Research into myocardial regeneration has an exciting future, shown by the results of experimental and clinical work challenging the dogma that the heart is a post mitotic non-regenerating organ. Such studies have initiated a lively debate about the feasibility of novel treatment approaches leading to the recovery of damaged myocardial tissue. The possibility of reconstituting dead myocardium by endogenous cardiomyocyte replication, transplantation, or activation of stem cells-or even cloning of an artificial heart-is being advanced, and will be a major subject of future research. Although health expenditure for heart failure in the industrial world is high, we are still a long way from being able to treat the cause of reduced myocardial contractility. Despite the hopes of some people, conventional treatment for heart failure does not achieve myocardial regeneration.
Let us assume that patient has structural heart disease with current or previous symptoms. He is a modern individual, who is well informed about not only conventional treatment, but also novel alternative options; he will ask for advice about the best choices for him now and in the near future. What options has a treating doctor? There are four choices:
(1) Conventional treatment;
(2) Induction of endogenous cardiomyocyte replication;
(3) The cloning of artifical organs;
(4) Application of stem cell biology – a modern method we will discuss in this review.

So called "flatworm approach": organ regeneration via stem cells
The patient is suggested to undergo bone marrow aspiration. The autologous mononuclear bone marrow cells would be enriched by flow cytometry with specific antibodies with which the cells of interest could be labelled in vitro. These cells would eventually be reinjected into his myocardium through the formerly occluded coronary artery. Cells could be reintroduced via a coronary catheter, or with a transendocardial approach via a specific injection device, or directly into the myocardium during bypass surgery. The cells would repopulate the dead scar region and transdifferentiate into vascular smooth muscle cells, endothelial cells, and cardiomyocytes (Figure 1).

Rationale
In 1998, Thomson and colleagues were the first group to clone human embryonic stem cells. These cells exhibit a multilineage differentiation potential giving rise to endodermal, exodermal, and mesodermal cell offspring. Shortly thereafter, researches showed that human embryonic stem cells are able to differentiate into cells with the structural and functional properties of cardiomyocytes. However, although there is no doubt that, unlike other cell types, human embryonic stem cells have the potential for unrestricted lineage differentiation, and, therefore, unlimited usefulness for medical research, the use of allogenic embryonic stem cells in patients raises serious ethical concerns. Thus, the search for alternative cell populations as sources for tissue regeneration has led to the discovery that murine, as well as human adults have mesenchymal stem cells in their bone marrow that have multilineage potential distinct from the expected haemopoietic differentiation. In theory, the described plasticity of embryonic or adult stem cells could be attributable to a heterogeneous population of the cell preparations used; apparent plasticity could be explained by selection for subpopulations already committed to differentiation to specific phenotypes. However, a plethora of studies within the past 6 years provided clear evidence that not only bone marrow, but also many other organs have pluripotent stem cells that can differentiate into multiple organ types. The label "stem cell" may represent a function rather than an entity of a cell. However, this idea has been undermined by recent studies that did not identify stem cell plasticity, as was shown for haemopoietic stem cells. Other investigators suggest cell nuclear fusion rather than cellular differentiation as a possible explanation for apparent stem cell plasticity. It was shown that cardiomyocytes could be grown in vitro from murine stromal bone-marrow cells, which shows that cells derived from bone marrow do have the potential to differentiate into cardiomyocytes in vitro. Moreover, in-vivo studies that used different injection protocols or cytokine-mediated mobilisation of systemic bone-marrow cells in a murine myocardial infarct model provided evidence for the potential of transdifferentiation of marrow cells into cardiac cells that can contribute to the generation of new vessels and contractile-muscle tissue. Finally, human mesenchymal stem cells have the propensity to differentiate into vascular cells or cardiomyocytes when injected into the adult murine heart.

Fig. 1. Bone marrow stem cells in myocardial regeneration
The extent of the transdifferentiation potential and its longevity are not yet known. This approach is not hampered by ethical constraints and, therefore, will be in the focus of regenerative medicine in the next few years. However, compared with embryonic cells, adult progenitor cells seem to have much reduced differentiation plasticity.

