Stratum corneum
The stratum corneum is the outermost part of the epidermis, made of flattened dead cells filled with keratin and embedded in a lipid matrix. It forms the skin's barrier, holding water in and keeping most external substances out.
This is the layer everything else is protected by, and it is not alive. The stratum corneum is the final state of cells that were produced at the base of the epidermis, matured on the way up, and ended as flattened envelopes packed with keratin.
The standard description is a brick wall. Cells are the bricks; a mixture of lipids between them is the mortar. Neither component works without the other, and the mortar is the part that fails first in several common conditions.
Its function runs both ways. Water is held inside, which is not trivial given the surface area involved, and most external substances are kept outside. Impair it and both halves suffer at once: skin dries out and becomes more reactive to things it previously tolerated.
Thickness varies enormously by site. It is heaviest on palms and soles, and thinnest on eyelids, which is why the same substance can be harmless in one place and provoke a reaction in another.
Two whole areas of dermatology sit here. Moisturisers act on this layer and essentially nowhere else, and common fungal infections live in it because its keratin is what they feed on.
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