Skin

Skin is the soft, flexible outer tissue covering the bodies of vertebrate animals. Its three main jobs are protection, regulation, and sensation. In mammals, it is an organ of the integumentary system, built from multiple layers of ectodermal tissue, and it shields the underlying muscles, bones, ligaments, and internal organs. The word *cutaneous* (from the Latin *cutis*, meaning "skin") refers to anything related to the skin.

Other animal coverings, like the exoskeleton of arthropods, come from different developmental origins, have different structures, and are made of different chemicals. Amphibians, reptiles, and birds each have their own distinct type of skin. Skin—including both cutaneous and subcutaneous tissues—is essential for forming, structuring, and operating extraskeletal features such as the horns of bovids (like cattle) and rhinos, the antlers of deer, the ossicones of giraffids, the osteoderms of armadillos, and the os penis or os clitoris.

All mammals have some hair on their skin, even marine mammals like whales, dolphins, and porpoises that look hairless. Skin is the body’s interface with the environment and its first line of defense against external threats. For instance, it protects against pathogens and prevents excessive water loss. It also provides insulation, helps regulate temperature, allows sensation, and supports the production of vitamin D folates. Severe damage to skin can heal by forming scar tissue, which may be discolored or depigmented. Skin thickness varies across the body. In humans, the thinnest skin—about 0.5 mm thick—is under the eyes and around the eyelids, making it one of the first areas to show aging signs like crow’s feet and wrinkles. The thickest skin, at about 4 mm, is on the palms and soles. Estrogen promotes faster and better wound healing in skin.

Fur is dense hair that mainly improves insulation but can also serve as a secondary sexual characteristic or camouflage. Some animals have very hard, thick skin that can be processed into leather. Reptiles and most fish have tough protective scales made of beta-keratins, while birds have hard feathers made of the same material. Amphibian skin is a weak barrier, especially for chemicals, and is often subject to osmosis and diffusion. For example, a frog sitting in an anesthetic solution will be sedated quickly as the chemical passes through its skin. Amphibian skin is crucial for daily survival and for exploiting a wide range of habitats and ecological conditions.

Biologists reported in January 2024 the discovery of the oldest known skin, fossilized about 289 million years ago, possibly from an ancient reptile.

**Etymology** The word *skin* originally referred only to dressed and tanned animal hide; the usual word for human skin was *hide*. It was borrowed from Old Norse *skinn*, meaning "animal hide, fur," which comes from the Proto-Indo-European root *sek-*, meaning "to cut"—likely because animal hide was commonly cut off to make garments.

**Structure in Mammals** Mammalian skin has two primary layers: the epidermis, which provides waterproofing and acts as a barrier to infection, and the dermis, which houses the skin’s appendages.

**Epidermis** The epidermis is the outermost layer, forming a protective barrier that keeps water in and pathogens out. It is a stratified squamous epithelium made of proliferating basal and differentiated suprabasal keratinocytes. Keratinocytes make up 95% of the epidermis; Merkel cells, melanocytes, and Langerhans cells are also present. The epidermis has several strata (from outermost inward): stratum corneum, stratum lucidum (only on palms and soles), stratum granulosum, stratum spinosum, and stratum basale (also called stratum germinativum). Keratinocytes in the stratum basale divide by mitosis; daughter cells move upward, changing shape and composition as they differentiate, eventually becoming anucleated. During this process, they form desmosomes (cellular junctions), secrete keratin proteins and lipids, and build an extracellular matrix that gives skin mechanical strength. Keratinocytes from the stratum corneum are eventually shed (desquamation). The epidermis has no blood vessels; cells in the deepest layers get nutrients by diffusion from blood capillaries in the upper dermis.

**Basement Membrane** A thin sheet of fibers called the basement membrane separates the epidermis and dermis, formed by the action of both tissues. It controls the movement of cells and molecules between the dermis and epidermis and also serves as a reservoir for cytokines and growth factors, releasing them during remodeling or repair.

**Dermis** The dermis lies beneath the epidermis and consists of connective tissue that cushions the body from stress and strain. It gives skin tensile strength and elasticity through an extracellular matrix made of collagen fibrils, microfibrils, and elastic fibers, embedded in hyaluronan and proteoglycans. Skin proteoglycans vary and have specific locations: hyaluronan, versican, and decorin are throughout the dermis and epidermis, while biglycan and perlecan are only in the epidermis. The dermis also harbors many structures.