Skin is an important part of one’s personality and character; a lot can be learned by observing an individual’s skin and its abnormalities. Wrinkles are an indication of one’s mood, age, social habits, or overexposure to the sun. The skin color reflects one’s ethnicity as a result of the melanin content; the skin texture can reflect one’s life occupation that involves repeated mechanical forces or weather exposure. Skin reflects one’s emotions as it moves fluidly with the underlying muscles and connective tissue. Skin abnormalities can be a response to a disease process, injury, allergy, or medication. But what does the skin have to do with wound healing? In order to be considered closed, a wound has to have full re-epithelialization, defined as new skin growth, and no drainage or weeping from the pores. An appreciation for the anatomy and physiology of the integumentary system and the skin’s role in healing is needed to understand wound closure, complete with optimal aesthetics and function.

The skin is a complex, dynamic, multilayered organ that covers the body, making it the largest single organ. It comprises 15–20% of the total body weight; if laid out flat, the skin would cover a surface of 1.5–2 m².¹ Embedded in the layers are a plethora of cells, vessels, nerve endings, hair follicles, glands, and collagen matrixes, each performing a specific task that as a whole enables the skin to protect and preserve the rest of the body. Both the cellular and non-cellular components of the epidermis and dermis are described in TABLES 1-1 and 1-2.

The layers of the skin are organized into the outermost epidermis and the underlying dermis. Beneath the dermis is a structure called the hypodermis or subcutaneous layer, although it is not a true part of the skin (FIGURE 1-1). The junction of the epidermis and dermis is reticular, with an individualized pattern that forms dermatoglyphs, or the fingerprints and footprints, of the hands and feet.¹ The reticular structure allows the skin to withstand the repeated friction and shear forces that occur with activities of daily living; however, as the skin ages the ridges flatten out and the skin is more susceptible to frictional tears and blistering. Between the epidermis and dermis is a laminar adhesive layer termed the basement membrane that binds the two layers of the skin.

Epidermis

The layers of the epidermis are, from innermost to the surface, stratum basale, stratum spinosum, stratum granulosum, stratum lucidum, and stratum corneum; in totality the layers are 50–150 µm in thin skin, 400–1400 µm in thick skin^1,2 (FIGURE 1-2). The primary cells composing the epidermal layers are keratinocytes, with melanocytes, Langerhans cells, and Merkel cells embedded in layers. The keratinocytes are mitotically active in the stratum basale, but through a process defined as stratification, they migrate outward to the avascular stratum spinosum and begin to flatten out and become less active. When they reach the outer stratum corneum, the keratinocytes are termed corneocytes, dead flat cells that form the outer protective layer of the skin.

The keratinocytes are composed of keratin protein filaments that are present in greater concentrations as the cells migrate toward the stratum corneum. In the stratum basale, the keratinocytes are bound to the basal lamina by hemidesmosomes; and in all the epidermal layers, to each other by desmosomes. These cell-to-cell adherent discs are composed of transmembrane glycoproteins, termed cadherins, and include four desmoglein proteins (FIGURE 1-3).³

As the keratinocytes move into the stratum spinosum, they become active in keratin or protein synthesis. The keratin forms filament bundles called tonofibrils that converge on the hemidesmosomes and desmosomes to give the skin strength to withstand friction or shear force. As the keratinocytes migrate into the stratum granulosum, filaggrin (derived from “filament-aggregating protein”) binds to the tonofibrils, thereby forming an insoluble keratin matrix that “acts as a protein scaffold for the attachment of cornified-envelope proteins and lipids that together form the stratum corneum.”⁴ Also in the stratum granulosum, lamellar granules containing many lamellae of lipids undergo exocytosis, releasing a lipid-rich material into the intercellular spaces and forming envelopes around the protein-filled cells that are undergoing keratinization.¹ This combination of tightly adhered filaments and lipid-rich envelopes is what gives the skin its ability to serve as both a barrier to loss of water from the body and protection from extrinsic foreign material.

The stratum lucidum is present primarily in the thick, hairless skin of the palms and soles (termed glabrous skin) and consists of dead, clear keratinocytes, thus the term “clear layer.” The stratum lucidum is between the stratum granulosum and the stratum corneum and provides the palms and soles more protection from friction and serves as a greater moisture barrier.

When the keratinocytes enter the stratum corneum, they are flat, stacked, and embedded in lipid layers to form the main protective shield of the skin. The keratinocytes are held together with modified adhesive desmosomes, termed corneodesmosomes.⁵ As the keratinocytes migrate to the surface they are termed corneocytes, the corneodesmosomes are degraded in a process carefully controlled by a number of proteases and their inhibitors, and as a result the cells desquamate or slough off.⁶ Over a period of 30 days, the entire process of migration and desquamation is completed and the epidermis is renewed.