The epidermis forms the outer layer of skin, creating a tough, renewable, waterproof barrier against the environment. It is a type of epithelium, the tissue that makes up surfaces and linings in the body. Over most of the body the epidermis is relatively thin, while on areas such as the palms of the hands and soles of the feet it is much thicker and hairless. Depending on its location, the epidermis may contain hair follicles, nails, and sweat and sebaceous glands. The study of skin is known as dermatology.
With its multiple layers of epithelial cells, the top layer of skin protects the body from the world outside. The main type of cell it contains is the keratinocyte, so called because it makes tough proteins called keratins which help to strengthen the skin. As well as being tough, this layer has to constantly renew itself and repair any damage resulting from injuries. It does so by continually growing new layers of cells.
The deepest layer of cells is known as the basal layer. Basal cells divide to form new keratinocytes and, as each successive layer forms, the layer above is pushed nearer to the surface of the skin. The closer cells are to the surface, the flatter they become, until the outermost layers are composed of what is called squamous epithelium. This is made from flat dead keratinocytes known as squames.
While those cells at the surface are dead and constantly being shed, the layers of keratinocytes lower down are still alive and active. What is called the granular cell layer seals off the living cells, forming a waterproof barrier. This means that although water can cause the surface skin cells to swell when taking a bath, it may not penetrate the deeper epidermal layers. The presence of the granular cell layer also prevents unregulated loss of water from the body.
In addition to keratinocytes, there are a number of other cells in the epidermis. Melanocytes are partly responsible for the color of the skin, through their production of the pigment melanin. Merkel cells are associated with sensing light touch, and Langerhans cells form part of the body's immune response.
Sweat glands are present in the epidermis and are important in regulating the temperature of the body by allowing sweat to evaporate from its surface. Hair follicles and sebaceous glands occur together, with the glands producing oil that helps stop skin from drying out. Following an injury involving extensive skin removal, any epithelial cells that have been left behind in slightly deeper sweat glands and hair follicles may be used by the body to grow new skin.
