Regeneration of ischemic cardiac muscle and vascular endothelium by adult stem cells

High frequency of chimerism in transplanted livers

Recent studies have shown that primitive stem cells can mobilize and differentiate into hepatocytes. We investigated the time and extent in which cells of recipient origins could differentiate into hepatocytes and other cells in human liver allografts. Microsatellite analysis, which can assess quantitatively the proportions of recipient and donor DNA, was performed in posttransplantation liver biopsy specimens from 17 patients at various times. Combined fluorescence in situ hybridization (FISH) for Y chromosome and immunofluorescence for different cell types was also performed in 10 of these cases with sex mismatch. Organ chimerism in the transplanted livers was found to be of variable extent, and the recipients' DNA in the posttransplantation liver biopsy specimens (excluding portal tracts) amounted up to 50%. The recipient DNA in the posttransplantation liver biopsy specimens increased after liver transplantation by as early as 1 week, peaked at around 30 to 40 weeks, and could be shown 63 weeks after transplantation. Most (64%-75%) of the recipient-derived cells showed macrophage/Kupffer cell differentiation. Only up to 1.6% of the recipient-derived cells in the liver grafts showed hepatocytic differentiation in the liver grafts and made up 0.62% of all hepatocytes of both donor and recipient origins. These livers had mild or minimal injury histologically. In conclusion, our results show that most of the recipient-derived cells in the liver allografts were macrophages/Kupffer cells and only a small proportion of hepatocytes was recipient derived. However, with regard to recipient-derived hepatocytes, our data cannot distinguish between transdifferentiation and cell fusion.

Little evidence of bone marrow-derived hepatocytes in the replacement of injured liver

We have tested the ability of bone marrow (BM) cells (BMCs) to form hepatocytes in liver injury models. We used three models: (i) carbon tetrachloride (CCl4) treatment, (ii) albumin-urokinase transgenic mouse [TgN(Alb1Plau)], and (iii) hepatitis B transgenic mouse [TgN(Alb1HBV)]. As a nonselective liver injury model, irradiated C57BL/6 (B6) mice were transplanted with BMCs from GFP transgenic mouse [TgN(ActbEGFP)] or beta-galactosidase transgenic mouse [TgN(MtnLacZ)] followed by the administration of CCl4. Irradiated TgN(Alb1HBV) and TgN(Alb1Plau) were also transplanted with BMCs from TgN(ActbEGFP) or TgN(MtnLacZ). Approximately 1.5 x 106 hepatocytes per liver were analyzed for GFP-positive cells, and the whole livers were inspected for beta-galactosidase expression. No GFP-positive hepatocytes and no gross blue staining of the livers with 5-bromo-4-chloro-3-indolyl beta-d-galactoside in any of the 18 recipient mice analyzed were detected. The livers from female animals with gender-mismatched BM transplantation were also tested with Y chromosome fluorescent in situ hybridization analysis to detect donor-derived cells. A total of five isolated hepatocytes were positive for Y chromosome in 4.1 x 105 hepatocytes analyzed. Our results demonstrate that there is little or no contribution of BMCs to the replacement of injured livers in these models. We conclude that BM-derived cells cannot generally lead to a cure of liver damage.

Neuroectodermal differentiation from mouse multipotent adult progenitor cells

We recently showed that a rare cell from murine bone marrow, which we termed multipotent adult progenitor cells (MAPCs), can be expanded for >120 population doublings. Mouse (m)MAPCs differentiate into mesenchymal lineage cells as well as endothelium and endoderm, and, when injected in the blastocyst, mMAPCs contribute to most if not all somatic cell lineages including the different cell types of the brain. Our results, reported herein, demonstrate that mMAPCs can also be induced to differentiate into cells having anatomical and electrophysiological characteristics similar to those of midbrain neurons. Differentiation to a neuronal phenotype was achieved by coculturing mMAPCs with astrocytes, suggesting that neuronal differentiation may require astrocyte-derived factors similar to what is required for the differentiation of embryonic stem cells and neural stem cells to neurons. Differentiation of mMAPCs to neuron-like cells follows similar developmental steps as described for embryonic stem cells and neural stem cells. MAPCs therefore may constitute a source of cells for treatment of central nervous system disorders.

