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 corneum
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.