Keratinocyte
From Wikipedia, the free encyclopedia
The keratinocyte is the major cell type of the epidermis, making up about 90% of epidermal cells. The epidermis is divided into four or five layers (depending on the type of skin) based on keratinocyte morphology:
- stratum basale (at the junction with the dermis)
- stratum spinosum
- stratum granulosum
- stratum lucidum (only present in thick skin - i.e. palms of hand and soles of feet)
- 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. While they move to the surface of the skin the keratinocytes are enucleated, flattened and highly keratinized. In this form they are called squamous cells. Eventually they die off and form the stratum corneum. This layer of dead cells 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 that approximate time frame can be accelerated in conditions of keratinocyte hyperproliferation, such as psoriasis.
