Epidermis
The epidermis is the outer layer of the skin, sitting above the dermis and containing no blood vessels of its own. It is built mainly from keratinocytes, which are produced at its base, mature as they rise, and are eventually shed from the surface.
Everything visible about skin colour, texture and surface begins here. The epidermis is thin, in most places thinner than a sheet of card, and it is where a large share of dermatology happens.
Its cells are produced at the base and travel upward. As they rise they fill with keratin, flatten, lose their nuclei and eventually form the compacted outer layer that is shed continuously. That journey takes several weeks in ordinary skin, and conditions that speed it up, psoriasis among them, produce visible consequences precisely because cells arrive at the surface before they have finished maturing.
No blood vessels run through it. The epidermis is supplied by diffusion from the dermis below, which is why a superficial graze bleeds so little and why the boundary between the two layers matters so much.
Other cell types live among the keratinocytes. Melanocytes make pigment. Immune cells sit here too, which is why the epidermis is where allergic contact reactions are set off rather than merely where they show.
The word appears constantly in clinical description, usually as a way of locating something. A process described as epidermal is happening in this layer, which tells you a good deal about how it is likely to look and how it is likely to behave.
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