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: Epidermis is the outermost layer of the skin. ...

1. stratum basale (at the junction with the dermis)
2. stratum granulosum
3. stratum spinosum
4. stratum lucidum (only present in thick skin - i.e. excluding palms of hand and soles of feet)
5. 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 keratinized cells called squamous cells. This layer forms an effective barrier to the entry of foreign matter and infectious agents into the body and minimises moisture loss. Stratum germinativum or stratum basale is the layer of keratinocytes that lies at the base of the epidermis immediately above the dermis. ... The dermis is the layer of skin beneath the epidermis, that consists of connective tissue and cushions the body from stress and strain. ... Stratum granulosum contains 3 to 5 rows of flattened cells whose cytoplasm contains small granules. ... It is a multiple-layered arrangement of cuboidal cells containing molecular bridges that conect them to adjacent cells. ... The Stratum Lucidum is one of the layers of the Epidermis. ... The stratum corneum (the horny layer) is the outermost layer of the epidermis, and comprises the surface of the skin. ... Mouse embryonic stem cells. ... Embryonic stem cells differentiate into cells in various body organs. ... Microscopy of keratin filaments inside cells. ...

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.