Adult stem cell plasticity and transdifferentiation. A number of experiments have suggested that certain adult stem cell types are pluripotent. This ability to differentiate into multiple cell types is called plasticity or transdifferentiation. The following list offers examples of adult stem cell plasticity that have been reported during the past few years.
Figure 2. Plasticity of adult stem cells.
Hematopoietic stem cells may differentiate into: three major types of brain cells (neurons, oligodendrocytes, and astrocytes); skeletal muscle cells; cardiac muscle cells; and liver cells. Bone marrow stromal cells may differentiate into: cardiac muscle cells and skeletal muscle cells. Brain stem cells may differentiate into: blood cells and skeletal muscle cells.
Current research is aimed at determining the mechanisms that underlie adult stem cell plasticity. If such mechanisms can be identified and controlled, existing stem cells from a healthy tissue might be induced to repopulate and repair a diseased tissue (Figure 2).
D. What are the key questions about adult stem cells?
Many important questions about adult stem cells remain to be answered. They include:
How many kinds of adult stem cells exist, and in which tissues do they exist? What are the sources of adult stem cells in the body? Are they "leftover" embryonic stem cells, or do they arise in some other way? Why do they remain in an undifferentiated state when all the cells around them have differentiated? Do adult stem cells normally exhibit plasticity, or do they only transdifferentiate when scientists manipulate them experimentally? What are the signals that regulate the proliferation and differentiation of stem cells that demonstrate plasticity? Is it possible to manipulate adult stem cells to enhance their proliferation so that sufficient tissue for transplants can be produced? Does a single type of stem cell exist — possibly in the bone marrow or circulating in the blood — that can generate the cells of any organ or tissue? What are the factors that stimulate stem cells to relocate to sites of injury or damage?
