The skin’s ability to heal from a cut or burn is a remarkable process. This renewal leads to the question of whether skin cells are a type of stem cell. The answer is complex; while most cells in our skin are not stem cells, the skin does harbor specific populations of them. Additionally, science can now artificially transform ordinary skin cells into stem cells in a laboratory.
The Different Kinds of Cells in Your Skin
The skin’s outermost layer, the epidermis, serves as our primary protective barrier and is primarily formed by specialized cells called keratinocytes. These cells make up about 90% of the epidermis and produce keratin, a protein that gives skin its strength. Keratinocytes are considered differentiated, meaning they have already matured into a specific role.
As new keratinocytes are born in the deeper epidermis, they migrate upwards over 40 to 56 days. During this journey, they flatten and eventually die, forming the tough outer layer of dead cells that we constantly shed. Because these cells are specialized for this specific life cycle, they are not stem cells.
The skin also contains other specialized cells. The epidermis has melanocytes, which produce pigment, and Langerhans cells, which are part of the immune system. Deeper in the dermis layer, fibroblasts produce collagen and other fibers that provide structure and elasticity.
The Stem Cells Residing in Skin
While most skin cells are specialized, the skin contains distinct populations of adult stem cells, also known as somatic stem cells. Their primary role is to maintain and repair the tissue where they are found. Unlike more versatile embryonic stem cells, adult stem cells are multipotent, meaning they are restricted to generating the cell types found in their home tissue.
These stem cells are located in protected areas within the skin. One location is the basal layer of the epidermis, the deepest layer connecting to the dermis. Here, epidermal stem cells are responsible for the constant renewal of the epidermis by producing new keratinocytes, ensuring a continuous supply of new cells.
Another reservoir of skin stem cells is found within hair follicles, in a region called the bulge. These hair follicle stem cells regenerate the hair follicle during its growth cycle. They can also be called upon during wound healing to help repair the follicle and the surrounding epidermis and sebaceous glands.
Turning Skin Cells into Stem Cells
A scientific development has added another layer to the relationship between skin and stem cells. It is now possible to take differentiated skin cells, such as fibroblasts, and reprogram them in a laboratory to become induced pluripotent stem cells (iPSCs). This was pioneered by scientist Shinya Yamanaka, who discovered that introducing a few specific genes into adult cells could rewind their developmental clock.
The resulting iPSCs are pluripotent, meaning they have the potential to develop into any cell type in the body, similar to embryonic stem cells. The process involves taking a skin biopsy, isolating cells like fibroblasts, and introducing “reprogramming factors” or genes into them. Within a few weeks, some of these cells transform into pluripotent stem cells.
This technology allows for the creation of patient-specific stem cells without the ethical controversies of embryonic stem cells. More recent techniques use gene-editing tools like CRISPR-Cas9 to activate the cell’s own genes for a more controlled method of reprogramming. Research has shown there are subtle molecular differences between iPSCs and embryonic stem cells.
Medical and Research Applications
Both natural and induced skin-derived stem cells have applications in medicine and research. The natural adult stem cells in the skin are used for therapeutic purposes. For example, epidermal stem cells can be cultured in a lab to grow sheets of new epidermis, which are then grafted onto burn victims to help regenerate their skin.
The applications for iPSCs generated from skin are even broader. Because iPSCs can be made from any individual, they are a useful tool for studying diseases. Scientists can take a skin sample from a patient with a genetic disorder, create iPSCs, and then differentiate them into the cell types affected by the disease. This allows researchers to model the disease in a lab, investigate it at a cellular level, and test new drugs.
Looking to the future, iPSC technology holds promise for regenerative medicine. The ability to generate patient-specific cells that will not be rejected by the immune system is a significant advantage. For example, a patient’s iPSCs could be genetically corrected in a lab before being used to grow healthy replacement skin for transplantation. This approach could one day be used to generate tissues and organs to treat many conditions, from diabetes to heart disease.