Adult human stem cells that are intrinsic to various tissues have been described and characterized, some of them only recently. These cells are capable of maintaining, generating, and replacing terminally differentiated cells within their own specific tissue as a consequence of physiologic cell turnover or tissue damage due to injury. Hematopoietic stem cells that give rise to blood cells and move between bone marrow and peripheral blood are the best-characterized adult stem cells in humans. Recent data suggest that adult stem cells generate differentiated cells beyond their own tissue boundaries, a process termed "developmental plasticity".

Adult Stem Cells and Their Potential for Developmental Plasticity
Stem cells are defined as cells that have clonogenic and self-renewing capabilities and that differentiate into multiple cell lineages. Whereas embryonic stem cells are derived from mammalian embryos in the blastocyst stage and have the ability to generate any terminally differentiated cell in the body, adult stem cells are part of tissue-specific cells of the postnatal organism into which they are committed to differentiate. Phenotypically characterized adult stem cells are listed below.

Table 1. Adult Human Stem Cells and Their Primary Direction of Differentiation

The hematopoietic system has traditionally been seen as an organized, hierarchic system with multipotent, self-renewing stem cells at the top, committed progenitor cells in the middle, and lineage-restricted precursor cells, which give rise to terminally differentiated cells, at the bottom. However, this classic paradigm of stem-cell differentiation restricted to its organ-specific lineage is being challenged by the suggestion that adult stem cells, including hematopoietic stem cells, retain a previously unrecognized degree of developmental plasticity that allows them to differentiate across boundaries of lineage, tissue, and germ layer. The hierarchical view no longer seems correct.
The molecular mechanisms of lineage switches within the hematopoietic system have been studied extensively, but the mechanisms that determine transitions in the fate of adult stem cells remain poorly understood. The results of recent studies of the plasticity of adult stem cells, which contradict the dogma that the differentiation and commitment of adult stem cells are restricted to their own tissue, are the subject of intense discussion. These findings demand the most stringent criteria for providing conclusive evidence. To prove that stem cells derived from bone marrow and peripheral blood, including hematopoietic stem cells, are indeed transformed into solid-organ-specific cells, several conditions must be met. First, the origin of the exogenous cell integrated into solid-organ tissue must be documented by cell marking, preferably at the single-cell level. Cells also should be processed with a minimum of ex vivo manipulation (e.g., culturing), which may make them more susceptible to crossing lineages. Second, the exogenous cell must be shown to have become an integral morphologic part of the newly acquired tissue. Third, and most important, the transformed cell must be shown to have acquired the function of the particular organ into which it has been integrated, both by expressing organ-specific proteins and by showing specific organ function.

Clinical in vivo Studies
Various stem-cell preparations originating from hematopoietic tissue have been used in in vivo studies of stem-cell plasticity, including unselected cells from bone marrow, purified hematopoietic stem cells, and single hematopoietic stem cells that originate from either bone marrow or peripheral blood. Because the experimental conditions in a preclinical setting are currently far more sophisticated than those available in a clinical setting, the mechanisms underlying the reported clinical observations should be interpreted with caution. The same is true when extrapolating knowledge about stem-cell function in nonprimate species to humans. Given the number and variety of clinical studies that have already been performed (Table below), and the lack of data from studies of large animals - in particular, primates - caution should be exercised in predicting a potential clinical benefit.

Table 2. Potential Clinical Applications for Tissue-Derived Adult Stem Cells for Tissue Repair or Replacement

Models of Differentiation of Adult Stem Cells into Solid-Organ-Specific Cells
The results of transplantation studies of peripheral-blood stem-cell transplantation suggest four possible explanations for how adult stem cells derived from bone marrow or peripheral blood differentiate into nonlymphohematopoietic tissue cells (Fig. 1).

Fig. 1. Various Models for Generating Solid-Organ Tissue Cells through Differentiation of Bone Marrow-Derived and Circulating Adult Stem Cells. In the first model, distinct stem cells differentiate, each into its own organ-specific cell (Panel A). In the second model, primitive somatic stem cells located in hematopoietic tissue differentiate into various organ-specific cells (Panel B). In the third model, stem cells, such as hematopoietic stem cells, differentiate along their predetermined pathway. Under certain, probably rare conditions, tissue injury or another stimulus causes some stem cells to deviate from their predetermined pathway and generate cells of a different tissue - a process known as transdifferentiation (Panel C). In the fourth model, mature cells dedifferentiate into cells with stem-cell-like characteristics and eventually redifferentiate into terminally differentiated cells of their own tissue or a different tissue (Panel D).

None of these four models of stem-cell differentiation have yet been proved to explain the mechanism underlying developmental stem-cell plasticity.

Potential Role of Circulating Adult Stem Cells in Tissue Repair
It is well known that circulating hematopoietic stem cells are critical for hematopoietic homeostasis. Similarly, one may hypothesize that circulating stem cells contribute to homeostasis in solid-organ tissue. The mechanisms by which circulating stem cells are recruited into various solid-organ tissues and tissue-specific cells are subsequently generated are not fully understood. Tissue injury that causes changes in the microenvironment may play an important part in stem-cell recruitment (Fig. 2).

Fig. 2. Possible Roles of Bone Marrow-Derived and Circulating Stem Cells in the Repair of Solid-Organ Tissue. After tissue injury, stem cells that are intrinsic to the tissue replace necrotic cells as a first line of defense. If the pool of endogenous stem cells is exhausted, exogenous circulating stem cells are signaled to replenish the pool and participate in tissue repair. Thus, circulating stem cells may serve as a backup rescue system.

