Stratum corneum

The human stratum corneum comprises several levels of flattened corneocytes that are divided into two layers: the stratum disjunctum and stratum compactum. The stratum disjunctum is the uppermost and loosest layer of skin. The skin's protective acid mantle and lipid barrier sit on top of the stratum disjunctum. The stratum compactum is the comparatively deeper, more compacted and more cohesive part of the stratum corneum. The corneocytes of the stratum disjunctum are larger, more rigid and more hydrophobic than those of the stratum compactum. Research on osmotic permeability suggests the stratum compactum consists of two layers. The stratum disjunctum above these layers can swell, as can the lowest layer of the stratum disjunctum up to two-fold. However, the first layer in the stratum compactum between them has limited swelling capacity and provides the stratum corneum's barrier. ==Function==

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). and 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 stratum corneum. The mechanical stress of heavy structural strain causes this stratum lucidum phase in these regions which require additional protection in order to grasp objects, resist abrasion or impact, and avoid injury. In general, the stratum corneum contains 15 to 20 layers of corneocytes. The stratum corneum has a thickness of between 10 and 40 μm. In reptiles, the stratum corneum 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 1,300 cells per cm2 per hour are shed. The stratum corneum protects the internal structures of the body from external injury and bacterial invasion. == Skin disease ==

showing prominent hyperkeratosis in skin without atypia. H&E stain. An inability to correctly maintain the skin barrier function due to the dysregulation of epidermal components can lead to skin disorders. For example, a failure to modulate the activity of kallikreins via the disruption of the protease inhibitor LEKTI causes the debilitating disorder Netherton syndrome. Hyperkeratosis is an increased thickness of the stratum corneum, and is an unspecific finding, seen in many skin conditions. ==See also==