The skin is the largest organ of the body, and it is composed of three main layers. The epidermis is the outermost layer of the skin. Keratinocytes constitute 95% of the cells of the epidermis. The epidermis also contains melanocytes (approximately 1-2% of epidermal cells), producing the protective pigment melanin which is transported to the surrounding keratinocytes via melanosomes. The epidermis contains also antigen presenting Langherans cells, it has no blood vessels. Cells in the deepest layers are nourished by diffusion from blood capillaries extending to the upper layers of the dermis. Keratinocytes move up the strata changing shape and composition as they differentiate and become filled with keratin. They eventually reach the top layer to form the stratum corneum, and get sloughed off, or desquamed. This process is called keratinization and takes place within weeks. The stratum corneum is composed of highly compressed layers of corneocytes in a "brick and mortar" configuration which provides skin waterproofing, prevents from infection and from penetration of chemicals. The surface of the skin is covered by intersecting grooves and ridges which produce characteristic skin surface patterns...