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The skin is composed of different tissue compartments that interconnect anatomically and interact functionally. It is difficult to envisage epidermal function without signals from the dermis or passenger leukocytes traveling to and from the skin. On the other hand, epidermis, dermis, and subcutaneous tissue are heterogeneous in nature and an analysis of pathologic processes involving the skin should therefore consider both this heterogeneity and the interactions of the individual cutaneous compartments; only then will it be understood why a few basic reactions lead to a multiplicity of reaction patterns within this tissue.
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Pathophysiologically, the skin can be subdivided into three reactive units that extend beyond anatomic boundaries (Fig. 6-1); they overlap and can be divided into different subunits that respond to pathologic stimuli according to their inherent reaction capacities in a coordinated pattern.
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The superficial reactive unit comprises the subunits epidermis, the junction zone, the subjacent loose, delicate connective tissue of the papillary body and its capillary network, and the superficial vascular plexus (see Fig. 6-1, SRU). The reticular layer of the dermis represents a second reactive unit and is composed of coarse connective tissue and the deeper dermal vascular plexus (see Fig. 6-1, DRU). The third reactive unit, the subcutaneous tissue, is also anatomically and functionally heterogeneous; septal and lobular compartments may be involved either alone or together (see Fig. 6-1, S). Hair follicles and glands are a separate (fourth) reactive unit embedded in these three basic units.
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Keratinocytes, which have the capacity to synthesize keratin ...