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Keratinocyte

A keratinocyte is the main cell type of the epidermis. Keratinocytes are produced at its base, fill with keratin as they rise towards the surface, flatten, lose their nuclei, and are eventually shed as part of the outermost layer.

Schematic cross-section of the skin, the epidermis highlightedFour stacked layers are drawn from the surface downwards: stratum corneum, epidermis, dermis and hypodermis. The epidermis is picked out in blue; the other layers are hatched. The drawing is schematic and not to scale.epidermis
epidermis — Where this sits in the skin: the epidermis. Schematic, not to scale.

Roughly nine out of every ten cells in the epidermis are keratinocytes, which makes them the material the skin’s surface is built from.

Their defining feature is a journey. A keratinocyte is produced by division at the base of the epidermis, and from that moment it travels upwards, being pushed along by the cells made after it. As it rises it fills with keratin, a tough structural protein, and changes shape. Near the surface it flattens, loses its nucleus, and becomes part of the compacted outer layer that is shed continuously.

In ordinary skin that journey takes several weeks. Conditions that accelerate it produce visible consequences for exactly that reason: cells arrive at the surface before they have finished maturing, and they accumulate rather than shedding cleanly. Psoriasis is the clearest example.

Keratinocytes are not passive building material. They participate in the skin’s immune signalling, and they receive melanin from neighbouring melanocytes, which is how pigment reaches the surface at all.

The word turns up in clinical description mainly to locate an origin. Cancers arising from keratinocytes are grouped separately from those arising from pigment cells, and the distinction matters because the two behave differently.

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