Piel

Introduction

The skin covers the entire external surface of the human body and is the principal site of interaction with the surrounding world. It serves as a protective barrier that prevents internal tissues from exposure to trauma, ultraviolet radiation, temperature extremes, toxins, and bacteria. Other important functions include sensory perception, immunologic surveillance,thermoregulation, and control of insensible fluid loss. For information on diseases and disorders of the skin, see the eMedicine Dermatology journal.

Anatomy of the skin.
Anatomy of the skin.

Anatomy of the skin.

Anatomy of the skin.

The integument consists of 2 mutually dependent layers, the epidermis and dermis, which rest on a fatty subcutaneous layer, the panniculus adiposus. Theepidermis is derived primarily from surface ectoderm but is colonized by pigment-containing melanocytes of neural crest origin, antigen-processing Langerhans cells of bone marrow origin, and pressure-sensing Merkel cells of neural crest origin. The dermis is derived primarily from mesoderm and contains collagen, elastic fibers, blood vessels, sensory structures, and fibroblasts.1

During the fourthweek of embryologic development, the single cell thick ectoderm and underlying mesoderm begin to proliferate and differentiate. The specialized structures formed by the skin, including teeth, hair, hair follicles, fingernails, toenails, sebaceous glands, sweat glands, apocrine glands, and mammary glands also begin to appear during this period in development. Teeth, hair, and hair follicles areformed by the epidermis and dermis in concert, while fingernails and toenails are formed by the epidermis alone. Hair follicles, sebaceous glands, sweat glands, apocrine glands, and mammary glands are considered epidermal glands or epidermal appendages, because they develop as downgrowths or diverticula of the epidermis into the dermis.1,2

The definitive multi-layered skin is present at birth,but skin is a dynamic organ that undergoes continuous changes throughout life as outer layers are shed and replaced by inner layers. Skin also varies in thickness among anatomic location, sex, and age of the individual. This varying thickness primarily represents a difference in dermal thickness, as epidermal thickness is rather constant throughout life and from one anatomic location to another.Skin is thickest on the palms and soles of the feet (1.5 mm thick), while the thinnest skin is found on the eyelids and in the postauricular region (0.05 mm thick). Male skin is characteristically thicker than female skin in all anatomic locations. Children have relatively thin skin, which progressively thickens until the fourth or fifth decade of life when it begins to thin. This thinning is alsoprimarily a dermal change, with loss of elastic fibers, epithelial appendages, and ground substance.3
Epidermis

The epidermis contains no blood vessels and is entirely dependent on the underlying dermis for nutrient delivery and waste disposal via diffusion through the dermoepidermal junction. The epidermis is a stratified squamous epithelium that consists primarily of keratinocytes inprogressive stages of differentiation from deeper to more superficial layers. The named layers of the epidermis include the stratum germinativum, stratum spinosum, stratum granulosum, and stratum corneum. The stratum germinativum or the basal layer is immediately superficial to the dermoepidermal junction. This single cell layer of keratinocytes is attached to the basement membrane via hemidesmosomes.As keratinocytes divide and differentiate, they move from this deeper layer to the more superficial layers. Once they reach the stratum corneum, they are fully differentiated keratinocytes devoid of nuclei and are subsequently shed in the process of epidermal turnover. Cells of the stratum corneum are the largest and most abundant of the epidermis. This layer ranges in thickness from 15-100 or...