epidermis
Meaning of epidermis: A layer with no blood supply of its own, rebuilt continuously from below and shed from above roughly once a month.
Definition of epidermis
The epidermis is the outer layer of the skin and the body's first barrier against injury, microbes and water loss. It is built almost entirely from keratinocytes, cells made in its deepest row that flatten and fill with tough protein as they are pushed toward the surface, where they are shed. It contains no blood vessels and is fed by the layer beneath.
Which cells live in it?
| Cell | What it does |
|---|---|
| Keratinocyte | Makes keratin and forms the barrier itself; the great majority of epidermal cells |
| Melanocyte | Makes pigment and hands it to neighboring keratinocytes, shielding their nuclei from ultraviolet light |
| Langerhans cell | Samples material crossing the surface and reports it to the immune system |
| Merkel cell | Sits at the base and contributes to the sense of light touch |
Skin color differences come from how much pigment each melanocyte produces and how it is packaged, not from how many melanocytes are present, which varies little between people. Each of these cell populations also gives rise to its own tumor: keratinocytes to squamous cell carcinoma, the deepest keratinocyte layer to basal cell carcinoma, and melanocytes to melanoma, which is why an understanding of skin cancer begins with the epidermis.
How is it layered?
Cells divide in the deepest row, sitting on a thin membrane that anchors the epidermis to the tissue below. As new cells appear beneath them, older ones are pushed upward through a series of layers, changing as they go. They flatten, stock themselves with keratin, release fatty material into the spaces around them, and finally lose their nuclei altogether. The surface layer is therefore made of dead, flattened cells packed in a water-resistant lipid mortar, an arrangement often compared to bricks and cement. The whole journey takes roughly a month. Palms and soles carry an extra layer and a far thicker surface, which is why they can withstand pressure that would blister skin elsewhere.
Why does the depth of an injury matter?
An injury that stops within the epidermis, such as a graze or a superficial burn that reddens without blistering, heals from the surviving basal cells and leaves no scar. Deeper damage reaches the dermis, where the blood vessels, nerves, sweat glands and hair follicles sit. Those structures are lined with cells that can regrow skin, so partial loss still heals from below, but when they are destroyed the wound closes with scar tissue instead. This is why burns are graded by depth rather than by area alone, and why the epidermis alone cannot bleed: the blood in a scraped knee comes from the layer underneath.
Used in a sentence
The biopsy sample included both the epidermis and a portion of the underlying dermis.
Good to know
A wound or burn confined to the epidermis generally heals differently than one extending into the dermis, since the deeper layer contains structures involved in skin regeneration that the epidermis alone does not.
Word origin
Where “epidermis” comes from
Formed from Greek epi-, meaning upon, and derma, meaning skin, so the epidermis is the layer sitting upon the skin proper.