Aging has a variety of definitions and implications depending on one's perspective. From an actuarial viewpoint, it may be defined as an increasing probability of death with the passage of time. There are two major theories of organism's aging: evolutionary and damage-based. The former posits that natural selection favors a genetic composition that enhances reproductive fitness and fecundity. According to the theory, since the effect of this genetic repertoire on vitality following the reproductive lifespan is immaterial to the species, genes have been selected and accumulated which favor reproductive success but have negative effects in later life, thus limiting lifespan. However, as has been pointed out, selection of genes favoring reproduction could also select for genes promoting longevity provided that they prolong the time a species may reproduce. The second theory argues that accumulated cellular damage over a lifetime limits the effective functioning of key organ systems. Compromised organ function, according to the theory, in turn limits longevity.

Long-lived cells are subjected to damage. An important focus for accumulated damage may be stem cell populations, since by definition they are long-lived and exposed to the noxious effects of both extrinsic and intrinsic effectors of damage. An example of the former on skin stem cell populations is ultraviolet light; and, for example, chemotherapeutic drugs used in cancer treatment may have long-lasting deleterious effects on hematopoietic stem cells. Reactive oxygen species (ROS), the byproducts of cellular metabolism, have long been regarded as the principal cell intrinsic effector of damage in many cells. Since mitochondria are the sites of oxidative metabolism, proximity to ROS may make damage to this organelle and its DNA especially acute.

Of course, there are other long-lived cells in the body in addition to stem cells that must also cope with lifelong exposure to damaging agents such as ROS. To use an example enjoying a current renaissance in popular culture, the persistence of tattoos attests to the long lifespan of phagocytic connective tissue cells in the dermis. Memory cells in the lymphoid system are the crucial repository of the catalog of antigens to which an animal has been exposed, and persist for the better part of an organism's lifespan. Neurons of the central nervous system have until recently been regarded as the archetypal postmitotic cell population, unable to be replenished. However, accumulating evidence has shown that neuron replacement in some brain regions not only occurs following experimental manipulations, but may also occur under steady-state conditions throughout life. As in other organs where worn-out cells are continuously replaced by new ones, stem cells are the cellular source enabling this process. In contrast to long-lived postmitotic cells, acquired stem cell damage is especially onerous because damage to cellular components, especially DNA but also mitochondria and some other structural components, are passed on to progeny. Although the clone size contributed to by single stem cells is dependent on a number of variables including aging, large numbers of descendants will be affected. It has recently been calculated that the number of stem cells per mammal has been evolutionarily conserved, such that mouse, man, and presumably elephants may begin life with the same number of stem cells. Thus, the average number of mature blood cell progeny produced by a stem cell may be in proportion to the size of the animal, since blood volumes are scaled to body size. Although the circulating lifespan of erythrocytes are about twofold different between mouse and man (45 and 110 days, respectively), this does little to close the gap between the number of cells in comparative blood volumes of 1.5 mL and 5.0 L respectively.

Evidence against the aging of stem cells
In view of the apparently extensive investment in protective mechanisms and the ability of stem cells to self-renew, what is the evidence that stem cells show any age-related defects whatsoever? After all, germline stem cells not only remain viable for the reproductive life of a mammal, but also are perpetuated through generation after generation of progeny via germ cells that they produce. Attributes of stem cells of adult organs, although more modest, are nonetheless impressive. A good example is the replicative and differentiative requirement put on hematopoietic stem cells following bone marrow transplantation. A minuscule fraction of a donor's total stem cell pool is required to completely replenish the ablated lympho-hematopoietic system of the transplant recipient. This feat is made more remarkable by the fact that it can be repeated in mice by serially transplanting marrow from primary to secondary hosts, etc. Thus, the original small population of stem cells can produce mature progeny during a period far in excess of the original donor's lifespan. Don't these collective developmental powers put to rest any thoughts of stem cell aging? Moreover, the life-ending diseases of most mammals, including humans (heart disease, Alzheimer's disease, end-stage renal failure, etc.), have etiologies that apparently don't involve stem cells. With respect to the hematopoietic system, diseases associated with a loss of stem cells, aplastic anemia and complete bone marrow failure are at the same time relatively rare and not especially age-dependent. Moreover, experimental comparison of the engraftment properties of young and old marrow in a large-animal model, the dog, failed to show any decrement in stem cell function with age. The proposition that the depletion of stem cell function could lead to organ failure and thereby be involved in determining lifespan is highly speculative. Nevertheless, regardless of the degree to which stem cells may be involved in particular disease processes current stem cell concepts must be incorporated into any comprehensive consideration of aging.

