Stratum corneum
Template:Short description Template:More citations needed Template:Infobox anatomy
The stratum corneum (Latin for 'horned/horny layer') is the outermost layer of the epidermis of the skin. Consisting of dead tissue, it protects underlying tissue from infection, dehydration, chemicals, and mechanical stress. It is composed of 15 to 20 layers of flattened cells with no nuclei or cell organelles.
Among its properties 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.<ref>Template:Cite journal</ref> The cytoplasm of corneocytes, its cells, shows filamentous keratin. These corneocytes are embedded in a lipid matrix composed of ceramides, cholesterol, and fatty acids.<ref>Template:Cite book</ref>
Desquamation is the process of cell shedding from the surface of the stratum corneum, 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.<ref name="ovaere">Template:Cite journal</ref>
Structure
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.<ref>Template:Cite journal</ref> The stratum compactum is the comparatively deeper, more compacted and more cohesive part of the stratum corneum.<ref>Template:Citation</ref> The corneocytes of the stratum disjunctum are larger, more rigid and more hydrophobic than those of the stratum compactum.<ref>Template:Cite journal</ref>
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.<ref name="Richter Peuckert Sattler Koenig 2004 pp. 246–257">Template:Cite journal</ref>
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).<ref name="ovaere" /> This complex surrounds cells in the stratum corneum 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.<ref name="ovaere"/>
Cells of the stratum corneum 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 stratum corneum varies throughout the body. In the palms of the hands and the soles of the feet (sometimes knees, elbows,<ref>Template:Cite book</ref> 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.<ref>Template:Cite book</ref> The stratum corneum protects the internal structures of the body from external injury and bacterial invasion.
Skin disease
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.<ref name="pmid15619623">Template:Cite journal</ref>
Hyperkeratosis is an increased thickness of the stratum corneum, and is an unspecific finding, seen in many skin conditions.
See also
References
External links
- Template:LoyolaMedEd
- Template:BUHistology - "Integument: thick skin"