Skin stem cells

The skin epidermis, like many other epithelia, continues to self-renew throughout the life

due to the presence of skin stem cells that provide new cells to replace the damaged or dead cells. Following wounding, the skin is able to regenerate itself to some degree. However, when the wound is too extensive, such as in third-degree burns or in some skin genetic diseases, the skin cannot repair itself properly without medical interventions. Approximately 20 different cell types emanating from different embryonic origins reside within the skin. Skin stem cells residing mainly in the basal cell layer  of epidermis and the outer root sheath of the hair follicle bulge ensure the maintenance of adult skin homeostasis and hair regeneration, but they also participate in the repair of the epidermis after injuries. Clonal analysis of bulge cells demonstrate that the progenies of one single bulge stem cell can repopulate all the epidermal lineages of the skin epidermis, demonstrating that bulge stem cells are truly multipotent. The use transplants composed of cultured epithelium for epidermal regeneration in patients suffering from massive burns represents the first therapeutic use of stem cells in burns management. Transplants containing skin stem cells and keratinocytes, either in cell suspension or cell sheet and combined with the use of fibrin matrices to facilitate cell delivery has been a well-established treatment for extensive burn wounds in a number of burn centres around the world. In general, this method provides early and efficient wound coverage and shows satisfactory functional result of epithelialization. However, current skin substitutes do not completely restore the normal skin structures, lacking the normal appendages of skin including hair follicles, sebaceous glands, and sweat glands as well as the normal mechanical properties of the skin. Through combined efforts and with the technological advancement of stem cell biology, tissue engineering and materials science, clinical scientists will be able to generate multi-functional full-thickness composite skin tissues that are structurally and functionally indistinguishable from normal skin.