O N 3 Overview of Biology , Development , and Structure of Skin

Skin is a complex organ that protects its host from its environment, at the same time allowing interaction with the environment. It is much more than a static, impenetrable shield against external insults. Rather, the 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, Chap. 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 Chap. 6). Whereas the epidermis and its outer stratum corneum provide a large part of the physical barrier provided by skin, the structural integrity of the 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 Chap. 10). Protection from UV irradiation is provided in great measure by the most superficial cells of the epidermis. Inflammation begins with the keratinocytes of the epidermis or immune cells of the dermis, and sensory apparatus emanates from nerves that initially traverse the hypodermis to the dermis and epidermis, ending in specialized receptive organs or free nerve endings. The largest blood vessels of the skin are found in the hypodermis, which serve to transport nutrients and immigrant cells (see Fig. 6-1, Chap. 6). The cutaneous lymphatics course through the dermis and hypodermis, serving to filter debris and regulate tissue hydration. Epidermal appendages provide special protective or sensory functions. Skin also determines a person’s physical appearance, influenced by pigmentation provided by melanocytes, with body contours, appearance of age, and actinic damage influenced by the epidermis, dermis, and hypodermis. The skin begins to be organized during embryogenesis, where intercellular and intracellular signals, as well as reciprocal crosstalk between different tissue layers, are instrumental in regulating the eventual maturation of the different components of skin. What follows is an integrated description of the major structural features of the skin and how these structures allow the skin to achieve its major functions, followed by a review of their embryologic origins. Also highlighted are illustrative cutaneous diseases that manifest when these functions are defective. Understanding the genetic and molecular basis of skin disease has confirmed, and in some cases revealed, the many factors and regulatory elements that play critical roles in skin function. EPIDERMIS