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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 2 of 13
Historically, the barrier has always been described as a fairly passive system. That is the old “bricks-and-mortar” model: dead cells, surrounded by lipids, preventing water loss and the entry of toxic molecules or pathogens.
Born within the epidermis, keratinocytes progressively move toward the skin surface and undergo a series of transformations (including the loss of their nucleus) that turn them into corneocytes, a breed of very important and very functional “dead” cells, constituting the skin barrier.
The differentiation is the process by which a keratinocyte will transform into a corneocyte. That process occurs during the move in the epidermis from basal layer to the stratum corneum and results in the loss of the nucleus and all other organelles of the keratinocytes, as well as the initial production of lipid and the keratinization of the cell.
A very essential notion is that of the movement upward to the stratum corneum. It is important to understand that although the term “migration” is often used, the movement is not a migration. Cells migrate when they move from one place to another under the influence of chemotactic factors, for instance, using their own means of motility. Cell motility involves deformation, extension and so on. In the case of the keratinocytes’ move to the surface and differentiation into corneocytes, the movement depends on a push created by the formation of new keratinocytes. Like a crowd moves under its own force because people in the back push other people that are in the front, so do keratinocytes push the corneocytes above them.
And the speed at which the cells move impacts the time they have to go through the differentiation process. So, the more keratinocytes are formed, through proliferation, the quicker the movement will be and the less time will be left for the differentiation process, which is dependent on the time it takes for the corneocytes to reach the surface. (More on the implications of this follows in A Story of Time and Synchronization.)
Lipids: A Key in Control Transepidermal Water Loss (TEWL)
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