Dermatopathology Basics
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Direct answer
Blisters split at definable levels, and naming that level is half of diagnostic dermatopathology: subcorneal splits house pemphigus foliaceus and impetigo, suprabasal acantholysis with a row of tombstone basal cells defines pemphigus vulgaris, and subepidermal blisters contain bullous pemphigoid and dermatitis herpetiformis. The rest of the subject is organised by reaction pattern — spongiotic (eczematous) dermatitis with intercellular oedema, psoriasiform dermatitis with regular acanthosis and parakeratosis, and interface dermatitis with vacuolar degeneration of the basal layer as in lichen planus and drug eruptions. Granulomatous patterns separate sarcoidosis (naked tubercles) from tuberculosis (caseation) and leprosy (perineural and neurotropic infiltrates), while melanocytic lesions are reported by Breslow thickness measured from the top of the granular layer.
What you must remember
- Spongiotic pattern: intercellular oedema squeezing keratinocytes apart, with eosinophils in allergic contact and atopic dermatitis; chronicity adds acanthosis and hyperkeratosis.
- Psoriasiform pattern: regular elongated rete ridges, diminished granular layer, parakeratosis with neutrophils (Munro microabscesses in the stratum corneum, Kogoj spongiform pustules in the spinous layer).
- Interface pattern: basal-layer vacuolar change and apoptotic Civatte bodies; lichen planus adds a sawtooth profile and band-like subepidermal infiltrate.
- Blister ladder: subcorneal — pemphigus foliaceus, staphylococcal scalded skin, bullous impetigo; intraepidermal suprabasal — pemphigus vulgaris; subepidermal — bullous pemphigoid, dermatitis herpetiformis, porphyria and epidermolysis bullosa.
- Granuloma key: sarcoidosis shows naked epithelioid tubercles with scant lymphocytes; tuberculosis shows caseation; leprosy shows granulomas around and within nerves, with Fite stain for organisms across the spectrum.
- Melanoma reporting essentials: Breslow thickness in millimetres from the granular layer, ulceration status, mitotic rate and margins — Clark level is essentially historical.
Reading an inflammatory panel
A 40-year-old has itchy violaceous polygonal papules on the flexor wrists; biopsy shows a band-like lymphocytic infiltrate hugging the dermo-epidermal junction, basal-layer vacuolar change, sawtooth rete ridges and occasional colloid (Civatte) bodies — lichen planus, and the direct immunofluorescence shows fibrinogen along the basement-membrane zone rather than the immunoglobulin patterns of the bullous diseases. Contrast a tense bulla in an elderly patient: a subepidermal split rich in eosinophils, with linear C3 and IgG at the basement membrane on immunofluorescence — bullous pemphigoid. Change the cell to neutrophils at the tips of dermal papillae with granular IgA on immunofluorescence, and the patient has dermatitis herpetiformis, sending the clinician toward coeliac screening. Dermatopathology earns its keep in exactly this way: morphology proposes, immunofluorescence confirms, serology closes.
Where candidates slip
Munro and Kogoj are swapped — Munro microabscesses are neutrophils trapped in the parakeratotic stratum corneum, Kogoj spongiform pustules are neutrophils within the spongiotic spinous layer. Acantholysis is treated as pemphigus alone, when it also explains the subcorneal split of pemphigus foliaceus and the transient acantholytic dermatoses. Among the granulomas, the "naked" quality of sarcoidal granulomas is missed, and leprosy is forgotten whenever a perineural cellular cuff is present — perineural involvement is the single most useful leprosy clue in Indian skin, where the disease is endemic enough to justify Fite stains on most granulomatous panels. Finally, Breslow thickness must be measured to the deepest invasive tumour cell, from the granular layer — overlying crust or ulceration has its own rule, and imprecision here changes staging.
Frequently asked questions
At which level does pemphigus vulgaris split?
Suprabasally, producing a row of intact tombstone basal keratinocytes on the blister floor, with acantholytic cells in the cavity.
What distinguishes a Munro microabscess from a Kogoj spongiform pustule?
Munro microabscesses are neutrophil collections in the parakeratotic stratum corneum, while Kogoj spongiform pustules are neutrophils within spongiotic spinous keratinocytes.
Which histological features favour dermatitis herpetiformis?
Neutrophil microabscesses at the tips of dermal papillae in a subepidermal split, with granular IgA deposition on direct immunofluorescence.
What is a naked tubercle?
An epithelioid granuloma with sparse surrounding lymphocytes, the sarcoidosis pattern, contrasting with the lymphocyte-rich rimming and caseation of tuberculosis.
How is Breslow thickness measured?
From the top of the granular layer of the epidermis (or the base of an ulcer) vertically down to the deepest invasive melanoma cell, reported in millimetres.
Which pattern defines lichen planus?
Interface dermatitis with band-like lymphocytic infiltrate, sawtooth rete ridges, basal vacuolar change and Civatte bodies, plus Wickham striae clinically.