Physical composition and function of the skin

The two major layers of the skin include the epidermis and the deeper dermis. Deeper still is the subcutaneous fat and fascia, which together are also often referred to as the hypodermis, though this is not part of the integument unlike the epidermis and dermis. A drawing of a three-dimensional (3D) model depicts various major components at different levels of the skin ( Fig. 5.1 ).

Structural components of skin.

In a 3D cross section of skin, various components and appendages are housed with the three major layers of the skin known as the epidermis, dermis, and hypodermis. The most superficial layer of skin, the epidermis, is anchored to the deeper layer, the dermis, at the dermal-epidermal junction (DEJ) by intercalating dermal papillae and ridges. The dermis consists of extracellular matrix (ECM) that houses various functional components such as blood vessels, lymphatics, nerves, glands, and skin appendages. The deepest layer of skin that is sometimes included as a skin layer though it is not part of the integument is the hypodermis. This layer is made of subcutaneous fat and fascia.

The epidermis is largely composed of stratified squamous keratinized cells called keratinocytes that populate each of the sublayers birthed at the deepest cell layer against the basement membrane and become more differentiated as the cells become more superficial. Within the epidermis there are four sublayers (strata): From most superficial to deepest they are stratum corneum, stratum granulosum, stratum spinosum, and stratum basale. An additional dense layer called stratum lucidum consists of enucleated flat keratinocytes between stratum granulosum and stratum corneum and exists in thick skin such as in palms and soles. The histologic features, main functions, and specialized cells of each strata are described in Table 5.1 . Basal stem cells are bound to the basement membrane by hemidesmosomes, and keratinocytes are bound to adjacent cells by desmosomes and other intercellular proteins contributing to the permeability barrier created from epidermal layers. As cells travel superficially, they undergo differentiation called keratinization (also known as cornification) and enhance the amount and types of keratin filaments while they flatten, dehydrate, and eventually dispose of their nucleus and organelles at their terminal stage in the stratum corneum. At the surface, cells are surrounded by a keratinized envelope composed of transglutaminase cross-linked proteins and lipids, notably loricrin and involucrin ( ) ( Fig. 5.2 ).

Layers of the epidermis.

The epidermis consists of four major sublayers or strata composed of keratinocytes. As cells migrate from the deepest to the most superficial strata, they undergo differentiation in a process called keratinization. The most superficial of the strata is the stratum corneum, which possesses terminally differentiated, enucleated, and dead keratinocytes that are flattened, dehydrated, and surrounded by a dense envelop of protein and lipids that serves as a water-resistant seal. Beneath it is the stratum granulosum named for its prominent keratinohyaline granules and Golgi-derived lamellar granules containing lipids that contribute to the water barrier. Deeper yet, the stratum spinosum is the thickest of the strata and is characterized by the intercellular bridges created by desmosomes, which anchor adjacent cells to one another. The stratum basale is the deepest of the strata and is a single layer of columnar cells attached to the basement membrane by hemidesmosomes. This is where the basal stem cells produce the new keratinocytes through mitosis. An additional dense layer called stratum lucidum (not pictured in Fig. 5.2 ) consists of enucleated flat keratinocytes just deep to the stratum corneum and exists in thick skin such as in palms and soles.

The DEJ is where the basement membrane lies and has a specialized 3D structure that anchors the epidermis to the dermis. Epidermal papillae and ridges show a wavy appearance in a superficial-to-deep cross section or a Swiss cheese model in a cross-sectional plane taken parallel to the skin surface at the DEJ.

The dermis consists of largely ECM made up of different protein fibers that provide the structural component of the skin. The dermal papillae consisting of the looser areolar connective tissue and its own vascular and lymphatic plexuses sit atop the reticular layer of the dermis, which contains more acellular irregular dense connective tissue. Fibroblasts are the main cells of the dermis producing ECM. Collagen is the predominant component of matrix responsible for the biomechanical properties associated with the strength of the dermal matrix by forming dense bundles of fibers. Other fibrillar proteins such as elastin, fibronectin, laminin, and fibrillins arrange around collagen to create a loose network that allows for the elasticity and flexibility of skin. Filling the spaces between fibers are the extrafibrillar matrix consisting of liquid binding glycosoaminoglycans (e.g., hyaluronan), proteoglycans, and glycoproteins that create a fluid medium for the movement of water, molecules, and immune cells. Additional cells in the dermal layer include macrophages, mast cells, adipocytes, Schwann cells, and stem cells.

Within the dermis there are several types of epidermally derived skin appendages, including sebaceous glands, sweat glands, and hair follicles. Blood vessels, lymphatics, and nerves also reside in the dermal and subcutaneous layers. Table 5.1 details the function of these structures ( ). To sum up, the skin provides several protective barriers to the outside world, most notably the physical barrier provided by the many layers of skin, thermoinsulation from fat, water barrier created by keratinization and lipid-rich layer in epidermal stratum granulosum, and immunologic barrier created by various antigen-presenting cells and players in innate and adaptive immunity (Langerhans cells in the stratum spinosum, dendritic cells, macrophages, mast cells, and even recently discovered skin-resident T cells in the dermis and epidermis) ( ). A properly functioning immunologic barrier is crucial in maintaining healthy skin, and these immune factors are discussed in greater detail in Chapter 7, Chapter 13, Chapter 14 . In addition to the native components of skin, the healthy skin microbiome is part of a functional barrier of skin, which is elucidated in Chapter 6 .