The epidermis is the skin, or, more specifically, the outer layer of mammalian skin cells. The skin is ectodermal in origin, and composed of a several differentiated layers of epithelium. Each layer is made up of flat, scale-like squamous cells which typify superficial epithelium. A stratified, squamous epithelium is a perfect construction for the epidermis, which is subject to constant abrasion. Squamous cells are easily sloughed off and renewed by the body.
Renewal happens through cell division within the basal layer, the lowest layer in the stratified epidermis. This layer creates cells, which then differentiate and migrate towards the more superficial layers of the skin. In healthy skin, cell proliferation matches the rate at which superficial cells are lost to the environment. Cells of the skin have an average lifespan of around 30 days.
The epidermis is the body’s first line of defense against foreign organisms and an environment that is drastically different, in temperature and humidity, from the body’s internal atmosphere. It is avascular and contains no blood vessels. Nourishment for the epidermis arrives through diffusion from the underlying vascular dermis.
The squamous cells of the epidermis come in four varieties: keratinocytes, melanocytes, Langerhans cells, and Merkel cells. Keratinocytes make up the majority of epidermal cells, comprising 95% of the epidermis.