The diagnosis of skin disease is based on color, morphology, and distribution of cutaneous lesions. The structure of the skin and associated appendages relates directly to these characteristics.
Folliculitis presents with papules or pustules. Follicular accentuation is characteristic of any eruption in darker-skinned races. In patients with miliaria, involvement of the sweat gland ostia results in erythematous papules, pustules, or superficial vesicles in areas of heavy sweating. The vesicles of miliaria crystallina are irregular in shape because the stratum corneum fails to impede the spread of the blister in random directions. This is in stark contrast to spongiotic and subepidermal blisters, which are distinctly round—as in acute dyshidrotic eczema or bullous pemphigoid.
The color of a cutaneous eruption relates to various pigments. Brown pigments include melanin, lipofuscin, and hemosiderin. Brown pigments located deeper in the dermis impart a blue hue because of diffraction of light. This is evident in blue nevi as a result of deep melanin and as a result of lipofuscin present in the sweat within nodular hidradenomas. Red pigment relates to oxygenated hemoglobin and blue to deoxygenated hemoglobin. Dilatation or proliferation of blood vessels and the rapidity of blood flow produce various shades of red and blue. Yellow pigments relate to lipid deposition or carotenoids dissolved in the cytoplasm of epithelial cells and histiocytes. In granulomatous disease, diascopy removes the visible appearance of oxygenated hemoglobin, allowing the observer to see the apple jelly yellow appearance of carotenoids within the cytoplasm. This section of the atlas will focus on the structure of the skin and how that structure translates to clinical manifestations of disease.