Skin is the largest organ of the human body. It provides the first line of defense against infectious agents, and prevents dehydration and injury. Skin has a number of appendages, such as hair, sebaceous and sweat glands, that also aid in the protection of inner organs from environmental assaults. Therefore, it is of extreme importance to maintain a healthy skin, which could be achieved through replacing damaged skin cells.


The cartoon shown in Figure depicts the structure of the skin. Shown, are two layers: the dermis or inner layer that originates from mesodermal embryonic cells, and the epidermis or outer layer that are formed from ectodermal embryonic cells. These two layers are separated from each other by a basement membrane. Epidermis is a stratified tissue composed of cells called keratinocytes. The layer of keratinocytes that is closest to the basement membrane is called basal, and contains a number of constantly proliferating cells. It is these cells that migrate to the surface and form a protective layer, maintaining the outer covering of epidermis intact.

 

Epidermal stem cells are located in the basal layer of the epidermis. These cells give rise to transient amplifying cells that migrate to the surface and form a protective layer.

There are three main locations in the adult skin where epidermal stem cells can be found; the bulge region of the hair follicle, the interfollicular epidermis (IFE), and the sebaceous gland. Further investigation showed that the bulge region contains cells that have the ability to give rise to the stem cells in the IFE region and sebaceous gland, indicating that basal (bulge region) stem cells are more primitive than their counterparts in the other two areas.

Indeed, basal stem cells of the adult hair follicle can participate in tissue repair by responding to signals to regenerate the epidermis, hair follicles, and sebaceous glands.

 

A schematic diagram of an epidermopilosebaceous unit in hair-bearing skin. The unit consists of the epidermis and the hair follicle with its associated sebaceous gland. The bulge contains a population of putative keratinocyte stem cells that can give rise to (pathway 1) a population of pluripotent and rapidly dividing progenitor (transit amplifying) cells in the matrix that yields the hair shaft. Alternatively, the bulge stem cells can give rise to the stemyprogenitorcells oftheepidermis(pathway2). It is hypothesized here that the epidermal stem cell represents a form of bulge-derived, young transit amplifying cell (SCyTA1, 2….?). The long, curved arrow denotes the demonstrated capability of adult epidermal cells to form anewhair follicle in response to appropriate mesenchymal stimuli. B, bulge; E, epidermis; FP, follicular or dermal papilla; M, matrix keratinocytes; ORS, outer root sheath; S, hair shaft; SC, stem cells; SG, sebaceous gland; TA, transit amplifying cells.

In the adult, stem cells reside in the epidermal basal layer, at the base of the sebaceous gland and in a niche within the hair follicle known as the bulge. Despite intensive studies, we still know very little about how stem cells and these niches become established and maintained. Genetic marking and molecular approaches stem cells within the bulge typically cycle infrequently. In response to a skin injury, these stem cells can be mobilized to move upward, proliferate and repair epidermal wounds or replenish the sebaceous gland. In normal homeostasis, these stem cells fuel the hair cycle, where they become activated to proliferate and regenerate the hair follicle with each new anagen phase. It has been known for nearly a decade that that the transition from dormant to activated follicle stem cells involves changes in signaling by Wnts, BMPs, and other factors.


 
 

Following information is summarized to demonstrate the applicability of stem cell research to skin in animal testing by other scientists specially in medical science field.


We do NOT test on animals at any stage in ingredient nor in finished product development.


Wound repair


Epidermal stem cells play a central role in homeostasis and wound repair, and represent a cellular sources of tumor initiation and as a vehicle for gene therapy. These stem cells also have the potential for treating burn victims. Figure demonstrate hair follicle epidermal stem cells that have been used in preparing skin equivalents, forming epithelium in deep burn wounds after implantation. These experiments show hope of being able to culture autologous epithelial grafts in vitro within a short time for implantation into patients. As they show multipotency to differentiate into almost all epithelial cell types, hair follicle stem cells can be used in preparing composite skin substitutes. Scientists demonstrated that epidermal stem cells can regenerate a fully stratified epidermis by in vitro methods. Other studies found that the progenies of epidermal stem cells (transient amplifying and differentiating cells) have regenerative capacity as well(Ref.: Cell Tissue Res. 2006 Dec;326(3):725-36.).


 

Hair Restoration
Epidermal stem cells can also form hair follicles and have the potential to treat baldness. Researchers at the Howard Hughes Medical Institute isolated murine epidermal stem cells, and showed that a single epidermal stem cell can differentiate into skin, hair and oil glands. Elaine Fuchs and colleagues took stem cells from normal mice, and grafted them on the backs of hairless mice. They were able to show the generation of normal skin with hair ( Ref.: Cell Cycle. 2006 Feb;5(3):232-3, Ref.: PNAS 2005 Oct 11;102(41): 14653-8).



Epidermal stem cells are necessary for the maintenance of the skin. They continually self- renew and differentiate into keratinocytes in order for the skin to protect against insults. Epidermal stem cells continuously undergo self-renew to maintain the integrity of the epidermis. Ongoing research studies are in the process of identifying markers of these stem cells, although a specific marker has not been identified.


Epidermal stem cells have been demonstrated to be useful for therapeutic purposes. Due to their plasticity, it may be possible these stem cells have the ability to treat injured tissue throughout the body. Epidermal stem cells show great promise in treatment of burn patients, which is supported by results showing the ability of these cells of the hair follicle to regenerate epithelium of deep burn wounds. Also, these cells may be useful in the treatment of baldness.