The skin

In humans and other mammals, the skin has three parts - the epidermis, the dermis and the subcutis (or hypodermis). The epidermis forms the surface of the skin. It is made up of several layers of cells called keratinocytes. The dermis lies underneath the epidermis and contains skin appendages: hair follicles, sebaceous (oil) glands and sweat glands. The subcutis contains fat cells and some sweat glands.

The skin and its structure: The skin has three main layers - the epidermis, dermis and subcutis. The epidermis contains layers of cells called keratinocytes. BL = basal layer; SL = spinous layer; GL = granular layer; SC= stratum corneum. Image adapted by permission from Macmillan Publishers Ltd: Nature Reviews Genetics 3, 199-209 (March 2002), Getting under the skin of epidermal morphogenesis, Elaine Fuchs & Srikala Raghavan; doi:10.1038/nrg758; Copyright 2002.

In everyday life your skin has to cope with a lot of wear and tear. For example, it is exposed to chemicals like soap and to physical stresses such as friction with your clothes or exposure to sunlight. The epidermis and skin appendages need to be renewed constantly to keep your skin in good condition. Whats more, if you cut or damage your skin, it has to be able to repair itself efficiently to keep doing its job protecting your body from the outside world.

Skin stem cells make all this possible. They are responsible for constant renewal (regeneration) of your skin, and for healing wounds. So far, scientists have identified several different types of skin stem cell:

Some studies have also suggested that another type of stem cell, known as mesenchymal stem cells, can be found in the dermis and hypodermis. This remains controversial amongst scientists and further studies are needed to determine whether these cells are truly mesenchymal stem cells and what their role is in the skin.

Epidermal stem cells are one of the few types of stem cell already used to treat patients. Thanks to a discovery made in 1970 by Professor Howard Green in the USA, epidermal stem cells can be taken from a patient, multiplied and used to grow sheets of epidermis in the lab. The new epidermis can then be transplanted back onto the patient as a skin graft. This technique is mainly used to save the lives of patients who have third degree burns over very large areas of their bodies. Only a few clinical centres are able to carry out the treatment successfully, and it is an expensive process. It is also not a perfect solution. Only the epidermis can be replaced with this method; the new skin has no hair follicles, sweat glands or sebaceous glands.

One of the current challenges for stem cell researchers is to understand how all the skin appendages are regenerated. This could lead to improved treatments for burn patients, or others with severe skin damage.

Researchers are also working to identify new ways to grow skin cells in the lab. Epidermal stem cells are currently cultivated on a layer of cells from rodents, called murine cells. These cell culture conditions have been proved safe, but it would be preferable to avoid using animal products when cultivating cells that will be transplanted into patients. So, researchers are searching for effective cell culture conditions that will not require the use of murine cells.

Scientists are also working to treat genetic diseases affecting the skin. Since skin stem cells can be cultivated in laboratories, researchers can genetically modify the cells, for example by inserting a missing gene. The correctly modified cells can be selected, grown and multiplied in the lab, then transplanted back onto the patient. Epidermolysis Bullosa is one example of a genetic skin disease that might benefit from this approach. Work is underway to test the technique.

Originally posted here:
Skin stem cells: where do they live and what can they do ...

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