<P> The skin is the largest organ in the human body . For the average adult human, the skin has a surface area of between 1.5 - 2.0 square meters (16.1 - 21.5 sq ft .). The thickness of the skin varies considerably over all parts of the body, and between men and women and the young and the old . An example is the skin on the forearm which is on average 1.3 mm in the male and 1.26 mm in the female . The average square inch (6.5 cm2) of skin holds 650 sweat glands, 20 blood vessels, 60,000 melanocytes, and more than 1,000 nerve endings . The average human skin cell is about 30 micrometers in diameter, but there are variants . A skin cell usually ranges from 25 - 40 micrometers (squared), depending on a variety of factors . </P> <P> Skin is composed of three primary layers: the epidermis, the dermis and the hypodermis . </P> <P> 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 which also serves as a barrier to infection and is made up of stratified squamous epithelium with an underlying basal lamina . </P> <P> The epidermis contains no blood vessels, and cells in the deepest layers are nourished almost exclusively by diffused oxygen from the surrounding air and to a far lesser degree by blood capillaries extending to the outer layers of the dermis . The main type of cells which make up the epidermis are Merkel cells, 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". 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 . </P>

What is the outer skin layer in human body called