Human skin

Structure

Human skin shares anatomical, physiological, biochemical, and immunological properties with other mammalian lines, particularly pig skin, which has similar epidermal and dermal thickness ratios, hair follicle and blood vessel patterns, and dermal collagen and elastin content. The skin contains mesodermal cells that produce pigmentation, such as melanin from melanocytes, which absorb some ultraviolet radiation. It also holds DNA repair enzymes that help reverse UV damage; people lacking these enzymes have high rates of skin cancer, including the invasive and often deadly melanoma.

Skin pigmentation varies substantially between populations, leading to classification based on skin colour. The skin is the largest organ, comprising roughly 16% of body mass, with a surface area of 1.5–2.0 m² for the average adult. Thickness varies over the body and between sexes and ages; for example, forearm skin averages differ in males and females.

One square inch of skin contains 650 sweat glands, 20 blood vessels, 60,000 melanocytes, and over 1,000 nerve endings. The average human skin cell diameter ranges from 25 to 40 μm. Skin is composed of three primary layers: the epidermis (the outermost, waterproof layer made of stratified squamous epithelium), the dermis, and the hypodermis (subcutaneous layer).

Development

Human skin shows high colour variety, from darkest brown to lightest pinkish-white, more than any other single mammalian species, as a result of natural selection. Skin pigmentation evolved primarily to regulate ultraviolet radiation penetration and its biochemical effects. The most important determinant of skin colour is melanin, produced in melanocytes; darker skin is mainly determined by melanin, while lighter skin colour is influenced by bluish-white connective tissue under the dermis and haemoglobin in dermal veins.

At least five pigments determine skin colour: melanin (brown, in the basal epidermis), melanoid (diffuse throughout the epidermis), carotene (yellow-orange, in stratum corneum and fat cells), haemoglobin (purple, in blood), and oxyhemoglobin (red, in blood). Geographic distribution of UV radiation correlates with indigenous skin pigmentation: higher UVR areas near the equator have darker-skinned populations, while areas farther from the tropics have lighter-skinned populations. Within the same population, adult females are considerably lighter in pigmentation than males, possibly because females need more calcium during pregnancy and lactation, and vitamin D from sunlight aids calcium absorption.

Functions

The skin provides an anatomical barrier against pathogens and damage, with Langerhans cells in the adaptive immune system and perspiration containing lysozyme that breaks bacterial cell walls. It contains nerve endings for sensing heat, cold, touch, pressure, vibration, and tissue injury. The skin regulates heat through a blood supply larger than its needs, allowing precise control of energy loss via radiation, convection, and conduction; dilated vessels increase heat loss, while constricted vessels conserve heat. It offers a semi-impermeable barrier to fluid loss, crucial in burns.

Skin also serves aesthetic and communication functions, stores lipids and water, synthesizes vitamin D with UV exposure, and excretes urea in sweat (at 1/130th the concentration of urine). The outermost 0.25–0.40 mm of skin is almost exclusively supplied by external oxygen, though this contributes negligibly to total respiration. Medicine can be administered through the skin via ointments or adhesive patches. The skin acts as a water-resistant barrier to prevent nutrient loss.

Permeability

Human skin has low permeability, with the stratum corneum blocking most inorganic nanosized particles and protecting internal tissues from toxins. However, transcutaneous particle delivery is being researched for medical applications, such as targeting cancer cells in the epidermis and other active cell division layers. Nanoparticles 40 nm in diameter and smaller can penetrate the skin; those larger than 40 nm do not go past the stratum corneum.

Most penetrating particles diffuse through skin cells, though some travel down hair follicles to the dermis. The shape of nanoparticles affects permeability: spherical particles penetrate better than oblong (ellipsoidal) ones, reaching deep into the epidermis and dermis, while ellipsoidal particles mainly stay in the stratum corneum and epidermal layers. Nanorods, used for their fluorescent properties, show mediocre penetration.

Did You Know?

• The skin supports its own ecosystems of microorganisms, including yeasts and bacteria, which cannot be removed by any amount of cleaning.
• Estimates place the number of individual bacteria on the surface of human skin at 50 million per square inch, though this figure varies greatly over the average 20 square feet of human skin.
• Dermatomes are helpful in the diagnosis of vertebral spinal injury levels; certain diseases like shingles cause pain and rashes involving dermatomal distribution.