Clinical and experimental study on therapeutic effect of umbilical cord blood transplantation on severe viral hepatitis

AIM: To investigate the therapeutic effect of umbilical cord blood transplantation (UCBT) on patients with severe viral hepatitis and on liver lesions in rats.

METHODS: One hundred and fifty three patients with severe viral hepatitis were included in the study between April 1990 and July 2002. The patients were treated with adult plasma transfusion (control), UCBT, plasma exchange (PE) and UCBT combined with PE (UCBT+PE) respectively. The therapeutic effectiveness was evaluated by serial determinations of liver function, lipids and immune function in all patients before and after the treatment. The model of experimental hepatic failure was constructed in SD rats by injecting carbon tetrachloride. Then, the rats were given normal saline, adult plasma or neonate cord blood intraperitoneally. After detection of liver function, the rats were killed and morphological changes of the liver were microscopically observed.

RESULTS: UCBT group and UCBT+PE group had much better improvement in liver and immune functions than control group and PE group. The patients in UCBT+PE group had the best clinical efficacy. UCBT was safe and had no side effects. The animal experiment showed significant improvements in liver function and survival rate in neonate cord blood group as compared with adult plasma group. The histopathology of rat’s liver indicated that neonate cord blood application could decrease the liver injury and increase hepatocellular regeneration.

CONCLUSION: UCBT demonstrated a good therapeutic effect on severe viral hepatitis and no obvious side effects. Umbilical cord blood can attenuate the liver lesions and reproduce hepatocyte. The treatment of UCBT combined with PE was much better than that of single plasma exchange, thus UCBT can enhance the therapeutic effect of plasma exchange on severe viral hepatitis.

Mesenchymal progenitor cells differentiate into an endothelial phenotype, enhance vascular density, and improve heart function in a rat cellular cardiomyoplasty model

BACKGROUND

Cellular cardiomyoplasty is a promising approach to improve postinfarcted cardiac function. The differentiation pathways of engrafted mesenchymal progenitor cells (MPCs) and their effects on the left ventricular function in a rat myocardial infarct heart model were analyzed.

METHODS AND RESULTS

A ligation model of left coronary artery of Lewis rats was used. MPCs were isolated by bone marrow cell adherence. Seven days after ligation, MPCs labeled with 4',6-diamidino-2'-phenylindole were injected into the infarcted myocardium (n=8). Culture medium was injected in the infarcted myocardium of control animals (n=8). Thirty days after implantation, immunofluorescence studies revealed some engrafted cells expressing a smooth muscle phenotype (alpha SM actin+), as similarly observed in culture. Other engrafted cells lost their smooth muscle phenotype and acquired an endothelial phenotype (CD31+). Furthermore, vessel density was augmented in the MPC group in comparison with the control group. After 30 days, echocardiography showed an improvement on left ventricular performance in the MPCs compared with the control group.

CONCLUSIONS

In vivo administration of syngenic MPCs into a rat model of myocardial infarcted heart was safety demonstrated. Some engrafted cells appeared to differentiate into endothelial cells and loss their smooth muscle phenotype. MPC engraftment might to contribute to the improvement on the cardiac function in such a setting.

Transplantation of autologous fresh bone marrow into infarcted myocardium: a word of caution

BACKGROUND

As the benefits of extemporaneous transplantation (Tx) of fresh (unfractionated) autologous bone marrow (BM) have been primarily studied in the setting of acute myocardial infarction, we assessed whether this approach could be effective for regenerating chronically infarcted myocardium.

