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.
Principal Skin Structures.
Dermis and Subcutaneous Tissue
The superficial dermis is thrown into a series of papillae into which the epidermis is molded, the papillary dermis. The deeper reticular layer consists of dense connective tissue containing blood vessels, lymphatics, nerves, and considerable elastic tissue. In its deeper portion are collagenous bundles mixed with elastic fibers. Between the meshes of this layer are sweat glands, sebaceous glands, hair follicles, and fat cells. The reticular layer merges with the deeper and looser subcutaneous layer. The dermis is thickest over the back; it is extremely thin over the eyelids, scrotum, and penis. In general, the dermis is thicker over dorsal and lateral than over ventral and medial surfaces. Dermal appendages include hair apparatus as well as eccrine, apocrine, and sebaceous glands.
The astute diagnostician always examines the fingernails. With the exception of the eye, there is no region of comparable size in which so many physical signs of generalized disease can be found. The nails continue to grow throughout life, providing a record of brief or prolonged disturbances of nutrition. They also serve as windows through which to view capillary changes associated with constitutional disease.
The nail plate is a horny, semitransparent rectangle, convex rectangle, with a smaller radius of curvature transversely (Fig. 6–2). The nail plate rests on and adheres to the nail bed, a layer of modified skin on the dorsal aspect of the terminal phalanx. The bed is studded with small longitudinal ridges containing a rich capillary network that shows through the nail plate as a pink surface. Roughly, the proximal third of the nail bed is composed of partially cornified cells containing granules of keratohyalin; this specialized layer is the matrix where new nail is made and added to the nail plate, forcing it distally. The matrix is viewed through the nail plate as the white lunula. The proximal part of the nail plate, buried in a dermal pouch, is the root. The dermal lip of the pouch is called the mantle; it terminates in the cuticle, a sharp cornified rim. The distal nail plate not adherent to the bed is the free edge and the body is the intervening portion. The sides of the nail plate are buried in lateral nail folds of skin and cuticle.
Fingernail Anatomy.The nail plate is formed by the cells of the matrix and extruded distally to the free margin where the plate separates from the nail bed. The lunula marks the extent of the matrix under the nail plate.
The nail plate grows continuously by elongation from the root and thickening from the matrix. The average time for growing a new fingernail is approximately 6 months; growth is faster in youth ...