Every clinician should be able to examine the skin and nails identifying the primary skin diseases and cutaneous signs of systemic disease. This chapter will help you characterize lesions sufficiently to either make a diagnosis or determine that referral to a dermatologist is indicated. The assistance of dermatologic atlases and textbooks listed in the bibliography is encouraged.
The skin covers the entire body surface protecting the underlying tissues from injury, infection, heat and fluid loss, and supporting the peripheral nerve endings. It is contiguous with the orifice mucous membranes at sharply demarcated borders. It is essential for temperature regulation dissipating heat via radiation, conduction, and convection (aided by the production of sweat), and providing insulation with the dermal and subcutaneous fat. Integrity of the epidermis depends upon tight intercellular adhesion to form an impermeable barrier. The dermis is rich in blood vessels, which dilate or constrict to dissipate or conserve body heat. Integrity of the dermis depends upon interlacing collagen bundles and elastic tissue.
In addition to its physical protective functions, the skin forms an immunologic barrier as well. Within the epidermis are Langerhans cells, antigen-presenting cells that migrate to the regional lymph nodes when activated by foreign antigens.
The skin also contains many specialized structures. Some of these are skin appendages, including the hair and glands and special sensory organs of the nervous system, often uniquely aggregated in specific locations. So, in addition to its protective functions, the skin and hairs also function as a sensory organ.
Morphologically, the three chief layers of the skin are the epidermis, the dermis, and the subcutaneous tissue.
The epidermis (cuticle) (Fig. 6–1) is the most superficial layer. It has four strata. The keratin layer (stratum corneum) is made of overlapping stratified keratinized nonliving cells that sequentially separate and drop off (desquamation). Underlying the horny layer are the granular layer (stratum granulosum), the spinous layer (stratum spinosum), and the basal layer (stratum basale). These layers consist mostly of keratinocytes, living cells deriving their nourishment from the dermis, since the epidermis is avascular. Melanocytes in the lower epidermis contain melanin, a brown or black pigment whose concentration is determined by heredity, exposure to sunlight, injury and repair, and hormonal control. The epidermis contains a network of furrows or rhomboid lines, visible with the unaided eye through the keratin layer; on relaxed surfaces the furrows are narrow, while over joints they are widened. The epidermis is thickest in areas of high friction, such as the palms and soles. The epidermis is separated from the dermis by the basement membrane and attached to it through hemidesmosomes.