The epidermis is the outermost layer of the skin, above the dermis. It contains no blood vessels, but is nourished by diffusion from the dermis. The epidermis itself is comprised of several layers and is made up to 90% of keratinized skin cells called keratinocytes.
The keratinocytes originate in the basal layer from the division of keratinocyte stem cells. They are pushed up through the layers of the epidermis, undergoing gradual differentiation. While they move to the surface of the skin the keratinocytes are enucleated, flattened and highly keratinized. In this form they are called squamous cells. Eventually they die off and these dead skin cells form the outermost layer of the epidermis, the stratum corneum. They are shed and replaced continuously from the deeper layers. The time of transit from basal layer to shedding is approximately one month.
This layer of continually replaced dead cells forms an effective barrier to the entry of foreign matter and infectious agents into the body and minimizes moisture loss. In some parts of the skin, e.g. the soles of the feet, the stratum corneum can be several millimeters thick.
In the deeper layers of the epidermis are the Merkel cells, which are associated with the sense of touch.
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