Evidence favoring stem cell aging
A closer examination of the available data reveals significant age-related changes in stem cell populations (Fig. 1). While it is true, in the most impressive demonstration of the staying power of stem cells, that they are capable of serial passage through a succession of mouse recipients, they are not immortal. Under the best of circumstances, it is generally not possible to exceed five successful passages, and the number is often considerably less. Moreover, after the second or third transplant, the few host stem cells surviving the lethal radiation dose have an increasing competitive advantage over the serially passaged donor cells and, despite their numerical disadvantage, assume more of the burden of hematopoiesis. It has been argued that the limitation is due to the manipulations associated with the transplant procedure, rather than to curtailed developmental potency of the stem cell. Nevertheless, engraftment of even primary transplant recipients does not restore the hematopoietic system to the normal state. Despite the apparently complete and long-term recovery of progenitor cell numbers and the return to normalcy of mature blood cell counts, stem cells recover to only a small fraction of the total number found in an unmanipulated animal. But to reiterate, even an incomplete restoration of stem cell numbers is nonetheless impressive and attests to stem cells' extensive powers of self-renewal. Failure to fully regenerate the stem cell population may be due to extrinsic factors associated with the transplant procedure, particularly the reassociation with bone marrow stromal components and their integuments, including cytokines and extracellular matrices that they produce. It is for these reasons that successively larger numbers of marrow cells must be transplanted with each passage to obtain engraftment.

Fig. 1. Diminished functional capacity of hematopoietic stem cells during aging. Few apoptotic cells, a mix of quiescent and active cells, the latter with robust self-renewal and differentiation capacity characterize a young stem cell population. Young stem cells show balanced differentiation into lymphoid and myeloid lineages (a). It is hypothesized that an aged stem cell population has a higher rate of apoptosis due to acquired cellular damage. There are fewer quiescent cells in the stem cell reserve and the active stem cells have restricted lineage potential (b). When an old stem cell population comes under hematopoietic stress, it is proposed that the rate of apoptosis increases as quiescent cells unsuccessfully make the activation step, further depleting the quiescent reserves. Active stem cells not only are hampered by lineage restrictions, but the number of differentiated progeny each produces is diminished, leading to a slow and blunted recovery (c).

In the absence of definitive proof of the involvement of apoptosis in age-related changes in hematopoietic stem cells, Figure 1 proposes that aging, and especially the effects of replicative stress on an aged stem cell population, cause an increase in apoptosis. As stem cells that have accumulated intracellular damage as a result of the rigors of aging are activated in response to hematopoietic stress, it is hypothesized that surveillance pathways leading to apoptosis, lead to increases in apoptosis as compromised stem cells are removed from the pool and thus prevented from initiating a clone with potentially dysfunctional or tumorigenic progeny.

A further possible change during aging, or following repeated replicative stress, in the efficiency with which stem cells find their way to the proper marrow microenvironment, similarly has not been adequately addressed. There is evidence that the homing efficiency of old stem cells is considerably diminished compared to stem cells from young mice. The role, if any that this may play naturally in unmanipulated mice is uncertain. Small numbers of stem cells normally circulate in the blood, but whether there is a natural and continuous redistribution of stem cells between the marrow and the blood is not clear. When the circulatory systems of two mice, distinguishable by a cell surface marker (Ly5), were joined, stem cells from each parabiont quickly and extensively colonized the bone marrow of the partner. Aside from the minor surgery associated with the surgical conjoining of the mice and the behavioral adjustment having a conjoined twin, the mutual cross-engraftment took place without further manipulation. The result demonstrates the potential importance of stem cell migration in the normal physiology of hematopoiesis, and perhaps other continuously renewing tissues.

