When a stem cell divides by mitosis, it does not formed two identical daughter cells as in ordinary mitosis (symmetrical mitosis), instead, one identical stem cell is formed, and one cell that goes on to differentiate into the type (or one of the types) of cell that is formed by this particular stem cell. This is known as asymmetrical mitosis.
There are particular stem cells stored in bone marrow called erythroblasts, or haematopoietic stem cells. These are pluripotent, not multipotent, as they have a much wider range of possible cells to be created than most stem cells in the body. They assist in replenishing the supply of red blood cells (erythropoiesis), platelets (thrombopoiesis), white blood cells (leucopoiesis) and granulocytes (granulopoiesis).
The process begins when an erythroblast divides into another of itself and a progenitor cell (see above). These are known as early progenitors, because they are not differentiated enough to split to form normal (non-stem) cells. These early progenitor cells have receptors on their membranes (as with most cells) for cytokines, the proteins that stimulate cytokinesis. Once they are stimulated, they will again perform split and form two committed progenitors. These will each have the ability to form only one of four different types: white blood cells, red blood cells, platelets and granulocytes (another type of white blood cell, with granules in the cytoplasm), and then they may or may not differentiate further into more specialized types of cells.
For the development of red blood cells (erythrocytes), a glycoprotein called EPD Procrit must be detected. This encourages the committed progenitor to form red blood cells upon mitosis. All other types have similar glycoproteins, which encourage differentiation into other particular cells.
There are many ways in which scientists plan to utilise the unique properties of stem cells for medical use, particularly erythroblasts as they are known to be the least diverse adult stem cells. (The best stem cells available would be from an undeveloped embryo, inside the blastocyst, but there are many ethical problems involved with their use, so finding an equally useful alternative would be much more viable for the future.)
Much sought after is a cure, or viable treatment, for type one diabetes, in which the body’s own T-cells have destroyed insulin producing beta cells in the pancreas. A transplantation of bone marrow containing pluripotent stem cells into diabetic, non-obese mice and discovered a reduction in blood sugar levels after two weeks. Apparently the bone marrow stem cells began producing pancreatic cells that produced insulin, and were not destroyed by the body’s T-cells. In the future, it is hoped that a human donor bone marrow transplant can take place and produce the same result.
Another bright looking possibility is a treatment for Parkinson’s disease, a disorder in which the cells which produce dopamine for the brain no longer function – current treatment replaces the chemical but reduces in effectiveness over time. A transplant of stem cells (not bone marrow stem cells as these do not have the ability to develop into brain cells) in rats has produced functioning dopamine-producing cells.
The future of HIV/AIDS treatment could also be dramatically changed with the development of stem cell technology: a drug, commercially known as Neupogen, is a human-engineered cytokine that stimulates the production of granulocytes, an important type of white blood cell that AIDS and HIV sufferers lack. It can also be used to treat patients who have been exposed to large amounts of radiation, either accidentally or through chemotherapy. It is currently in wide use, and more, similar, drugs are being tested that will perform the same type of actions for other types of blood cell.
Foetal haemoglobin is much more effective at oxygen retrieval, in order to be able to override the mother’s hold on oxygen in her own haemoglobin. Transplant of foetal erythroblasts in sickle-cell disease sufferers can work to replenish their own defective cells, and will quickly improve the sufferer’s oxygen retrieval abilities.
A therapy much celebrated by scientists and disliked by pro-life campaigners is therapeutic cloning. This is a type of cloning that aims to reproduce an organ, tissue or group of cells using a person’s own stem cells. This method almost eradicates the risk of rejection by the body (as the t-cells will recognise the cells as non-foreign, but the method used causes some foreign mitochondria to be involved in the daughter cells, so there is still some risk, although much reduced). A donor stem cell from the patient is fused, through somatic cell nuclear transfer, with an egg cell stripped of its nucleus and treated with electric shock treatment, to stimulate the egg into growth. Embryonic stem cells can then be harvested and treated (encouraged, using similar chemical gradient properties and utilising homebox genes) to develop into the desired cells or organ. These can then be transplanted into the patient and, theoretically, function as normal. Heart and lung transplants seem far into the future, but miniature, functioning kidneys and pancreatic cells have been produced.
Bibliography
Salters-Nuffield Advanced Biology As student book
Collins Advanced Science - Biology
