FIGURE 1. The structure of human skin.

This diagram of human skin shows the two main layers of skin — the upper epidermal barrier layer and the lower, much thicker, dermis. The epidermal barrier layer is relatively thin (0.1–0.2 mm in depth) and securely attached to the underlying dermis by a specialized basement membrane zone. This consists of several different types of collagen fibre, which attach cells securely to the underlying dermis, and is visible at the electron-microscope level. The dermis varies in thickness depending on its site in the body and is composed primarily of collagen I, with dermal inclusions of hair shafts and sweat glands, which are lined with epidermal keratinocytes. The dermis is well vascularized and also contains receptors for touch, temperature and pain. Keratinocytes in the epidermis rely solely on diffusion from the adjacent dermal capillary network. These cells progressively differentiate from cells in the basal layer, which is located on the basement membrane and gives rise to daughter keratinocytes, which are pushed upwards. These stratify, lose their nuclei and eventually become an integrated sheet of keratin, which is later shed. The upper keratinized epidermal layers provide the barrier layer, which resists bacterial entry and prevents fluid and electrolyte loss. (Image adapted, with permission, from ref. 79.)