Skin layers

Skin is composed of three primary layers: the epidermis, which provides waterproofing and serves
as a barrier to infection; the dermis, which serves as a location for the appendages of skin; and
the hypodermis (subcutaneous adipose layer).

Epidermis

Epidermis, "epi" coming from the Greek meaning "over" or "upon", is the outermost layer of the
skin. It forms the waterproof, protective wrap over the body's surface and is made up of stratified squamous epithelium with an underlying basal lamina.

The outermost epidermis consists of stratified squamous epithelium with an underlying connective tissue section, or dermis, and a hypodermis, or basement membrane. The epidermis contains no blood vessels, and cells in the deepest layers are nourished by diffusion from blood capillaries extending to the upper layers of the dermis. The main type of cells which make up the epidermis are keratinocytes, with melanocytes and Langerhans cells also present. The epidermis can be further subdivided into the following strata (beginning with the outermost layer): corneum, lucidum (only in palms of hands and bottoms of feet), granulosum, spinosum, basale. Cells are formed through mitosis at the basale layer. The daughter cells, (see cell division) move up the strata changing shape and composition as they die due to isolation from their blood source. The cytoplasm is released and the protein keratin is inserted. They eventually reach the corneum and slough off (desquamation). This process is called keratinization and takes place within about 30 days. This keratinized layer of skin is responsible for keeping water in the body and keeping other harmful chemicals and pathogens out, making skin a natural barrier to infection.

Hygiene

Unclean skin favors the development of pathogenic organisms – the dead cells that continually
slough off of the epidermis mix with the secretions of the sweat and sebaceous glands and the
dust found on the skin to form a filthy layer on its surface. If not washed away, the slurry
of sweat and sebaceous secretions mixed with dirt and dead skin is decomposed by bacterial
flora, producing a foul smell. Functions of the skin are disturbed when it is excessively
dirty; it becomes more easily damaged, the release of antibacterial compounds decreases,
and dirty skin is more prone to develop infections. Cosmetics should be used carefully because
these may cause allergic reactions. Each season requires suitable clothing in order to facilitate
the evaporation of the sweat. Sunlight, water and air play an important role in keeping the skin healthy.

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 one square inch (6.5 square cm) of human skin
at 50 million though this figure varies greatly over the average 20 feet2 (1.9 m²) of
human skin. Oily surfaces, such as the face, may contain over 500 million bacteria per
square inch (6.5 cm²). Despite these vast quantities, all of the bacteria found on the
skin's surface would fit into a volume the size of a pea. In general, the microorganisms
keep one another in check and are part of a healthy skin. When the balance is disturbed,
there may be an overgrowth and infection, such as when antibiotics kill microbes, resulting
in an overgrowth of yeast. The skin is continuous with the inner epithelial lining of the
body at the orifices, each of which supports its own complement of microbes.