUC 2024

CLINICAL SCENARIO

A 60-year-old retired farmer walks into your dermatology OPD. He has a warty, dark, 'stuck-on' plaque on his cheek (seborrheic keratosis), a pearly nodule on his nose (BCC), a dark irregular lesion on his forearm (melanoma until proven otherwise), and what looks like a flesh-coloured skin tag on his neck. Same organ — four completely different lesions. Your job in the practical exam is to distinguish them in seconds. This SDL builds exactly that skill.

WHY THIS MATTERS

Skin tumors appear in two high-stakes settings for you as a Year-2 student: (1) the practical exam slide or specimen tray, where you must name and describe the lesion within minutes; (2) the clinical wards, where missing a melanoma or mislabeling a BCC as a seborrheic keratosis has real consequences. NMC competency PA33.4 explicitly requires you to identify, distinguish, and describe — that three-word mandate structures this entire SDL.

RECALL

Before we begin, refresh your memory on three concepts from SDL1 (Skin Diseases — General Pathology):

Epidermis layers: stratum basale → spinosum → granulosum → corneum. Tumors arise from specific layers.
Melanocytes sit in the basal layer and are derived from neural crest; nevi and melanoma arise from them.
UV radiation is the dominant environmental carcinogen for skin: causes cyclobutane pyrimidine dimers in DNA, drives AK → SCC and BCC.

If any of these feel unfamiliar, re-read SDL1 section on normal skin histology before continuing.

Benign Epithelial Tumors — Seborrheic Keratosis

Seborrheic keratosis (SK) is the most common benign epidermal tumor in adults over 50, and a perennial practical-exam slide. Despite the name it has no relationship to sebaceous glands or true keratosis; the term is entirely historical.

Clinical features: Well-circumscribed, tan-to-dark-brown, velvety or verrucous plaque with a characteristic 'stuck-on' appearance — as if you could peel it off with a fingernail. Predilection for the face, trunk, and extremities. Multiple lesions may erupt suddenly (sign of Leser-Trélat) in association with internal malignancy, though isolated SK is entirely benign.

Two-panel medical illustration of seborrheic keratosis: Panel A shows a dark-brown warty plaque on the back with labeled leader lines for stuck-on appearance, irregular surface, and sharp border; Panel B shows a side-view schematic of the lesion sitting exophytically above the epidermal surface. — **Panel A:** Dark-brown sharply demarcated warty plaque on skin; leader lines labeling 'Stuck-on appearance', 'Irregular surface', 'Sharp border'. **Panel B:** Side-view cross-section schematic showing exophytic lesion sitting above the epidermal baseline with flat lower border.

Histological features (the high-yield pattern):

Exophytic proliferation of bland basaloid keratinocytes — the cells look like stratum basale cells, with no atypia.
Hyperkeratosis and acanthosis (thickened epidermis).
Horn cysts (fully formed keratin pearls within the tumor mass) and pseudo-horn cysts (keratin-filled invaginations from the surface).
A flat, sharp lower border (the lesion sits above a horizontal line).

Three-panel H&E histology illustration of seborrheic keratosis: Panel A shows the low-power exophytic dome of basaloid keratinocytes above a flat lower border with labeled horn cyst, pseudo-horn cyst, and hyperkeratosis; Panel B is a magnified view of an enclosed horn cyst; Panel C is a magnified view of an open pseudo-horn cyst communicating with the surface. — **Panel A:** Low-power overview: exophytic basaloid keratinocyte mass projecting above surrounding skin; flat lower border (dashed reference line); surface hyperkeratosis; horn cyst (enclosed pale inclusion, labeled); pseudo-horn cyst (surface-opening pale inclusion, labeled); inset boxes marking Panels B and C source regions.. **Panel B:** Magnified horn cyst: concentric lamellar pale-pink keratin filling a completely enclosed rounded cavity; rim of small uniform basaloid keratinocytes with round blue-purple nuclei; no surface communication.. **Panel C:** Magnified pseudo-horn cyst: flask-shaped surface invagination open at top; pale-pink keratin fill; flanking walls of basaloid keratinocytes; visible epidermal surface opening distinguishes it from a true horn cyst..

Memory hook: SKs are stuck-on, basaloid, and full of horn cysts.

Benign Epithelial Tumors — Acanthosis Nigricans, Acrochordon, and Epidermal Inclusion Cyst

Acanthosis nigricans (AN) presents as velvety, hyperpigmented, thickened skin in flexural areas (axillae, neck, groin). Histology shows papillomatosis and mild hyperkeratosis — but minimal true acanthosis despite the name. Its significance is as a marker rather than a primary tumor: it is associated with insulin resistance/type 2 diabetes, obesity, and (in adults over 40) occult internal malignancy (especially gastric adenocarcinoma). A brief recognition is sufficient for the practical.

Acrochordon (skin tag / fibroepithelial polyp) is a pedunculated, flesh-coloured soft papule hanging from a thin stalk, found most commonly on the neck, axillae, and eyelids. Histology is unexciting: a fibrovascular core covered by normal squamous epithelium. No malignant potential. Clinically distinguished from a melanocytic lesion by its soft, pedunculated, skin-coloured nature.

