Stem Cells
What are stem cells?
Stem cells are cells that have remained unspecialised. Central to their role is the fact that they have the ability to develop into all or many different types of specialised cell - they are said to be pluripotent (multi-powerful). At the beginning of life all cells are pluripotent, but as the embryo develops more and more cells become specialised to produce different tissues, such as nerve tissue, skin or muscle. In the adult pockets of stem cells persist where they provide a renewable source of cells. Stem cells in the bone marrow continue to produce many different varieties of blood cell throughout life, and groups of stem cells constantly renew the skin and gut lining. Experimentally, stem cells derived from bone marrow transplants in mdx mice were found to take part in muscle development i.e. some of the stem cells from the bone marrow had the potential to become muscle cells. However, only a few fibres were actually found to be dystrophin positive.
The Mechanism
Theoretically, if stem cells containing normal dystrophin genes could be delivered to muscle in DMD, they could regenerate fully functional muscle fibres. Stem cell therapy could be combined with gene therapy, whereby stem cells could be harvested from the patient, isolated and grown and then treated with a virus containing the target dystrophin gene. These treated cells could then be reintroduced to the patient either via the blood stream or local injection into a muscle. Once at their target they would gradually produce new groups of healthy muscle cells to replace the damaged fibres.
Problems
Although it has been shown that stem cells isolated from newborn mice can give rise to muscle fibres in mdx mice following injection into the blood stream, results so far have been disappointing. The stem cells appeared to stay very close to the injection site rather than migrating, and only gave rise to very small clusters of one to five fibres. Furthermore, introducing any foreign cell into the body again gives rise to an immune response.
Many basic scientific questions remain unanswered concerning stem cell biology:
What are the signals involved in the development of stem cells into other cell types? What regulates the proliferation and maintenance of stem cells?
How are stem cells attracted to the correct tissue?
Once cells have reached muscle, is there a major problem with development into muscle cells?
Research into stem cells is ongoing in many different medical fields, and with greater understanding of the basic science, a number of these questions will, no doubt, become clearer.