Structure and Function:
95% of the cells in the epidermis are keratinocytes. These cells are found in the basal layer of the stratified epithelium that comprises the epidermis, and are sometimes referred to as basal cells, or basal keratinocytes. Squamous keratinocytes are also found in the mucosa of the mouth and esophagus, as well as the corneal, conjunctival, and genital epithelia.
These cells are responsible for forming tight junctions with the nerves of the skin. They also keep Langerhans cells of the epidermis, and lymphocytes of the dermis, in place.
Immune role of keratinocytes:
In addition to their structural role, keratinocytes play a role in immune system function. The skin is the first line of defense, and keratinocytes serve as a barrier between an organism and its environment. In addition to preventing toxins and pathogens from entering an organisms body, they prevent the loss of moisture, heat, and other important constituents of the body. In addition to their physical role, keratinocytes serve a chemical immune role as immunomodulaters, responsible for secreting inhibitory cytokines in the absence of injury, and stimulating inflammation and activating Langerhans cells, in response to injury. Langerhans cells serve as antigen-presenting cells when there is a skin infection, and are the first cells to process microbial antigens entering the body from a skin breach.
Differentiation of Keratinocytes:
Keratinocyte stem cells reside in the basal layer of the epidermis, which is the lowest layer of the stratified epithelia. These cells divide to give rise to transient amplifying cells which divide further, and differentiate, as they move upwards in the epidermis. The differentiating cells produce compounds and other proteins which are critical to the integrity of the outermost layer of the skin, the stratum corneum. The keratinocytes in the stratum corneum are dead squamous cells that are no longer multiplying. Once keratinocytes reach the corneum, they are said to be keratinazed, or cornified, creating the tough outer layer of skin.
There are major proteins found in keratinocytes are keratins. These proteins form the cytoskeleton of keratinocytes, and keratin expression changes as transient amplifying cells differentiate and move to the most superficial stratum corneum. These keratins are what make up our hair and nails, which is why defects in keratin expression result in various diseases of the epidermis, as well as the hair and nails.