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Summary: What is an Adult Stem Cell?

  • Adult stem cells can proliferate without differentiating for a long period (a characteristic referred to as long-term self-renewal), and they can give rise to mature cell types that have characteristic shapes and specialized functions.

  • Some adult stem cells have the capability to differentiate into tissues other than the ones from which they originated; this is referred to as plasticity.

  • Adult stem cells are rare. Often they are difficult to identify and their origins are not known. Current methods for characterizing adult stem cells are dependent on determining cell surface markers and observations about their differentiation patterns in test tubes and culture dishes.

  • To date, published scientific literature indicates that adult stem cells have been derived from brain, bone marrow, peripheral blood, dental pulp, spinal cord, blood vessels, skeletal muscle, epithelia of the skin and digestive system, cornea, retina, liver, and pancreas; thus, adult stem cells have been found in tissues that develop from all three embryonic germ layers.

  • Hematopoietic stem cells from bone marrow are the most studied and used for clinical applications in restoring various blood and immune components to the bone marrow via transplantation. There are at least two other populations of adult stem cells that have been identified from bone marrow and blood.

  • Several populations of adult stem cells have been identified in the brain, particularly the hippocampus. Their function is unknown. Proliferation and differentiation of brain stem cells are influenced by various growth factors.

  • There are now several reports of adult stem cells in other tissues (muscle, blood, and fat) that demonstrate plasticity. Very few published research reports on plasticity of adult stem cells have, however, included clonality studies. That is, there is limited evidence that a single adult stem cell or genetically identical line of adult stem cells demonstrates plasticity.

  • Rarely have experiments that claim plasticity demonstrated that the adult stem cells have generated mature, fully functional cells or that the cells have restored lost function in vivo.

What Do We Need to Know About Adult Stem Cells?

  • 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? And if the latter is true—which seems to be the case—exactly how do adult stem cells arise, and why do they remain in an undifferentiated state, when all the cells around them have differentiated?

  • Is it possible to manipulate adult stem cells to increase their ability to proliferate in vitro, so that adult stem cells can be used as a sufficient source of tissue for transplants?

  • How many kinds of adult stem cells exist, and in which tissues do they exist? Evidence is accumulating that, although they occur in small numbers, adult stem cells are present in many differentiated tissues.

  • What is the best evidence that adult stem cells show plasticity and generate cell types of other tissues?

  • Is it possible to manipulate adult stem cells to increase their ability to proliferate in vitro so that adult stem cells can be used as a sufficient source of tissue for transplants?

  • Is there a universal stem cell? An emerging concept is that, in adult mammals, there may be a population of "universal" stem cells. Although largely theoretical, the concept has some experimental basis. A candidate, universal adult stem cell may be one that circulates in the blood stream, can escape from the blood, and populate various adult tissues. In more than one experimental system, researchers have noted that dividing cells in adult tissues often appear near a blood vessel, such as candidate stem cells in the hippocampus, a region of the brain [75].

  • Do adult stem cells exhibit plasticity as a normal event in vivo? If so, is this true of all adult stem cells? What are the signals that regulate the proliferation and differentiation of stem cells that demonstrate plasticity?

Stem Cells 101