The skin surface sets the boundary between the body and the external environment and serves three main functions. It protects against outside damage from physical, chemical, or infection-related stress, and it regulates body temperature and fluid balance. To fulfill these diverse functions, the skin has three anatomic layers: the epidermis, dermis, and subcutis, which serve independent and joint functions.

The epidermis, the outmost coating of the skin, is formed by amultilayer of keratinocytes. The epidermis is nonvascularized and separated by a basement membrane from the underlying zone of highly vascularized connective tissue, called the dermis. The dermo-epithelial zone is not linear, but rather is an interdigitated interface. Keratinocytes form the rete ridges, which protrude into the dermis. Concurrently, between these rete ridges, the dermis extends upward and forms the papillae. Morphologically and functionally distinct from this so-called papillary dermis is the deeper portion of the dermis, the reticular dermis, which harbors eccrine and apocrine glands (for sweat and scent production, respectively). The subcutaneous fat layer (subcutis) below resides on the muscle fascia. Terminal hairs root in the fat layer; the sebaceous glands pertinent to the hair follicle reside within the dermis (Figure 156.1; for color reproduction, see Color Plate 156.1).

VASCULAR ARCHITECTURE

The knowledge about the structure and organization of the cutaneous vasculature has been developed over the past 25 years. Before that, it was assumed that cutaneous vessels form a random network. Irwin Braverman was the pioneer in setting up a model for the complex vascular network of the skin (1). This chapter focuses on skin blood vessels; the characteristics of the lymphatic vascular system are described in Chapter 169.