Clinical meaning
Skin lesion morphology reflects specific pathological processes occurring at defined layers of the integumentary system, and precise morphological description using standardized terminology is the foundation of dermatological diagnosis. Primary lesions arise de novo from previously normal skin and indicate the fundamental disease process. A macule (<1 cm) or patch (>1 cm) is a flat, non-palpable change in skin color caused by alterations in melanin (hyperpigmentation from increased melanocyte activity or melanin deposition in the dermis), hemoglobin (erythema from vasodilation or vascular proliferation), or pigment deposition (jaundice from bilirubin, hemosiderin from extravasated red blood cells). A papule (<1 cm) or plaque (>1 cm) is a palpable, elevated, solid lesion — the elevation results from epidermal hyperplasia (acanthosis, as in psoriasis where accelerated keratinocyte proliferation shortens the cell cycle from 28 days to 3-4 days), dermal cellular infiltration (inflammatory cells in eczema, malignant cells in cutaneous lymphoma), or dermal collagen deposition (morphea, keloids). A nodule (>1 cm, extends into dermis or subcutis) represents deeper pathology — granulomatous inflammation (sarcoidosis, foreign body reaction), neoplasia (basal cell carcinoma, dermatofibroma), or cyst formation. Vesicles (<1 cm) and bullae (>1 cm) are fluid-filled elevations whose anatomical level of separation is diagnostically critical: intraepidermal blisters (pemphigus vulgaris — caused by IgG autoantibodies against desmoglein-3, disrupting desmosomal adhesion between keratinocytes, producing flaccid, easily ruptured blisters with positive Nikolsky sign) versus subepidermal blisters (bullous pemphigoid — IgG autoantibodies against hemidesmosomes at the dermal-epidermal junction, producing tense blisters that are more difficult to rupture). A pustule is a vesicle containing purulent exudate — neutrophil-predominant (bacterial infection, pustular psoriasis) or eosinophil-predominant (erythema toxicum neonatorum). A wheal (hive/urticaria) results from transient dermal edema caused by mast cell degranulation releasing histamine, which increases vascular permeability of post-capillary venules — the wheal blanches with pressure because the edema compresses superficial blood vessels. Secondary lesions result from the evolution or disruption of primary lesions. Scale represents abnormal accumulation of stratum corneum (keratinization disorder) — the silvery, micaceous scale of psoriasis results from parakeratosis (retention of nuclei in the stratum corneum from accelerated epidermal turnover), while the greasy, yellow scale of seborrheic dermatitis reflects lipid-rich desquamation associated with Malassezia yeast colonization. Crust is dried serum, blood, or purulent exudate on the skin surface (impetigo produces honey-colored crusts from dried serous exudate mixed with bacterial debris). An erosion is loss of epidermis above the basement membrane (heals without scarring), while an ulcer extends through the basement membrane into the dermis or deeper (heals with scarring) — this distinction reflects the depth of tissue destruction and has prognostic significance. Lichenification (thickened, accentuated skin markings) results from chronic rubbing or scratching that stimulates epidermal hyperplasia and dermal fibrosis — the hallmark of chronic eczema. Atrophy indicates loss of tissue substance: epidermal atrophy (thin, translucent, cigarette-paper wrinkling from potent topical corticosteroid use suppressing keratinocyte proliferation) or dermal atrophy (depressed lesions from collagen loss).