The morphology of skin lesions is extremely important in the diagnosis of dermatologic conditions. This chapter describes the morphology and diagnostic significance of primary and secondary skin lesions.

Papule

Morphology. Papules are small, solid eruptions in the epidermis and are approximately 1 cm in diameter or smaller. Papules are erythematous, reddened swellings produced by tissue infiltration of inflammatory cells, epidermal edema, or epidermal hypertrophy. Papules, when present as a group, may form erythematous plaques or rashes. The elevation may not be visible but can usually be felt with the fingertips. The surface epithelium of true papules remains intact unless secondarily traumatized.

Diagnostic Significance. Papules are the basic lesions in many allergic and parasitic skin diseases (e.g., flea allergy dermatitis and sarcoptic mange). Papules are also the earliest stage in pustule formation and may be confused with the initial stages of small vesicular eruptions. Therefore, papules may be observed in pustular diseases such as bacterial folliculitis and pemphigus foliaceous. It is very important to examine the stratum corneum covering the eruption. With pustules and vesicles, the stratum corneum is usually separated from the underlying epidermal layers and the resulting cleft may be filled with various amounts of fluid or inflammatory exudates. Most diseases with papular eruptions are pruritic. Since scratching may dramatically alter the appearance of the skin, lesions should be sought in nontraumatized areas.

Pustule

Morphology. Pustules are small circumscribed or larger asymmetric eruptions on the skin. The cavity of the pustule is filled with inflammatory (suppurative) exudate. Pustules may be superficial (just under the stratum corneum, i.e., subcorneal) or deep (extending down to the hypodermis). Pustules may be yellow, erythematous, or hemorrhagic in appearance. Rupture of the pustule may leave a superficial erosion or, with deep pustules, a necrotic fistulating tract.

Diagnostic Significance. Subcorneal pustules are associated with superficial bacterial infection (superficial staphylococcal dermatitis, folliculitis), pemphigus foliaceous, and subcorneal pustular dermatoses. Deep pustules are seen with deep pyodermas and certain fungal diseases such as sporotrichosis. In the differential diagnosis of pustular eruptions, the contents of an intact pustule should be microscopically examined for bacteria and acantholytic epithelial cells and aseptically cultured for bacteria. Pemphigus foliaceous forms pustules that are sterile.

Vesicle

Morphology.