"This assay has opened up a whole new avenue of research," said Ruby Ghadially, MD, SFVAMC staff physician and UCSF associate professor of dermatology. "If you can determine how many stem cells you have, then you can identify distinguishing characteristics that will allow you to isolate the cells. We could then potentially use these cells as effective carrier cells for gene therapy and, someday, use them to produce new stem cells for treating burns and wounds in the skin," Ghadially said.
The study was published online September 17 in the Proceedings of the National Academy of Sciences's online early edition, and will appear in the print version of the journal September 30.
Stem cells are the body's unspecialized cells, which give rise to the specialized cell types that make up an organism. Embryonic stem cells emerge in the first days of an embryo's development, and have the potential to differentiate, or specialize, into each of the 200 types of tissue in the body. Adult stem cells are unspecialized cells found in specialized tissues throughout the body, including bone marrow, skin and the pancreas, among others. They can reproduce themselves as well as give rise to all the cell types of the tissue in which they are found. Scientists are working to take advantage of the natural properties of stem cells in order to develop ways of repairing or replacing the cells of damaged tissues and organs.
Knowing the number of stem cells allows researchers to look for cell-surface molecules, or markers, that distinguish stem cells from specialized cells. This allows researchers to isolate stem cells from specialized cells and investigate ways of taking advantage of their ability to make new specialized cells. For example, stem cells isolated from the blood are now being used to treat cancer patients whose blood cells are damaged by radiation treatment or chemotherapy. Stem cells taken from either the patient before treatment or from a donor are transplanted into the patient following treatment where they make new blood cells.
Using the new assay for skin stem cells, the researchers found that the bottom layer of skin, called the basal epidermis, had the same number of stem cells as found in bone marrow: about one in every 10,000 cells. The assay relies on the same concepts as those used to quantify stem cells in blood. Researchers placed skin cells from two donor mice onto a patch of denuded skin of a third mouse. The cells from one donor were labeled with green fluorescent protein (GFP). The differentiated cells that make up the layers of the skin died off as expected, while stem cells, which are permanent, produced new differentiated cells that replaced the ones that died. So, to confirm the presence of stem cells in their test mice, researchers looked for those cells that still glowed green after a month.
To estimate the number of cells in basal epidermis, researchers kept the number of cells of the non-labeled donor constant over a number of host mice. But, they varied the number of the GFP cells to see how small a sample they could add before they saw no green stem cells after a month's time. This process, called limiting dilution, gave them the ratio of stem cells to differentiated cells in their GFP samples.
According to Ghadially, research using stem cells from the blood is 20 years ahead of other stem cell research, largely because researchers have a similar assay they use to quantify the number of stem cells in a given sample. "We know a lot about stem cells in the blood and that's because we can get our hands on them. Now we can determine which markers distinguish stem cells from differentiated skin cells, which will eventually allow us to isolate skin stem cells," Ghadially said.
The eventual isolation of skin stem cells, Ghadially said, promises to allow the treatment of wounds, including burns, through transplantation of stem cells directly onto the damaged area where new skin will grow. It may take decades, but Ghadially predicts researchers also will be able to prompt skin stem cells to produce more stem cells in the same way researchers have been able to do with stem cells taken from the blood. Also, isolating epidermal stem cells will allow skin researchers to better understand the process by which skin cells differentiate and, since skin cancer likely originates in stem cells, better understand--and maybe someday better treat--skin cancer.
Additional authors include Tracy E. Schneider, BS, Chantal Barland, MD, and April M. Alex, MS, of the UCSF Department of Dermatology; Ying Lu, PhD, statistician and James E. Cleaver, PhD, researcher of the UCSF Comprehensive Cancer Center; H. Jeffrey Lawrence, MD, SFVAMC staff physician and UCSF professor of medicine; and Maria L. Mancianti, MD, of the Department of Pathology, Alta Bates Medical Center, Berkeley, CA.
This research was supported by two grants to Ghadially from the National Institutes of Health and a Department of Veterans Affairs Merit Review Program Award.