The skin is the body’s largest organ. It creates a barrier between the external environment and the internal organs. The skin has several important functions vital to human life. Its thickness varies depending on where it is located on the body. For example, the skin on the face is thin compared to the skin on the back.
The epidermis and dermis are the 2 main layers of the skin. They lie on top of a third layer called the subcutis.
The epidermis is the thin outer layer of the skin. It contains no blood vessels and relies on the dermis for its nutrients and waste removal. The epidermis is thinnest on the eyelids and thickest on the palms of the hands and soles of the feet. The epidermis layer itself is made up of layers of cells (basal cells and squamous cells) that work together to continually rebuild the surface of the skin. The epidermis also includes 2 other types of specialized cells: Langerhans cells (involved in immune response) and Merkel cells (believed to play a role in making the skin sensitive to touch).
The basal cell layer is the deepest part of the epidermis. This layer sits on top of the dermis. The round cells in this layer are called basal cells. Basal cells continually divide, producing new cells that undergo a maturing, or keratinisation, process as they push the older cells toward the surface of the skin. These older cells eventually become flattened squamous cells.
Melanocytes are another type of cell found mainly in the basal cell layer. Melanocytes produce melanin, which is the substance that gives skin its colour. When the skin is exposed to the sun’s ultraviolet rays, the melanocytes produce more melanin. Melanin production helps protect the body by lessening the damaging effects of the sun’s ultraviolet radiation (UVR). The melanocytes in dark-skinned people are more active, so more melanin is produced. Therefore, dark-skinned people have more protection from the sun than light-skinned people.
Freckles, birthmarks and age spots are areas of the skin where melanocytes have produced more melanin than in the surrounding skin.
The squamous cell layer is located above the basal cell layer and occupies the major part of the epidermis. The main cells in this area are called keratinocytes. These cells contain a protein called keratin. Keratin is a tough substance that helps to protect the skin from injury. Keratin is also found in hair and nails.
As keratinocytes mature and move toward the surface of the skin, they undergo gradual changes in composition and appearance. Shortly before they reach the surface, the cells die and take on a scale-like appearance (squamous cells). The surface of the skin is covered in dead cells, which are shed and replaced every 3–4 weeks by the newly divided cells in the basal cell layer that will be pushed up.
The dermis is the second layer of the skin, beneath the epidermis. The dermis is the thickest of all 3 layers. It is made up of a papillary layer and a reticular layer. Collagen and elastin are produced by fibroblasts in the dermis to provide structure to the skin.
Most of the skin’s specialized structures are found in the dermis: blood vessels, lymph vessels, hair follicles, sweat glands, sebaceous (oil) glands and nerve endings.
Beneath the dermis lies a fat layer known as the subcutis or hypodermis. This layer is made up mainly of fat, or adipose tissue. It helps to conserve the body’s heat and protect the organs of the body.
The skin has many functions, including:
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