During cornification, the process whereby living keratinocytes are transformed into non-living corneocytes, the cell membrane is replaced by a layer of ceramides which become covalently linked to an envelope of structural proteins (the cornified envelope). This complex surrounds cells in the<unk>  and contributes to the skin's barrier function. Corneodesmosomes (modified desmosomes) facilitate cellular adhesion by linking adjacent cells within this epidermal layer. These complexes are degraded by proteases, eventually permitting cells to be shed at the surface. Desquamation and formation of the cornified envelope are both required for the maintenance of skin homeostasis. A failure to correctly regulate these processes leads to skin disorders developing. Cells of the<unk>  contain a dense network of keratin, a protein that helps keep the skin hydrated by preventing water evaporation. These cells can also absorb water, further aiding in hydration. In addition, this layer is responsible for the "spring back" or stretchy properties of skin. A weak glutenous protein bond pulls the skin back to its natural shape. The thickness of the<unk>  varies throughout the body. In the palms of the hands and the soles of the feet (sometimes knees, elbows, knuckles,) this layer is stabilized and built by the stratum lucidum (clear phase) which allows the cells to concentrate keratin and toughen them before they rise into a typically thicker, more cohesive<unk> . The mechanical stress of heavy structural strain causes this SL phase in these regions which require additional protection in order to grasp objects, resist abrasion or impact, and avoid injury. In general, the<unk>  contains 15 to 20 layers of corneocytes. The<unk>  has a thickness between 10 and 40 μm. In reptiles, the<unk>  is permanent, and is replaced only during times of rapid growth, in a process called ecdysis or moulting. This is conferred by the presence of beta-keratin, which provides a much more rigid skin layer. In the human forearm, about 1300 cells per cm2 per hour are shed.<unk>  protects the internal structures of the body from external injury and bacterial invasion.
Stratum corneum
Stratum granulosum
Stratum lucidum
Neuromatrix
Episcleral layer
Inserm
Dermoepidermal junction
Stratum spinosum