Stratum corneum

The stratum corneum (Latin for 'horned/horny layer') is the outermost layer of the epidermis of the tetrapod skin. In human skin, it consists of a stratified squamous epithelium made of 15 to 20 layers of corneocytes, flattened, terminally differentiated keratinocytes that have lost their nuclei or organelles. The filamentous keratin within the cytoplasm of these cells forms a semi-impermeable cuticle that protects underlying tissues from external insults such as dehydration, infection, direct chemical exposure and mechanical abrasion. Hyperkeratosis and hyperplasia of the stratum corneum, often in response to excessive abrasions or impacts, can lead to the formation of hardened skin conditions such as calluses, corns and hangnails. Among the properties of the stratum corneum are mechanical shear, impact resistance, water flux and hydration regulation, microbial proliferation and invasion regulation, initiation of inflammation through cytokine activation and dendritic cell activity, and selective permeability to exclude toxins, irritants and allergens. The corneocytes are embedded in a lipid matrix composed of ceramides, cholesterol, and fatty acids, preventing desiccation and enhancing the overall cohesive strength of the stratum corneum while allowing some pliability. The natural shedding process of surface dead corneocytes off the stratum corneum is known as desquamation or "sloughing", while deliberate, artificial removal of dead skin cells from the stratum corneum is called exfoliation. Adequate desquamation can help balancing proliferating keratinocytes that form in the stratum basale. These cells migrate through the epidermis towards the surface in a journey that takes approximately fourteen days.