Stem cell plasticity, meet aging
Constitutive stem cell migration may involve not only hematopoietic stem cells, but stem cells of other organs. Accumulating evidence is consistent with (but has not yet risen to the level of proof of) the notion that stem cells found in some organs may colonize others and, most surprisingly, may sufficiently adjust developmental programs to produce differentiated cells characteristic of the new resident organ. At the risk of oversimplifying the aggregate findings, reciprocal transplantation variously between brain, bone marrow, liver, skeletal muscle, heart, skin, vascular endothelium, and intestinal lining has provided evidence that cells residing in these organs may be found in other organs long after transplant. The controversy surrounds the degree to which the transplanted cells adopt the functional identities of their new environments. Transplanted cells have altered their gene expression patterns to correspond with, and contribute to, new organ function. In some instances, donor cells make up a significant fraction of the organ and were quite likely generated by transplanted stem or progenitor cells. In other instances, donor cells were widely dispersed with little or no evidence of proliferation or of generating significant numbers of functional progeny. Recent extensive studies reporting stem cell plasticity showed purified murine hematopoietic stem cells to seed the liver and generate significant numbers of hepatocyte progeny; and in patients who have undergone allogeneic bone marrow transplant to treat hematologic malignancy, donor stem cells have colonized the brain and produced new neurons, particularly in the hippocampus and cerebral cortex. These two examples provide evidence of transengraftment and transdifferentiation between organs derived from all three embryonic germ layers. An alternative conceptual explanation is that generic, totipotent stem cells are found in all of these organs and, when transplanted, account for the apparent transdifferentiation of organotypic stem cells.

How might aging fit into this? Despite the apparently stunning new properties ascribed to stem cells depending on the microenvironment in which they reside, there is evidence in at least the hematopoietic system that stem cells progressively lose the breadth of their developmental potency (see Fig. 2). Thus, serially transplanted bone marrow stem cells rapidly lose the capacity to produce the normal spectrum and proportions of blood cell lineages. Stem cell differentiation in secondary transplants is skewed toward myeloid cells at the expense of T and B cell production. Similarly, and in direct support of an effect of age, a loss of lymphoid potency, and accentuated myeloid differentiation, was found for stem cells from aged mice. The capacity of human CD34+ cells to generate T cells in vitro was found to be diminished independently by age and exposure to prior chemotherapy, and synergistically in combination.

Fig. 2. Restriction in developmental potency of stem cells during aging. Cells of the inner cell mass of mammalian embryos have complete developmental potential (totipotency), through embryogenesis, to produce a viable and complete organism (a human in this example). Embryonic stem cell (ESC) lines derived from the inner cell mass retain totipotency and can be maintained in culture (a). Hematopoietic stem cells (HSC) in the bone marrow of young adults have a balanced mutlipotency to produce lymphoid (L) and myeloid (M) progeny in adequate numbers to fight infection, carry oxygen, and clot blood. Under certain experimental, and possibly physiological, conditions, stem cells in young bone marrow have expanded developmental potential to produce, for example, hepatocytes, muscle cells, endothelium, and neurons. Under no circumstances do these stem cells retain the potency to produce a new individual (b). In a continuum of restriction in developmental potential, stem cells derived from the bone marrow of the old may lose plasticity. Moreover, differentiation in hematopoietic lineages is skewed toward myeloid differentiation, at the expense of lymphopoiesis (c).

In aggregate, these results demonstrate that replicative stress, aging, and possibly the rigors of the transplant procedure itself are cumulative and quickly diminish the developmental potency of at least hematopoietic stem cells. There is a large literature on the effects of age on the immune system. Suffice it to say that diminished immune function with age is a hallmark that could clearly limit longevity. T cell function, including the arm of cellular immunity recognizing and destroying preneoplastic and frankly tumorigenic cells, is particularly affected. If aging diminishes the multilineage developmental potential of hematopoietic stem cells, what of stem cells found elsewhere in the body? And would it be too much of a stretch to predict that stem cell plasticity would be diminished during aging? Information is lacking on the effects of age on stem cell plasticity, although it could and should be experimentally tested.

Whatever plasticity a stem cell possesses is extant when young and diminishes, or is lost, with age (Fig. 2). The continuous circulation of either totipotent stem cells or "plastic" stem cells (the difference begs distinction) in the young may provide a homeostatic means of meeting stem cell crises in one or another organ in response to stress. During aging, this mode of mutual stem cell replenishment among organs may be lost and, by necessity, individual organs may be forced to become self-reliant on their own stem cell reserves. If an organ-specific stressor depletes those, it may become the weak link in the chain of physiological fitness and threaten the longevity of the organism.