## Histopathology of Lichen Planus **Key Point:** The classic histological triad of lichen planus is: 1. **Hyperkeratosis** (thickened stratum corneum) 2. **Acanthosis** (epidermal thickening) 3. **Saw-tooth appearance** of rete ridges (also called "lichenoid" pattern) ### Characteristic Features **High-Yield:** The pathognomonic finding is **liquefaction degeneration (basal cell apoptosis)** of the basal layer, which creates a clear subepidermal zone. This is accompanied by a dense band-like lymphocytic infiltrate in the superficial dermis (lichenoid interface dermatitis). ### Microscopic Pattern | Feature | Finding | |---------|----------| | Epidermis | Hyperkeratosis, acanthosis, saw-tooth rete ridges | | Basal Layer | Liquefaction degeneration, apoptotic bodies | | Dermis | Dense band-like lymphocytic infiltrate (lichenoid) | | Dermal-Epidermal Junction | Obscured by inflammation | **Clinical Pearl:** The saw-tooth pattern is best appreciated on vertical sections and is a hallmark distinguishing lichen planus from other lichenoid dermatoses. [cite:Robbins 10e Ch 25] 
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