The number of times that stem cells in self-renewing tissues divide over a lifetime is unknown. Estimates of stem cell numbers and cell division rates require accurate identification of stem cells, a major challenge in all studies of stem cell biology.1 The problem of identifying stem cells is exemplified by the hematopoietic system, the self-renewing tissue best studied in both humans and mice. Our scientists are working to understand how to grow large quantities of adult stem cells, which have the potential to develop into more than 200 cell types in cell culture. Stem cells are powerful tools in biology and medicine. What can scientists do with these cells to harness their incredible potential? Stem cells are powerful.
They are pluripotent, meaning they can transform into any of the 220 cell types in the human body. A stem cell can also divide to produce millions more stem cells. The potential of stem cells to renew themselves or create new tissue is nearly endless. These properties make stem cells an important tool in the laboratory and in medicine. They offer scientists a better understanding of human development and a way to test drugs without putting human volunteers at risk, and they provide a way to replace damaged tissues, such as retinal cells, muscles or the spinal cord.
But how can we get the most out of stem cells? How can we make them grow where we want, grow how we want, and repair damaged or diseased tissue? Only when cells divide several times do they begin to lean toward one destination or another, expressing the genes specific to a cell type. Different types of stem cells have different degrees of potency, that is, the number of different cell types they can form. These adult stem cells reside in special stem cell niches, regions of certain tissues where they wait for signals from the body to replace or repair tissue. Throughout the life of the body, adult stem cell populations act as an internal repair system that generates replacement cells for which they are lost due to normal wear and tear, injury, or disease. The NIH awards fall into several categories of stem cell research based on NIH estimates of funding for various categories of research, conditions and diseases (RCDC).
Adult (non-embryonic) stem cells are unspecialized or undifferentiated cells, meaning that they have not yet developed into a specific cell type. However, they still don't fully understand where adult stem cells come from or how they differentiate when needed. It took many years of trial and error to learn how to obtain and maintain pluripotent stem cells in the laboratory without the cells spontaneously differentiating into specific cell types. Yamanaka and his colleagues had yet to demonstrate that their iPS cells were fully pluripotent, which meant demonstrating that they could form a complete animal in the same way that iPS cells do embryonic stem cells.
Brain neural stem cells can differentiate into several types of brain cells, but they cannot develop into liver cells, for example. The problem is that adult stem cells exist in very small numbers and are often buried deep in the tissue. In addition to the technical limitations related to cell culture, scientists are held back by human cultivation outside the laboratory because not everyone supports embryonic stem cell research. Telomere length was longer in fetal liver cells, intermediate in umbilical cord blood cells, and shorter in adult bone marrow cells.
For example, a doctor isolates a patient's hematopoietic stem cells, introduces into them a harmless virus that expresses a correct version of the mutated gene, and then re-administers the stem cells to patients. These cells, known as hematopoietic stem cells, give rise to red blood cells, white blood cells and all the cells of the immune system that carry oxygen.