One of the reasons for controversy about the role of stem cells in myocardial regeneration is the absence of evidence for the origin of stem cells. Evolving technologies, such as the ex vivo magnetic labelling of stem cells with the subsequent detection via MRI, might help. Recently, this approach was shown to be useful for the detection of mesenchymal stem cells injected into the infarcted myocardium of pigs. One of the key issues in this respect is whether progenitor cells have entered their differentiation process before homing to the heart, or whether they start their differentiation process after nesting into cardiac tissue. If so, what factor or factors trigger cardiac-specific differentiation?
In theory, an unselected differentiation process could be started in progenitor cells as soon as they enter the systemic circulation, and those cells that eventually repopulate cardiac tissue could home in on the myocardium just because they have begun the cardiac differentiation process well before they reach their destiny. Two groups of investigators have noted evidence for the setting as the trigger that incites progenitor cells to a specific transdifferentiation, even if they had already begun another cell type differentiation. In these studies, murine or human endothelial progenitor cells were shown to transdifferentiate into a cardiomyocyte phenotype when cocultured with isolated cardiomyocytes.
Is there evidence for the concept that progenitor cells migrate into the myocardium and regenerate damaged cardiac tissue in humans? A prudent setting to test this hypothesis is sex-mismatched cardiac transplantation in humans; that is, when a female heart is transplanted into a male host. The colonisation and differentiation of host cells homed to the transplanted heart can be identified by the presence of the Y chromosome in the host-derived cells (cardiac chimerism). Several groups of workers have tested the hypothesis using this model. Most probably because of the variation between microscopic techniques and the large differences in time between transplantation and sampling, the number of cardiomyocytes identified as being differentiated host progenitor-cells vary enormously between studies-from 0% to 30%. What holds true for cardiomyocytes seems to be the case with vascular smooth-muscle cells, as transplant vasculopathy has been shown to involve the homing of host progenitor cells within the vascular smooth muscles of the donor organ. In summary, although controversy remains about the extent to which it takes place, there seems to be little doubt that progenitor cells can home to myocardial tissue and then differentiate to cardiocytes (myocytes, smooth muscle cells, or endothelial cells) in human beings.
Despite uncertainties, several groups have started to investigate the safety and effectiveness of bone-marrow transplantation in the human heart. Strauer and colleagues were the first to show the feasibility of the intracoronary injection of autologous mononuclear bone marrow cells, first in a case report and subsequently in a trial in 20 patients with acute myocardial infarction, 10 of whom received intracoronary injection of bone-marrow cells. All patients underwent cardiac catheterisation with successful reopening of the infarctrelated artery in the post-acute phase (within a mean of 12 h of symptom onset). Following a study that had shown the feasibility of bone-marrow stem-cell transplantation to the myocardium in an animal model, enriched mononuclear bone-marrow cells derived from bone-marrow aspiration done several days after the infarction were injected via the infarct-related artery 5-9 days after the vessel had been reopened. The investigators reported no serious side-effects and could even show functional improvement at 3 months' followup, as assessed by several invasive and non-invasive procedures. Other group also reported similar results with six patients who had had a myocardial infarction 10 days to 3 months before receiving autologous bone-marrow cells (enriched for AC133+ cells) during the course of a subsequent coronary artery bypass surgery via direct intracardiac injection in the infarct border zone. Although these were not randomised placebo-controlled trials, their results do provide preliminary evidence for the feasibility and shortterm safety of bone-marrow stem-cell transplantation in the heart for patients with subacute myocardial infarction.
The intracardiac injection of bone-marrow cells has promise not only for patients shortly after myocardial infarction, but also for patients with chronic congestive heart failure. Researches did a study in which eight patients with chronic ischaemic heart failure received catheter-based transendocardial injections of autologous bone-marrow cells guided to the ischaemic areas via an electromechanical mapping system. Similar results were noted in a larger trial that enrolled 21 patients with end-stage ischaemic heart disease. Alternatively, autologous skeletal myoblast grafts carrying many muscle satellite cells were successfully injected into the myocardium during bypass surgery in a case report and in a trial. Of note is that although these patients had improved myocardial performance, the one group of workers reported an increased arrhythmogenic potential, necessitating the implantation of internal defibrillators in four of ten patients.
Collectively, these reports suggest that further larger randomised placebo-controlled double-blind studies with mechanistic and clinical endpoints are warranted. Furthermore, they provide the first preliminary evidence that injection of autologous bone-marrow derived mononuclear cells in the human heart is feasible. Whether this approach is safe in the long term and whether substantial improvement of function and clinical symptoms will ensue remain to be determined.
A recent report of clusters of satellite cells in human myocardium – ie, cells that amplify and commit to the myocyte lineage in response to increased workload – raises the hope for alternative interventions to reactivate endogenous myocardial sources for tissue repair.

Limitations
First, we do not know the efficacy of this approach because there are no large controlled studies. Second, it is unclear whether a potential functional improvement is caused by increased angiogenesis, cardiogenesis, or neither. Third, we know little about longevity of patients with this approach. The need for randomised controlled trials is evident, before more patients are exposed to the unknown risks of cell transplantation, these trials, including early risk assessment, should be initiated.

Cautious optimism
A new field in cardiovascular medicine has opened up. However, we should beware of unrealistic expectations lest the unfortunate events that arose around the advent of gene therapy be repeated. The rapid rise in the number of studies in this area of inquiry, which sometimes provide us with incredible results, should not prevent a healthy scepticism. Rather, cautious optimism should lead us into the exciting era of cellular therapy for heart failure.
However, on the way, we should do some homework. Several questions need to be addressed.
What are the factors controlling cardiomyocyte cell-cycle withdrawal?
Are stem-like cells present in all tissues?
What triggers a bone-marrow cell to migrate and home in on the heart?
What are the trophic factors responsible for the differentiation of a progenitor cell into a cardiomyocyte?
Are cellular transdifferentiation capacity and plasticity physiological aspects of tissue repair throughout life?
Which approach is better-endogenous (cardiomyocyte replication) or exogenous (transdifferentiation of progenitor cells) regeneration?
Do we really need embryonic stem cells after so many promising data lending support to the use of adult stem cells?
Will cellular therapy make conventional drug therapy redundant?