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The human stratum corneum comprises several levels of flattened corneocytes that are divided into two layers: the stratum disjunctum and stratum compactum. The skin's protective acid mantle and lipid barrier sit on top of the stratum disjunctum. [5] The stratum disjunctum is the uppermost and loosest layer of skin.
The topmost layer is called the stratum corneum. During sloughing, it is this layer that is removed. [1] As cells progress through the various layers to reach the stratum corneum, they undergo a process called cornification which transforms keratinocytes to corneocytes, effectively killing them.
Hyperkeratosis is thickening of the stratum corneum (the outermost layer of the epidermis, or skin), often associated with the presence of an abnormal quantity of keratin, [1] and is usually accompanied by an increase in the granular layer. As the corneum layer normally varies greatly in thickness in different sites, some experience is needed ...
The lipids ultimately form the lamellar lipid bilayer that surrounds corneocytes and also contributes to the permeability barrier homeostasis of the stratum corneum. [12] The homeostasis function is regulated by the calcium gradient in the epidermis. [17] Usually the calcium level is very low in stratum corneum, but high in stratum granulosum.
In normal skin, the rate of keratinocyte production equals the rate of loss, [4] taking about two weeks for a cell to journey from the stratum basale to the top of the stratum granulosum, and an additional four weeks to cross the stratum corneum. [2] The entire epidermis is replaced by new cell growth over a period of about 48 days. [13]
After reaching the top layer stratum corneum they are eventually 'sloughed off', or desquamated. This process is called keratinization and takes place within weeks. It was previously believed that the stratum corneum was "a simple, biologically inactive, outer epidermal layer comprising a fibrillar lattice of dead keratin". [9]
Scale forms on the skin surface in various disease settings, and is the result of abnormal desquamation. In pathologic desquamation, such as that seen in X-linked ichthyosis, the stratum corneum becomes thicker (hyperkeratosis), imparting a "dry" or scaly appearance to the skin, and instead of detaching as single cells, corneocytes are shed in clusters, which forms visible scales. [2]
The stratum corneum provides the most significant barrier to diffusion. In fact, the stratum corneum is the barrier to approximately 90% of transdermal drug applications. However, nearly all molecules penetrate it to some minimal degree. [3] Below the stratum corneum lies the viable epidermis.