The keratinocyte is the major cell type of the epidermis, making up about 90% of epidermal cells. The epidermis is divided into four layers based on keratinocyte morphology:
- basal layer (at the junction with the dermis)
- stratum granulosum
- stratum spinosum
- stratum lucidum
- stratum corneum
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 until they reach the stratum corneum where they form a layer of dead, flattened, highly keratinised cells called squames. This layer forms an effective barrier to the entry of foreign matter and infectious agents into the body and minimises moisture loss.
Keratinocytes are shed and replaced continuously from the stratum corneum. The time of transit from basal layer to shedding is approximately one month although this can be accelerated in conditions of keratinocyte hyperproliferation such as psoriasis.
