The skin is the site of many complex and dynamic processes as demonstrated in Figure 1-1 and Table 1-1. These processes include barrier and immunologic functions, melanin production, vitamin D synthesis, sensation, temperature regulation, protection from trauma and aesthetics.
| Component | Structure and Function |
|---|---|
| Stratum corneum | Semipermeable barrier with “bricks” (stacked cornified cells) and “mortar” (ceramides, cholesterol, and fatty acids) like construction |
| Stratum granulosum | Contains keratohyalin granules that produce profilaggrin |
| Stratum spinosum | Contains desmosomes for intercellular adhesion |
| Langerhans cells | Dendritic cells, important in the modulation of the adaptive immune response |
| Merkel cells | Specialized cells with neuroendocrine function |
| Melanocytes | Dendritic cells that produce melanin for ultraviolet light protection |
| Basal cell layer | Contains the stem cells that divide and produce the rest of the keratinocytes in the epidermis |
| Basement membrane | Interface between the epidermis and dermis |
| Ground substance | Amorphous gel of mucopolysaccharides that is the substrate for the dermis |
| Collagen | Network of fibrous proteins for skin tensile strength |
| Elastic fibers | Fibrous proteins responsible for skin elasticity |
| Fibroblasts | Cells that produce ground substance, collagen, and elastic fibers |
| Mast cells | Leukocytes that release histamine and heparin |
| Histiocytes/macrophages | Leukocytes that phagocytize and present antigen |
| Eccrine glands | Sweat glands that help with temperature regulation |
| Apocrine glands | Axillary and anogenital glands responsible for body odor |
| Sebaceous glands | Component of pilosebaceous unit that produces sebum |
| Hair follicle | Component of pilosebaceous unit that produces the hair fiber |
| Somatic sensory and sympathetic autonomic nerves | Supply blood vessels, glands, and hair follicles |
| Meissner corpuscles | Specialized nerve receptors for light touch |
| Pacinian corpuscles | Specialized nerve receptors for pressure and vibration |
| Blood vessels | Two horizontal plexies in the dermis that are connected and can shunt blood flow |
| Lymphatics | Parallel to blood vessels with 2 plexuses for flow of plasma |
| Fat | Provides protection from cold and trauma; Essential for storage of energy and metabolism of sex hormones and glucocorticoids |
The epidermal barrier protects the skin from microbes, chemicals, physical trauma, and desiccation due to transepidermal water loss.1–3 This barrier is created by differentiation of keratinocytes as they move from the basal cell layer to the stratum corneum. The keratinocytes of the epidermis are produced and renewed by stem cells in the basal layer resulting in replacement of the epidermis approximately every 28 days. It takes 14 days for these cells to reach the stratum corneum and another 14 days for the cells to desquamate.
Keratinocytes produce keratins, structural proteins that form filaments that are part of the keratinocyte cytoskeleton. In the stratum spinosum keratin filaments radiate outwards from the nucleus and connect with desmosomes which are prominent under the microscope giving a “spiny” appearance to cells. As cells move into the stratum granulosum, keratohyalin granules composed of keratin and profilaggrin are formed. Profilaggrin is converted into filaggrin (filament aggregation protein) that aggregates and aligns keratin filaments into tightly compressed parallel bundles that form the matrix for the cells of the stratum corneum. Filaggrin gene mutations are associated with ichthyosis vulgaris and atopic dermatitis. As keratinocytes move into the stratum corneum they lose their nuclei and organelles and develop a flat hexagon shape. These cells are stacked into a “bricks and mortar”–like pattern with 15 to 25 layers of cells (bricks) surrounded by lipids (mortar). The lipids consist of ceramides, free fatty acids, and cholesterol.
Epithelial cells at the interface between the skin and the environment provide the first line of defense via the innate immune system.4–6 Epithelial cells are equipped to respond to the environment through a variety of structures including Toll-like receptors (TLRs) of which there are at least 10, nucleotide-binding oligomerization domain-like receptors, C-type lectins, and peptidoglycan recognition proteins. TLR-mediated activation of epithelial cells is also associated with the production of defensins and cathelicidins, families of antimicrobial peptides.
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