Here is the detailed histopathological comparison of these three entities: --- 1. Epidermoid Cyst (Sebaceous Cyst Misnomer) Architecture - Unilocular cyst in the dermis or subcutis - Often connected to the surface by a punctum (follicular infundibulum) Wall - Lined by stratified squamous epithelium with a granular layer - Epithelium shows progressive keratinization (orthokeratosis) - The wall resembles the epidermis or follicular infundibulum Contents - Lamellated keratin — concentric layers of anucleate keratin - Often appears as cheesy, foul-smelling material grossly - May contain cholesterol clefts and foreign body giant cell reaction if ruptured Special Features - Rupture is common → elicits granulomatous inflammation with foreign body giant cells - May show calcification in long-standing lesions - No skin appendages in the wall --- 2. Trichilemmal (Pilar) Cyst Architecture - Unilocular cyst in the dermis/subcutis - No connection to the surface (no punctum) Wall - Lined by stratified squamous epithelium showing trichilemmal keratinization - NO granular layer — abrupt transition from nucleated cells to anucleate keratin (trichilemmal keratinization, like the outer root sheath) - Epithelial cells are pale-staining with distinct cell borders - Thick, homogeneous basement membrane (PAS-positive) Contents - Dense, homogeneous keratin — not lamellated - Often described as firm, dry, "crumbly" material Special Features - Proliferating trichilemmal cyst/tumor — if the wall shows broad anastomosing trabeculae and pushing borders, it is a proliferating variant (can be locally aggressive, rarely malignant) - Minimal inflammatory response unless ruptured - Common on scalp; often multiple --- 3. Pilomatricoma (Calcifying Epithelioma of Malherbe) Architecture - Solid tumor, not a true cyst (though may have cystic degeneration) - Well-circumscribed, lobulated mass in the dermis and subcutis - Often surrounded by a fibrous pseudocapsule Two Cell Types (Biphasic) 1. Basaloid cells — small, dark, uniform cells with scant cytoplasm, resembling hair matrix cells 2. Shadow/ghost cells (anucleate keratinized cells) — large, eosinophilic cells with distinct cell borders, no nuclei, resembling "ghosts" of basaloid cells Key Histological Features - Transition zone between basaloid cells and ghost cells is characteristic - Extensive calcification — dystrophic calcification within the ghost cell areas (appears as dark blue/purple deposits) - Foreign body giant cell reaction around keratin debris - Ossification may occur in long-standing lesions - Mitotic activity can be seen in basaloid cells (do not mistake for malignancy) Variants - Pilomatrical carcinoma — rare malignant counterpart; shows atypia, infiltrative growth, and high mitotic activity --- Histopathological Comparison Table Feature Epidermoid Cyst Trichilemmal Cyst Pilomatricoma Type True cyst True cyst Solid tumor (may have cystic change) Location Dermis/subcutis Dermis/subcutis Dermis/subcutis Wall epithelium Stratified squamous with granular layer Stratified squamous without granular layer Biphasic: basaloid + ghost cells Keratinization Orthokeratosis (gradual) Trichilemmal (abrupt, no granular layer) Matrical keratinization → ghost cells Contents Lamellated keratin Dense homogeneous keratin Ghost cells + calcification Calcification Rare/minimal Rare Extensive and characteristic Granular layer Present Absent N/A Punctum Often present histologically Absent Absent Inflammation Common if ruptured Minimal Foreign body reaction common --- Diagnostic Pearls - Granular layer present → Epidermoid cyst - No granular layer, thick basement membrane → Trichilemmal cyst - Basaloid cells + ghost cells + calcification → Pilomatricoma (pathognomonic) Would you like me to elaborate on any specific feature, such as immunohistochemistry or differential diagnosis with malignant mimics?

Create a comparison poster: Epidermoid vs Trichilemmal vs Pilomatricoma