What Allows Our Skin Cells to Shed?

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Layers of the Epidermis. Illustration by Jessica C. Kraft

The epidermis is a self-renewing system. Old cells (or ‘squames’) are shed from the skin surface as new cells (‘keratinocytes’) are produced in the underlying epidermis and pushed outward into the stratum corneum to become ‘corneocytes’. In normal skin, the process of desquamation is invisible and imperceptible.  But in some skin diseases, the production of cells may increase and/or the mechanism of shedding may be faulty, resulting in the visible build-up and eventual shedding of unsightly scales, such as what commonly occurs as previously sunburned skin peels away or…

less commonly in inherited skin disorders, like ichthyosis.

The Stratum corneum

The Stratum corneum. Illustration by Jessica C. Kraft.

The process of desquamation has been attributed to the progressive breakdown (‘proteolysis’) of proteins forming structures, called ‘corneodesmosomes,’ that link adjacent ‘corneocytes’ to one another.  We now show that this prior model does not fully account for desquamation, because the cells detach well above the sites where these junctions are degraded. In other words, proteolysis of corneodesmosomes may be necessary for desquamation, but it is not sufficient.

Instead, in this study we demonstrate that cells detach following hydration, which swells the extracellular spaces and separates adjacent corneocytes. Then, as water evaporates from between corneocytes, it is replaced by air, which eventually allows individual cells to detach from the skin surface. (This is how it would happen if we let our skin air dry after soaking in water.  Our washcloths and drying towels probably short circuit the process by brushing off the loosened squames.)

Bottom line: Bathing not only cleanses our skin of dirt and other external debris, it completes the normal process of desquamation, sweeping away spent and finished epidermal cells.

 

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