The skin is the most accessible organ of the human body. Its most basic function is simply a protective one. As a barrier, the skin holds off desiccation and disease by keeping moisture in and pathogens out. Nevertheless, characterization of the skin as mere “plastic wrap” is a gross underestimation of the anatomic and physiologic complexity of this vital structure.
Unlike parenchymal organs, end-organ dysfunction or failure is not a prerequisite for the diagnosis of a skin disease, because all skin diseases can be observed clinically irrespective of their functional effects. Among the spectacular array of neoplastic, inflammatory, infectious, and genetic cutaneous disorders, some elicit only trivial aberrations in skin structure or function, whereas others lead to profound and morbid consequences.
The integumentary system consists of a layer of tissue, 1–4 mm thick, that covers all exposed surfaces of the body. The skin merges uninterruptedly with the structurally similar envelope of the mucous membranes, but skin is distinct from mucosa in that it contains adnexal structures such as the eccrine units that exude sweat and the folliculosebaceous apocrine units that produce hairs and oils. There is considerable variation in skin thickness and composition, depending on the requirements of a particular body site. For example, the thinnest skin overlies the eyelids, where delicacy and mobility are essential. The thickest skin is present on the upper trunk, where sturdiness exceeds mobility in importance. The surfaces of the palms and soles are characterized by a high density of eccrine sweat units, reflecting the importance of this region in temperature regulation; an absence of hairs, which would interfere with sensation; and an accentuation of the cornified layer (see later discussion), contributing to the tackiness needed to handle objects deftly. The size of the structures between sites can also vary greatly, best illustrated by the contrast between the large terminal hair follicles found on the scalp, bearded areas, and genital skin and the small vellus hair follicles found at most other sites.
Using a light microscope, two important skin layers are easily identifiable: a stratified squamous epithelium, the epidermis; and a layer of connective tissue, the dermis. The subjacent adipose tissue is considered a third layer by some and is referred to as the subcutis.
The epidermis consists of keratinocytes arrayed in four distinct substrata: the basal, spinous, granular, and cornified layers (Figure 8–1). Basal keratinocytes include the proliferative pool of keratinocytes. These cells divide, giving rise to progeny that are displaced toward the skin surface. As the keratinocytes move outwardly, they progressively flatten and accumulate keratin filaments within their cytoplasm. Individual keratinocytes are tightly bound together by intracellular junctions called desmosomes (Figure 8–2). The desmosomal junctions appear as delicate “spines” between cells in conventional microscopic sections and are most conspicuous in the epidermal spinous layer (...