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Stem Cell Science & Age Management of Skin
By: Christine Heathman
Posted: May 26, 2010, from the June 2010 issue of Skin Inc. magazine.
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The skin. The skin is the largest and most dynamic immune organ, made up of billions of cells playing a protective and esthetic role where aging is clinically evident via wrinkles. Two types of adult stem cells have been identified within the skin’s ecosystem: epithelial skin cells located in the basal layer of the epidermis, and hair bulge stem cells situated in the hair follicle.
The skin’s top layer, the epidermis, is a stratified epithelium housing terminally differentiated cells that shed by the millions daily from the skin, continuously delivering new skin cells. Because of this differentiating cell dynamic, the importance of stem cells in the skin is scientifically substantiated, enumerating their relevance to skin age management.
The role of the epidermis stems directly from the terminal differentiation of keratinocytes into corneocytes to form what is the visible skin. This dynamic and complicated immune organ interfaces with a hostile environment, and its uppermost layer, the stratum corneum, is subject to continuous abrasion by chemical and physical injury. The stratum corneum, an essential part of the epidermis, is the outermost skin layer at the environment interface. This skin layer is also the principal permeability barrier to transepidermal water loss (TEWL) and a major cordon to percutaneous absorption of topically applied compounds, such as botanical stem cell extracts. The degree of moisture in the stratum corneum is an important factor when evaluating skin function because loss of moisture is a major factor in aging skin. This detail is important to note when treating and managing photoaging skin because there is a significant correlation between TEWL and the percutaneous absorption of topical ingredients.
Why is TEWL an important measurement in aging skin? Water in the stratum corneum is a dynamic equilibrium between the underlying tissues and the environmental atmosphere. The intricate stratum corneum barrier constitutes 70% of the epidermis, which is continuously rehabilitated from the granular layer. This layer is the first victim of UV assault that eventually results in photoaging of the skin. To protect the epidermis against invasion of microorganisms and toxic agents, as well as the loss of indigenous fluids residing in the stratum corneum, the horny layer of the skin must be perpetually renewed. This is where stem cells play an important role.
Stem cells’ role in the skin. The stem cell, which is responsible for cell renewal replacement in the epidermis, is an intermediate between the keratinocyte stem cell and terminally differentiating cells. The stem cell is the amplifying cell that undergoes limited cycles of replication. One of the key questions in stem cell research has been how stem cells know when it’s time to stop reproducing. In some cases, stem cells seem to be able to divide into two structurally different cells; one that remains a stem cell and another, called a progenitor cell, that goes on to generate specialized cells. Details still remain unclear, so this area of research remains active, however the study with skin stem cells reveals important information about other organs of the body.