Keratinocyte

[2] Keratinocytes form a barrier against environmental damage by heat, UV radiation, water loss, pathogenic bacteria, fungi, parasites, and viruses.

Keratinocytes differentiate from epidermal stem cells in the lower part of the epidermis and migrate towards the surface, finally becoming corneocytes and eventually being shed,[3][4][5][6] which happens every 40 to 56 days in humans.

[7] The primary function of keratinocytes is the formation of a barrier against environmental damage by heat, UV radiation, dehydration, pathogenic bacteria, fungi, parasites, and viruses.

[8] A number of structural proteins (filaggrin, keratin), enzymes (e.g. proteases), lipids, and antimicrobial peptides (defensins) contribute to maintain the important barrier function of the skin.

[3] Epidermal stem cells reside in the lower part of the epidermis (stratum basale) and are attached to the basement membrane through hemidesmosomes.

Keratinocytes form tight junctions with the nerves of the skin and hold the Langerhans cells and intra-dermal lymphocytes in position within the epidermis.

[citation needed] Keratinocytes contribute to protecting the body from ultraviolet radiation (UVR) by taking up melanosomes, vesicles containing the endogenous photoprotectant melanin, from epidermal melanocytes.

[32] A sunburn cell is a keratinocyte with a pyknotic nucleus and eosinophilic cytoplasm that appears after exposure to UVC or UVB radiation or UVA in the presence of psoralens.