Assembly of the epidermal cornified cell envelope.

The cornified cell envelope structure is formed beneath the plasma membrane in terminally differentiating stratified squamous epithelia. It provides a vital physical barrier to these tissues in mammals and consists of a 10 nm thick layer of highly crosslinked insoluble proteins. In the specialized case of the epidermis, a 5 nm thick layer of ceramide lipids is covalently bound to the proteins. These organize extracellular lipids into orderly lamellae and, together, the cell envelope and extracellular lipids are essential for effective physical and water barrier function in the skin. The assembly of the cornified cell envelope proceeds in three principal stages, starting in the upper spinous cell layers of the epidermis.