Epidermis

The epidermis covers the entire body with a layer of stratified squamous epithelium. Its principal component is the malpighian stratum, arranged in three poorly defined layers: (1) a basal layer called the stratum germinativum lying on the dermis, from which the epidermis gets its support and blood supply; (2) the stratum spinosum ; and (3) the stratum granulosum. Overlying the malpighian stratum is the stratum corneum , a relatively impermeable layer of desquamating, nonnucleated cells. It provides the surface covering for the skin. Beneath the malpighian layer and against the dermis is the stratum lucidum . The epidermis contains no blood vessels and depends on the dermis for nutrition.

It has been estimated that each cell in the stratum germinativum of the malpighian layer takes 19 days to reach the surface. As the cells are displaced outward, they become increasingly keratinized, the keratin either remaining soft, as in the skin, or becoming hard, as in the nails and hair. In either case, the stratum corneum forms a tough layer that serves as a barrier to the environment.

At the junction between the epidermis and the dermis, rete pegs project into the dermis among dermal papillae vascularized by capillary loops.

Dermis

The dermis , made up of collagen and elastic fibers in a diffuse ground substance, is the matrix for nerves, vessels, and glands. It is composed of two layers—(1) a superficial papillary layer of delicate fibers and (2) a deep reticular layer of much coarser branching fibers of collagen lying more or less parallel to the surface amid elastic tissue.

Near the epidermis, the collagen fibers in the papillary layer become finer to act as a protecting buffer between the coarse collagen fibers below and the epithelial cells above. The interspersed elastic fibers are interconnected and serve to return stretched collagen fibers to their resting position. The ground substance and accompanying fluid acts as a lubricant between the fibers, each fiber lying within its mucopolysaccharide sheath.

In the relaxed skin, the collagen fibers of the reticular layer are markedly coiled, especially in young individuals; in the stretched skin, they become parallel and resist further stretching. With age, they are straighter at rest, and the ground substance, previously gel-like, gradually becomes replaced by fibrous intercellular tissue, accounting for wrinkling of the skin. In any given area, the collagen bands of the reticular layer lie in parallel bundles that follow Langer lines (see Figure 5-3 ). Incisions that split the bundles longitudinally result in less scar formation than those that cut across them.

Three systems of vessels in succession distribute the blood to the skin after delivery by perforating arteries . These are (1) the subdermal vascular plexus (cutaneous rete) between the subcutaneous layer and the reticular layer, (2) the dermal vascular plexus between the dermal vascular layer and the papillary layer, and (3) the subpapillary vascular plexus at the junction of the papillary layer with the epidermis, from which the capillary loops emerge. These systems are interconnected by a complex network of vessels of varying sizes. If the dermis becomes excessively deformed, the rigidity of the surrounding collagen may compromise the lumens of these vessels with resulting ischemia. It must be appreciated that the principal function of the cutaneous vasculature is not for skin nutrition but for thermoregulation and is under neural control.

Hair follicles , present in most parts of the body, transfix the dermis and probably restrict its mobility. Sebaceous glands are part of the hair follicle structure. Sweat glands lie in the base of the dermis. They are both eccrine (secretory) glands, some of which respond to stress and some regulate temperature, and apocrine (shedding) glands that release the apical portion of the gland, producing a secretion with a characteristic odor.

After placement as a graft, the skin temporarily loses the normal lubrication from these glands and, unless protected with bland creams until glandular function returns, it becomes dry and susceptible to injury.

Subcutaneous layer

The subcutaneous layer is fatty and serves principally as insulation. It contains free and encapsulated nerve endings for several types of sensory input and for control of the vascular supply. The subcutaneous tissue sends protrusions of fat, the fat domes or adipose columns, into the dermis. When the skin is cut at this level, a collagen network, which has interstices into which fat protrudes, is exposed. Hair follicles descend into the fat domes, and sweat glands lie among them. These extensions of elements of the skin into the subcutaneous tissue account for the reepithelialization that occurs in the donor site after split-thickness grafts are harvested or after deep burns. Strands of collagen run through the subcutaneous layer to attach the dermis to the underlying muscle. These strands vary in size and length, and determine the mobility of the skin. For example, over the penis, they are small and flexible; over the ilium, they are dense and firm.

The skin maintains its physical characteristics on transplantation, making cosmetic matching difficult.