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Structure and Function of Skin at a Glance
  • Three major layers—epidermis, dermis, hypodermis:
    • Epidermis: major permeability barrier, innate immune function, adhesion, and ultraviolet protection.
    • Dermis: major structural element, three types of components—cellular, fibrous matrix, and diffuse and filamentous matrix. Also site of vascular, lymphatic, and nerve networks.
    • Hypodermis (subcutis): insulation, mechanical integrity, containing the larger source vessels and nerves.

Skin is a complex organ that protects its host from its environment, at the same time allowing interaction with its environment. It is much more than a static, impenetrable shield against external insults. Rather, skin is a dynamic, complex, integrated arrangement of cells, tissues, and matrix elements that mediates a diverse array of functions: skin provides a physical permeability barrier, protection from infectious agents, thermoregulation, sensation, ultraviolet (UV) protection, wound repair and regeneration, and outward physical appearance (Table 7-1). These various functions of skin are mediated by one or more of its major regions—the epidermis, dermis, and hypodermis (Fig. 7-1; see also Fig. 6-1, Chapter 6). These divisions are interdependent, functional units; each region of skin relies upon, and is connected with, its surrounding tissue for regulation and modulation of normal structure and function at molecular, cellular, and tissue levels of organization (see Chapter 6).

Table 7-1 Functions of Skin
Figure 7-1

The major regions of skin. Skin is composed of three layers: (1) epidermis, (2) dermis, and (3) hypodermis. The outermost epidermis is separated from the dermis by a basement membrane zone, the dermal–epidermal junction. Below the dermis lies the subcutaneous fat (hypodermis). Epidermal appendages, such as hair follicles and eccrine and apocrine sweat glands, begin in the epidermis but course through the dermis and/or the epidermis. Blood vessels, lymphatics, and nerves course through the subcutaneous fat and emerge into the dermis.

Whereas the epidermis and its outer stratum corneum provide a large part of the physical barrier provided by skin, the structural integrity of skin as a whole is provided primarily by the dermis and hypodermis. Antimicrobial activities are provided by the innate immune system and antigen-presenting dendritic cells of the epidermis, circulating immune cells that migrate from the dermis, and antigen-presenting cells of the dermis (see Chapter 10). Protection from UV irradiation is provided in great measure by the most superficial cells ...

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