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Examining Skin: A Functional Barrier
By: Marie Alice Dibon, PharmD
Posted: March 2, 2012, from the March 2012 issue of GCI Magazine.
page 3 of 13
The lipids that constitute the lipid mantle, coating the corneocytes, are produced and extruded by the keratinocytes in the basal layer, and are secreted throughout the whole differentiation process.
The lipids, organized in a lamellar fashion (in layers), are cerebrosides, ceramides, fatty acids and cholesterol. Present in the barrier, they constitute a hydrolipidic cement.
These layers of lipids, produced and secreted during the maturation of the keratinocytes, coat each cell in the stratum corneum in such a way that water can still circulate between the lipidic lamellae through a passive diffusion process. Water moves between corneocytes in a very slow and controlled fashion, resulting in a very controlled evaporation rate.
The proteins in the cornified envelope are linked to the lipids that coat the cells. Some ceramides are bound covalently to the proteins, especially involucrin—a structural protein. And transglutaminases controlled by calcium play an essential role in the liaison between involucrin and lipids.
An Active Proteinic Nature
The false notion that the stratum corneum is a dead structure comes from the fact that the corneocytes, born as keratinocytes in the basal layer, are highly proteinic entities that become cornified as they move up in the granular layer and lose their nucleus in the process. However, they are organized in a very complex way, and are the host of a frantic enzymatic activity—forming a very efficient, very active and very “lively” skin barrier.
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