Skin structure and function
The skin is an organ that provides the outer protective wrapping for all the body parts. It is the largest organ in the body. It is a waterproof, airtight and flexible barrier between the environment and internal organs. It keeps the internal environment of our body stable. The skin is divided into 3 layers, the epidermis, the dermis and the subcutaneous layer.
The diagram below shows how the different layers and parts of the skin are arranged.
Epidermis
The epidermis is the outer layer of the skin. It is a mosaic of cells glued together and its thickness depends on the location on the body. On the palms and soles the epidermis is thick, flexible and resists mechanical injury. On the eyelids it is very thin and allows maximum movement. The epidermis prevents loss of water and body fluids, resists mechanical and chemical injury and protects against bacteria, viruses and parasite infections. The pigment in the epidermis plays an important role in protecting the skin from ultraviolet radiation.
The hair follicles, sweat glands, sebaceous (oil) glands and apocrine glands develop from the epidermal cells, but their deeper parts extend into the dermis. The glands open onto the surface of the skin via small ducts.
Hair grows from the hair follicle, which is found in all skin except the palms and soles.
Nails are specialised plates of hard keratin that develop from the epidermis overlying the small bones at the ends of the fingers and toes.
There are 3 main groups of cells in the epidermis:
- Keratinocytes (skin cells)
- Melanocytes (pigment cells)
- Langerhans cells (immune cells).
The main cell in the epidermis is the keratinocyte, which develops from the bottom or basal layer and then migrates upwards over a period of about four weeks to the outer surface (stratum corneum) where it is shed.
Langerhans cells are specialised immune cells that are an important part of the body’s immune response to foreign materials and infections.
The melanocytes produce pigment. All humans have the same number of melanocytes. The difference in skin colour occurs because in darker skin melanocytes produce more pigment. The melanin pigment protects the cells of the epidermis and the tissues in the dermis from sun damage. Lighter skinned people are more susceptible to developing sun damaged skin because their melanocytes produce less melanin (skin pigment).
Dermo-epidermal junction
This is a complex region where the dermis and epidermis are attached to each other via specialised cells and molecules. It contains the basement membrane.
Dermis
The dermis lies beneath the epidermis and is 20 to 30 times thicker than the epidermis. It is composed of a dense network of specialised proteins (collagen and elastin) organised into fibres of differing sizes and properties. A complex gel of different proteins surrounds these fibres. All together this is known as the extracellular matrix.
Within the extracellular matrix are blood and lymphatic vessels, nerves, the bottom part of the hair follicles and sweat glands.
Subcutis (subcutaneous layer)
This is a specialised area under the dermis, which contains a network of collagen fibres and fat cells (adipocytes). It protects the body from external trauma and insulates from cold. It acts as a main storage site for fat and therefore energy. There are many blood and lymphatic vessels and nerves passing through the subcutis.
The thickness of the subcutaneous layer varies according to the location on the body and from person to person.
This information has been written by Dr Rashi Minochi and Dr James Choi