Under the surface
Skin’s most important role is to protect the body from the environment. It comprises three main layers: the epidermis, the dermis and subcutaneous fat. Most of the body is covered in hairy skin but the palms of the hands and the soles of the feet are covered in hair-free (glabrous) skin.
Epidermis The outermost layer of skin acts as a mechanical and antimicrobial barrier, and consists of several layers. Its top part, the stratum corneum, prevents water from leaving the body and toxic substances from entering.
Dermis Nerve endings in skin’s middle layer help people to feel sensations such as itching, pain, pleasure and heat. The dermis produces sweat and oils, and contains hair follicles. It also hosts a variety of immune cells.
Subcutaneous fat Skin’s deepest layer is sandwiched between the dermis and skeletal muscles. Its roles include fat storage, connecting the dermis to muscle and bone, and controlling body temperature.
Hairy skin More than 90% of the body is covered by hairy skin1. It is involved in perceiving a variety of tacile sensations, including those that form part of social exchanges, and the ability to detect the presence of foreign objects. In hairy skin, the epidermis is less than 0.1 millimetres thick and the dermis is 1–2 millimetres deep.
Glabrous skin Hair-free skin is found mainly on the palms and soles. It is innervated by specialized nerves that help us to understand subtle tactile details. Such skin is thicker than hairy skin; the epidermis is about 1.5 millimetres thick and the dermis is about 3 millimetres deep.
Skin’s somatosensory system comprises more than a dozen subtypes of sensory neuron, but only those involved in tactile sensation are well understood. Such neurons enable skin to react to and interpret myriad stimuli, including temperature gradients, pressure and physical damage.
C fibre These unmyelinated nerve fibres are found only in hairy skin. Although sensitive to indentation, they are most active when a stimulus moves slowly across the skin’s surface.
Ruffini ending Found in the dermis of both hairy and glabrous skin, these sensory receptors respond optimally to stretching of skin.
Pacinian corpuscle Located deep in the dermis of both types of skin, Pacinian corpuscles respond to high-frequency vibration.
Meissner corpuscle These nerve receptors lie just beneath the epidermis of glabrous skin, where they detect movement across the skin and fluttering touch.
Merkel cell Part of the stratum basale of the epidermis, these cells help to relay information about the texture, curvature and shape of objects. Merkel cells are most dense in glabrous skin.
Skin’s epidermis and dermis help to protect the body from microbes, pollutants, ultraviolet radiation and excessive loss or absorption of water.
Despite its many superpowers, the skin is not infallible. Because of its visibility, diseases that affect the skin can have psychological as well as physical effects.
Skin disease’s worldwide burden can be quantified in terms of the disability-adjusted life year (DALY), which reflects a lost year of healthy life. The main burden falls on people aged 15–19, mostly owing to acne vulgaris. From the age of 50, there is a slow increase in burden as skin loses function and the incidence of skin cancer rises. Dermatitis, including eczema, persists throughout life; people tend not to outgrow the condition but learn to better manage it.
Melanoma kills more people worldwide than does non-melanoma skin cancer, even though non-melanoma is much more common. However, deaths from melanoma are dwarfed by those from other cancers. Skin cancer was responsible for about 60% of US skin-related deaths in 2013.
Nature 563, S84-S85 (2018)