SKIN

" EARS (Hearing)

" EYES (Vision)

" MOUTH (Taste)

" NOSE (Smell)

" SKIN (Touch)

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Skin - Introduction

Skin, outer body covering of an animal. The term skin is commonly used to describe the body covering of any animal but technically refers only to the body covering of vertebrates (animals that have a backbone). The skin has the same basic structure in all vertebrates, including fish, reptiles, birds, and humans and other mammals. This article focuses primarily on human skin.

The skin is essential to a person’s survival. It forms a barrier that helps prevent harmful microorganisms and chemicals from entering the body, and it also prevents the loss of life-sustaining body fluids. It protects the vital structures inside the body from injury and from the potentially damaging ultraviolet rays of the sun. The skin also helps regulate body temperature, excretes some waste products, and is an important sensory organ. It contains various types of specialized nerve cells responsible for the sense of touch.

The skin is the body’s largest organ—that of an average adult male weighs 4.5 to 5 kg (10 to 11 lb) and measures about 2 sq m (22 sq ft) in area. It covers the surface of the body at a thickness of just 1.4 to 4.0 mm (0.06 to 0.16 in). The skin is thickest on areas of the body that regularly rub against objects, such as the palms of the hands and the soles of the feet. Both delicate and resilient, the skin constantly renews itself and has a remarkable ability to repair itself after injury.

STRUCTURE OF THE SKIN

The skin consists of an outer, protective layer (epidermis) and an inner, living layer (dermis). The top layer of the epidermis is composed of dead cells containing keratin, the horny protein that also makes up hair and nails.

The skin is made up of two layers, the epidermis and the dermis. The epidermis, the upper or outer layer of the skin, is a tough, waterproof, protective layer. The dermis, or inner layer, is thicker than the epidermis and gives the skin its strength and elasticity. The two layers of the skin are anchored to one another by a thin but complex layer of tissue, known as the basement membrane. This tissue is composed of a series of elaborately interconnecting molecules that act as ropes and grappling hooks to hold the skin together. Below the dermis is the subcutaneous layer, a layer of tissue composed of protein fibers and adipose tissue (fat). Although not part of the skin itself, the subcutaneous layer contains glands and other skin structures, as well as sensory receptors involved in the sense of touch.

Epidermis

About 90 percent of the cells in the epidermis are keratinocytes, named because they produce a tough, fibrous protein called keratin. This protein is the main structural protein of the epidermis, and it provides many of the skin’s protective properties. Keratinocytes in the epidermis are arranged in layers, with the youngest cells in the lower layers and the oldest cells in the upper layers. The old keratinocytes at the surface of the skin constantly slough off. Meanwhile, cells in the lower layers of the epidermis divide continually, producing new keratinocytes to replace those that have sloughed off. As keratinocytes push up through the layers of the epidermis, they age and, in the process, produce keratin. By the time the cells reach the uppermost layer of the epidermis, they are dead and completely filled with the tough protein. Healthy epidermis replaces itself in a neatly orchestrated way every month.

Scattered among the keratinocytes in the epidermis are melanocytes, cells that produce a dark pigment called melanin. This pigment gives color to the skin and protects it from the sun’s ultraviolet rays. After being produced in the melanocytes, packets of melanin called melanosomes transfer to the keratinocytes. There they are arranged to protect the deoxyribonucleic acid (DNA), or genetic material, of the keratinocytes.

All people have roughly the same number of melanocytes. Differences in skin color, such as that between light-skinned people of European descent and dark-skinned people of African descent, result from differences in the amount of melanin produced and how melanosomes are arranged in the keratinocytes. Particularly in people with light skin, melanin sometimes accumulates in patches, forming freckles, age spots, or liver spots.

In people of almost any skin tone, exposure to the sun causes melanocytes to increase their production of melanin to help protect the skin. This process results in a darkening of the skin tone to form a suntan. The suntan fades when keratinocytes containing the extra melanin are sloughed off. A type of melanin called pheomelanin makes redheaded people more sensitive to the sun. A total lack of melanin, a genetic condition called albinism, makes people extremely sensitive to the sun. People with albinism can be members of any race and have very light skin, hair, and eyes.

The epidermis also contains a type of immune cell known as a Langerhans cell. Produced in the bone marrow, Langerhans cells take up sentrylike positions in the epidermis, where they help cells of the immune system recognize potentially dangerous microorganisms and chemicals.

Another cell in the epidermis is the Merkel cell, found in sensitive, hairless areas such as the fingertips and lips. Located in the deepest layer of the epidermis, Merkel cells contact nerve endings in the dermis below and function as a type of touch receptor.

Dermis

Unlike the epidermis, the dermis or lower layer of the skin is richly supplied with blood vessels and sensory nerve endings. The dermis also contains relatively few cells compared to the epidermis—instead, it is made up mainly of fibrous proteins and other large molecules.

The main structural component of the dermis is a protein called collagen. Bundles of collagen molecules pack together throughout the dermis, accounting for three-fourths of the dry weight of skin. Collagen is also responsible for the skin’s strength. Another protein in the dermis, elastin, is the main component of elastic fibers. These protein bundles give skin its elasticity—the ability to return to its original shape after stretching. Collagen and elastin are produced by cells called fibroblasts, which are found scattered throughout the dermis.

The upper part of the dermis is known as the papillary layer. It is characterized by dermal papillae, tiny, fingerlike projections of tissue that indent into the epidermis above. In the thick skin on the palms and soles, the epidermis conforms to the shape of the underlying dermal papillae, forming ridges and valleys that we know as fingerprints. These ridges provide traction that helps people grasp objects and surfaces.

Some dermal papillae contain touch receptors called Meissner’s corpuscles, and many contain loops of tiny blood vessels. The extensive network of blood vessels in the dermal papillae plays an important role in the regulation of body temperature. The blood vessels dilate in hot environments to help dissipate heat, and they constrict to conserve heat in cold environments. Approximately one-fourth of the body’s blood flows through the skin at any given time.

The lower layer of the dermis is called the reticular layer. It is made primarily of coarse collagen and elastic fibers. Skin appendages such as glands and hair follicles are often anchored in the reticular layer of the dermis. The reticular layer also contains several different types of sensory receptors, nerve cells specialized to detect various stimuli, including pain, heat, cold, itch, and pressure. For example, Pacinian corpuscles are receptors found in the deep dermis of weight-bearing surfaces, such as the soles of the feet. They are composed of concentric layers of cells, much like an onion, and transmit vibrational stimuli. Sensory receptors are more dense in hairless areas, such as the fingertips and lips, making these areas especially sensitive.

World Health Organization