In this regard, the highest percentage of donor-derived hepatocytes (about 40%) was found in a liver-transplant recipient with fibrosing cholestatic recurrent hepatitis C. A low frequency of hepatocyte replacement by bone marrow-derived stem cells has also been reported (up to 2.2% of total hepatocytes), even in the absence of histologic evidence of acute hepatic tissue injury, suggesting that circulating cells take part in tissue homeostasis. Because the model for most experimental and clinical transdifferentiation studies is allogeneic stem-cell transplantation with use of the Y chromosome as a marker, human leukocyte antigen disparity cannot be ruled out as a trigger for circulating stem-cell transdifferentiation.
A concentration of stem cells at the site of tissue damage has been shown to contribute to tissue repair. The repair of infarcted cardiac tissue after either the injection of stem cells derived from bone marrow directly into heart tissue surrounding the infarct or a systemic increase in the number of peripheral-blood hematopoietic stem cells to a level approximately 250 times as high as the base-line level with use of a combined treatment of recombinant human granulocyte colony-stimulating factor and stem-cell factor was reported earlier.
The release of chemokines such as stromal-derived factor from injured hepatic tissue and interaction with its specific receptor, CXCR4, on hematopoietic precursor cells is another prototype of how tissue repair may function by aiding in the recruitment of circulating hematopoietic precursor cells to the site of tissue injury.
On a molecular level, transcriptional regulation of hematopoietic stem cells may be altered in response to signals from the local environment. Specifically, homing, growth, and differentiation factors that may be involved in transitions of cell fate have been described for neuronal cells as a prototype.

A Clinical Model of Solid-Organ Tissue Generated by Circulating Stem Cells
Under physiologic steady-state conditions, equilibrium is maintained whereby endogenous stem cells intrinsic to a specific tissue and, to a much lesser extent, blood-derived stem cells of heterologous lineage replenish apoptotic tissue. However, a rigorous demand for tissue repair that cannot be met by endogenous stem-cell differentiation may, as a backup system, trigger circulating stem cells to undergo differentiation into solid-organ tissue, though this is believed to be rare under physiologic steady-state conditions. Thus, the pool of tissue-intrinsic stem cells can be considered an intermediary between systemically circulating stem cells and terminally differentiated solid-organ-specific cells (Fig. 2). Adult cells derived from bone marrow contributed to the formation of up to 3.5% of differentiated muscle fibers. The triggering mechanisms for the plasticity of bone marrow-derived stem cells seemed to be an insufficient pool of stem cells intrinsic to this tissue in concert with an increased demand for the production of new cells and changes in the microenvironment as a result of tissue injury at the site.

Potential Clinical Applications
There are essentially two strategies for using adult stem cells derived from hematopoietic tissue for tissue repair. One approach is based on identifying and expanding in vitro multipotent adult progenitor cells that, like embryonic stem cells, are capable of generating mesodermal, ectodermal, and endodermal tissue. The other approach is based on the in vivo availability of a pool of systemic and circulating adult stem cells that can be manipulated to generate or repair solid-organ tissue.
Assuming that circulating stem cells generate cells specific to solid organs in vivo, a potential clinical concept of tissue repair would require three conditions. First, the stem-cell pool must be easily accessible, as is the case with the circulating stem-cell pool that is routinely used for harvesting hematopoietic stem cells. Second, the concentration of stem cells at the site of tissue regeneration must be sufficient, which can be accomplished either by cytokine-induced mobilization of hematopoietic stem cells, mesenchymal stem cells, angioblasts, and smooth-muscle progenitor cells from extravascular sites into the circulating blood or by directly delivering the cells to the site of tissue injury. Third, appropriate signals from the site of damaged tissue must direct exogenous stem cells to the site where they are needed. However, we do not yet clearly understand how to manipulate the microenvironment surrounding the area of tissue regeneration actively and signal exogenous blood-derived stem cells to participate in tissue regeneration in vivo.
As with transplantation of hematopoietic stem cells, solid-organ tissue repair can be accomplished by differentiation of autologous or allogeneic stem cells. Allogeneic transplants are advantageous in experimental settings in which the Y chromosome is used as a marker, as well as in hereditary stem-cell disorders, in which transplantation of normal stem cells is supposed to generate normally functioning tissue. Differentiation of autologous bone marrow or circulating stem cells into solid-organ tissue seems the preferred therapeutic approach, as it precludes immunology disparities.
We are only beginning to understand the circulating blood, not only as a distributor of hematopoietic progenitor cells but also as a systemic supplier of progenitor cells that have the potential to participate in the homeostasis of various solid-organ tissues and that are capable of nonlymphohematopoietic tissue repair. There is a growing body of evidence that the adult stem-cell system may be more flexible than previously thought, particularly under conditions of tissue stress. As a consequence, tissue-specific cell systems may be regarded as versatile and dynamic, irrespective of lineage-specific restrictions.
The observation that solid-organ tissues are colonized by organ-specific cells originating from the circulating blood suggests that tissue regeneration and repair may be feasible if we learn to direct progenitor cells from the circulating blood into areas of injured or diseased tissue and to modulate their proliferation and maturation once these cells have reached the target tissue. Despite the promising studies indicating the potential plasticity of adult stem cells, many obstacles remain, some of which may favor the misinterpretation of data that indicate such a phenomenon. The basic mechanisms of stem-cell differentiation that lead to the formation of solid-organ tissue are still not completely understood. However, translational research, including clinical studies, is already being performed to develop potential treatment strategies.