Two-panel illustration: Panel A shows multiple pedunculated flesh-coloured acrochordons on the neck with labels for stalk and pedunculated papule; Panel B shows a longitudinal cross-section schematic of a single skin tag with labels for epidermis, loose fibrovascular stroma, stalk, and dermis. — **Panel A:** Multiple acrochordons on neck skin; 'Stalk' label with leader line to the narrow fibrovascular pedicle at base; 'Pedunculated papule' label on the hanging soft tip; 'Normal skin' label on surrounding unaffected surface. **Panel B:** Longitudinal cross-section of single acrochordon; 'Epidermis' (outer layer); 'Loose fibrovascular stroma' (pale core); 'Stalk / fibrovascular pedicle' (narrow base); 'Dermis' (anchor tissue below).

Epidermal inclusion cyst (epidermal/sebaceous cyst) is a common intradermal or subcutaneous cyst lined by true stratified squamous epithelium with a granular layer. The lumen contains laminated keratin. A central punctum is often visible clinically. On rupture, the extravasated keratin triggers a granulomatous foreign-body reaction with multinucleated giant cells — this is the source of the classic 'infected cyst' presentation.

Two-panel H&E histology illustration of an epidermal inclusion cyst: Panel A shows a low-power cross-section of the entire cyst with the cyst wall and laminated keratin in the lumen labelled; Panel B shows a high-power view of the cyst wall layers including the basal, spinous, granular, and keratinizing layers with leader-line labels. — **Panel A:** Cyst wall (stratified squamous epithelium lining), Laminated keratin in lumen (concentric eosinophilic whorls), Fibrous capsule (outer boundary). **Panel B:** Basal cell layer, Spinous layer (prickle cells), Granular layer (keratohyalin granules — dark basophilic dots), Keratinizing surface / cyst wall (anucleate flattened cells), Laminated keratin (luminal side).

CLINICAL PEARL

Sign of Leser-Trélat — an abrupt eruption of multiple pruritic seborrheic keratoses is a rare paraneoplastic phenomenon; the associated tumor is most often gastric adenocarcinoma but also lymphomas. The same visceral malignancies also cause acanthosis nigricans. If you see both together on an exam MCQ, think GI malignancy.

Benign Adnexal (Appendageal) Tumors — Overview

Adnexal tumors arise from skin appendages: sweat glands (eccrine and apocrine), hair follicles, and sebaceous glands. A brief overview is sufficient at this level; detailed classification belongs to postgraduate dermatopathology.

Tumor	Appendage of origin	Key histological clue
Cylindroma	Apocrine sweat gland	'Jigsaw puzzle' islands of basaloid cells, each surrounded by a pink hyaline sheath
Syringoma	Eccrine sweat duct	Small ducts with a comma-shaped tail ('tadpole ducts') in a fibrous stroma
Pilomatricoma (calcifying epithelioma of Malherbe)	Hair follicle matrix	Basaloid cells + ghost/shadow cells + dystrophic calcification

H&E histology diagram of syringoma showing comma-shaped tadpole ducts embedded in fibrous stroma in the dermis, with a high-power inset labeling the dilated lumen head and solid epithelial tail. — **Panel A:** Multiple comma-shaped tadpole ducts (rounded head + tapering tail), dense fibrous stroma (pale pink collagen), dermis context, two-layer epithelial lining. **Panel B:** Head — dilated lumen (rounded end), Tail — solid epithelial strand (tapering end), Double-layer epithelium (bracketed), duct wall cellular detail.

Three-panel H&E histology diagram of pilomatricoma showing Panel A low-power overview of basaloid and ghost cell zones, Panel B high-power transition from nucleated basaloid cells to anucleate ghost cells, and Panel C dystrophic calcification deposits among ghost cells. — **Panel A:** Low-power H&E overview: peripheral palisading basaloid cells (dark, hyperchromatic), central anucleate ghost/shadow cells (pale eosinophilic outlines), overall nodular architecture of pilomatricoma. **Panel B:** High-power transition zone: nucleated basaloid cells (left), gradient arrow, anucleate ghost cells (right) — illustrating progressive nuclear loss. **Panel C:** High-power calcification zone: irregular basophilic dystrophic calcification deposits, surrounding ghost/shadow cell outlines, occasional foreign-body giant cells.

Practical tip: If a slide shows tadpole-shaped ductal structures → think syringoma. Jigsaw-puzzle basaloid islands with a pink rim → cylindroma. Ghost cells + calcification → pilomatricoma. These are pattern-recognition answers.

SELF-CHECK

A biopsy from a flesh-coloured eyelid papule in a 35-year-old woman shows small ductal structures with a comma-shaped tail embedded in a fibrous dermis. The most likely diagnosis is:

A. Cylindroma

B. Syringoma

C. Pilomatricoma

D. Seborrheic keratosis

Reveal Answer

Answer: B. Syringoma

Syringoma arises from eccrine sweat ducts and characteristically shows small comma-shaped ('tadpole') ducts in a fibrous dermis. It favors the lower eyelids of young women. Cylindroma shows jigsaw-puzzle basaloid islands with a hyaline sheath; pilomatricoma has ghost cells and calcification; SK has basaloid cells with horn cysts.