Location of Adult Stem Cells – A Number of Niches

A recent confirmation regarding adult stem cells is the location of an active population in the limbus which maintains and regenerates the corneal epithelium. Suggested forty years ago, or so, by Davanger and Evansen (1971) as a possible site of stem cells, recent research has found that the limbal epithelial stem cells (LESCs) are highly active and have a long cell cycle. This makes them ideal for transplant purposes and the relative transparency of the cornea also enables non-invasive imaging of the cells to be carried out. The LESCs are also thought to be held in an undifferentiated state by the pallisades of Vogt (folds in the sub-epithelial connective tissue). This protected enclave of stem cells has been termed a stem cell ‘niche’ and there is speculation over the existence in the body of other, as yet undiscovered, stem cell niches which could act as bountiful resources for stem cell extraction and stem cell therapy.

The use of LESCs has already helped restore a Canadian man’s vision as he received a stem cell transplant from his sister after becoming almost completely blind due to a progressive eye disease connected to contact lens use. The procedure, carried out in Toronto, is thought to be the first of its kind and may have removed the need for the man to have a corneal transplant. More patients are being scheduled for similar procedures and various visual problems may be helped by this resource of stem cells. It is not yet possible to say whether the LESCs could be used to treat disorders outside of optical disease but some scientists remain optimistic over their potential.

Neural Cell Regeneration – Stem Cells and Markers

Another exciting development in the field of adult stem cell research is the finding that bone marrow stem cells have the ability to travel to the brain post-transplant and can aid neuroregeneration. Scientists have conducted animal experiments using gender-mismatched bone marrow transplants and then isolated neural cells which contained Y chromosomes in the brains of those (female rodents) receiving the transplants. The possibility of using peripheral blood stem cells to form new neural tissue or regenerate damaged tissue is extremely exciting for those suffering with a degenerative brain illness, stroke patients, those with spinal cord injury, and many others.

The identification of adult stem cells remains a complex issue with a number of strategies employed by researchers but no general consensus on the best markers of adult stem cells. Most stem cell markers lack the desired specificity and cells outside of the stem cell ‘zone’ under scrutiny may carry the same expression of these markers. An example of this is in the colon where CD44 and CD133, amongst others, have been suggested as markers of stem cells; these occur outside the area of stem cells however, making isolation difficult.

The Future of Adult Stem Cell Research

As scientists refine the methods of isolating, extracting, culturing, and reintroducing adult stem cells into the body the potential benefits for health appear almost limitless. With restrictions having been partially lifted on the funding of stem cell research using human embryonic stem cells (hESCs) however, the future of adult stem cell research may start to look a little different. Other than bone marrow transplants, few clinical procedures using autologous stem cells are available in the US and other heavily-regulated countries. As clinical trials are underway using adult stem cells for spinal cord injury treatment, stroke, cardiac problems, and a number of other applications, the next few years look likely to be full of stem cell treatment developments offering possible hope for those with previously intractable diseases.

References

Anne McIlroy, Stem cell transplant from sister helps athletic student see again, Wednesday, Dec. 29, 2010, http://www.theglobeandmail.com/life/health/stem-cell-transplant-from-sister-helps-athletic-student-see-again/article1852714/

Ed. Masters, J.R.W., and Palsson, B., (2009), Human adult stem cells, Springer.