STEM CELLS
Stem cells are primitive cells that have no specific function but can develop into mature cells with specialized functions. In a fetus, stem cells develop into the various organs of the body to produce a baby. In an adult, stem cells are used to replace worn out cells when they die. Although stem cells act in similar ways, there are types of stem cells in terms of where they come from: adult stem cells, which are present in the body throughout adult life; embryonic stem cells, which are only found in the embryo; and induced pluripotent stem cells (iPSCs), which can be created in the lab from ordinary adult cells and reverted back to a stem cell.
Medical and scientific interest in stem cells is based on a desire to find a source of new, healthy tissue to treat diseased or injured human organs. It is known that some organs, such as the skin and the liver, are adept at regenerating themselves when damaged. Scientists do not yet understand how these cells differentiate during development, but they do know that stem cells are a key to these regenerative properties. So when they do understand these developmental processes, they may be able to apply them to stem cells grown in vitro and potentially regrow cells such as nerve, skin, intestine, liver, etc for transplantation.
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LIMB & ORGAN REGENERATION
Tissue regeneration is the most important use of stem cells. A person who needs a new kidney, for example, must wait for a donor and then undergo a transplant. If scientists can instruct stem cells to differentiate in a certain way, they could use them to grow a specific tissue type or organ. Humans are able to regenerate some simple tissues — shallow cuts, for example — but deep cuts and other injuries are healed with scar tissue rather than with regenerated tissue. Other animals, such as crabs and salamanders, recruit stem cells to the injured area or missing limb to heal the injury without scar tissue, or to regenerate the entire limb.
Humans have some stem cells in their bodies, but the number is limited and the stem cells are not readily available to help with healing. One theory is that humans lack the genetic code that triggers regeneration of a lost limb. Or they have that genetic code but it's switched off. Perhaps scar tissue is an adaptation but one that prevents regeneration.
Another theory is that the cellular machinery that triggered regeneration was abandoned during the evolution of humans because the growth of cells can look a lot like cancer. Rapid cell division is associated with tissue regeneration but it's also a feature of cancer. Perhaps evolution suppressed rapid cell division to combat cancer at the cost of losing our ability to regenerate tissue.
The reason for that theory is that our immune system is more highly developed than animals who can regenerate limbs. In a salamander, for example, if its tail is cut off, the stem cells are triggered to grow, multiply, and specialize. The salamander's more primitive immune system is not triggered to destroy the proliferation of these abnormal cells. Salamanders regenerate tissue but hardly ever get cancer. Our immune system, however, is likely to see a proliferation of undifferentiated cells as an alien invasion and send Killer T cells to destroy them before they differentiate into bone, blood, nerves and skin of a new limb.
CELL SIGNALING & CANCER
Cell signaling is essential for an effective immune system. Damaged cells release a chemical message to alert neighboring healthy cells that healing must be activated. In order to detect the chemical warning, a neighbor cell must be a target cell; that is, have the right receptor for that signal. Receptors sit on the surface of a cell, half outside, half inside. When a signaling molecule binds to the target cell's receptor, it triggers a change inside the cell, telling the cell to grow, stop growing, survive or die. "Killer" cells use cell-surface markers to distinguish normal cells from abnormal cells or cells infected by pathogens.
Researchers discovered that undifferentiated stem cells have not yet used cell signaling to become an adult stem cell with a specialized function. This discovery has applications in cancer research because a large percentage of cancer cells are also undifferentiated cells. If a mutation causes a cell to ignore messages that it's time to stop growing or that it's time to die, a tumor develops. Cancer cells have ten ways to ignore chemical signals from surrounding cells.
STEM CELLS & BRAIN DISORDERS
Since stem cells can grow into brain cells, they have the potential to repair damage that results in neurological diseases such as Parkinson's (uncontrolled muscle movement) and Alzheimer's (dementia). All Food and Drug Administration approved treatments currently on the market are aimed at minimizing the symptoms of Alzheimer's, because the cause of Alzheimer's disease is still unknown.
What scientists do know is that the condition is characterized by two types of abnormal brain structures — amyloid-beta (AB) plaques and neurofibrillary tangles. AB plaques are sticky clumps of protein fragments that accumulate around and attack brain cells, leading to their death. Neurofibrillary tangles are twisted fibers of protein that build up inside the neurons of Alzheimer's patients. The memory loss and communication problems caused by these abnormal brain structures don't appear until after age sixty because it takes time for these structures to amass.
The reason why some people get Alzheimer's and some do not has been elusive because samples of brain tissue cannot be removed from living patients, making comparisons impossible. However, with the advent of induced pluripotent stem cell technology — the biological reprogramming of mature cells into stem cells — stem cells from a patient's skin can be generated and changed into brain cells that do and do not result in Alzheimer's, and study the results.
This approach to studying amyloid-beta plaques and neurofibrillary tangles has led to treatments that focus on the cause, not just the symptoms, of Alzheimer's disease. Stem cell therapy involves the systemic introduction of stromal stem cells into the body via IV. When introduced in large quantities, these stem cells can find inflammation within the body and repair it. This unique property of stem cells is what makes them a potentially viable treatment for Alzheimer's Disease.
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