METHODS AND RESULTS

Myocardial infarction was created in 18 sheep by ligation of circumflex arterial branches. Three weeks later, BM was aspirated from the iliac crest, washed, labeled with the fluorescent dye Dil and reinjected (mean: 422 x 10(6) cells in 3 mL) in 10 sites across the infarcted area through the reopened thoracotomy (n=9). Nine controls received culture medium. Left ventricular (LV) function was assessed before and 2 months after Tx by two-dimensional echocardiography whereas transmural velocity gradients were measured using M-mode tissue Doppler imaging at the center of the infarcted/grafted area. Formalin-fixed hearts were processed for the detection of grafted cells and angiogenesis. LV ejection fraction deteriorated similarly in the Tx and control groups (from 42+/-5% to 30+/-4% and from 40+/-4% to 31+/-1%, respectively; P=0.86). Likewise, BM Tx failed to prevent LV dilatation and impairment of the global wall motion score. The decrease in regional systolic velocity gradients (s(-1)) featured a similar pattern (Tx group: from 0.77+/-0.11 to 0.31+/-0.07; control group: from 0.73+/-0.10 to 0.50+/-0.07; P=0.06). Histologically, there was neither BM tissue engraftment, except for a few scattered Dil-positive macrophages in the infarcted fibrotic areas nor transdifferentiation of BM cells into endothelial cells.

CONCLUSIONS

These data caution against the functional efficacy of extemporaneous Tx of fresh unfractionated BM into postinfarction scars.

Myoendothelial differentiation of human umbilical cord blood-derived stem cells in ischemic limb tissues

Human umbilical cord blood (UCB) contains high numbers of endothelial progenitors cells (EPCs) characterized by coexpression of CD34 and CD133 markers. Prior studies have shown that CD34+/CD133+ EPCs from the cord or peripheral blood (PB) can give rise to endothelial cells and induce angiogenesis in ischemic tissues. In the present study, it is shown that freshly isolated human cord blood CD34+ cells injected into ischemic adductor muscles gave rise to endothelial and, unexpectedly, to skeletal muscle cells in mice. In fact, the treated limbs exhibited enhanced arteriole length density and regenerating muscle fiber density. Under similar experimental conditions, CD34- cells did not enhance the formation of new arterioles and regenerating muscle fibers. In nonischemic limbs CD34+ cells increased arteriole length density but did not promote formation of new muscle fibers. Endothelial and myogenic differentiation ability was maintained in CD34+ cells after ex vivo expansion. Myogenic conversion of human cord blood CD34+ cells was also observed in vitro by coculture onto mouse myoblasts. These results show that human cord blood CD34+ cells differentiate into endothelial and skeletal muscle cells, thus providing an indication of human EPCs plasticity. The full text of this article is available online at http://www.circresaha.org.

Reprogramming immune responses: enabling cellular therapies and regenerative medicine

Recent advances in cellular therapies have led to the emergence of a multidisciplinary scientific approach to developing therapeutics for a wide variety of diseases and genetic disorders. Although most cell-based therapies currently consist of heterogeneous cell populations, it is anticipated that the standard of care will eventually be well-characterized stem cell lines that can be modified to meet the individual needs of the patient. Many challenges have to be overcome, however, before such "designer cells" can become a clinical reality. One of the major hurdles will be to prevent immune rejection of the therapeutic cells. A patient's immune system may react to genetically modified or allogeneic cells as foreign, leading to their destruction. We propose that specific reprogramming of the immune system to accept cellular therapies can be accomplished by establishing hematopoietic chimerism. Successful engraftment of hematopoietic stem cells (HSCs), which have the same origin as those cells intended for therapeutic use, should lead to a re-education of the immune system so that the donor cells are recognized as self and will not be rejected. Developing safe, nontoxic protocols for reprogramming the immune system is critical to the success of this approach. Two major requirements exist for achieving stable HSC engraftment: (A) depletion or displacement of host stem cells, and (B) adequate immune suppression. Available data indicate that an agent such as busulfan is effective in depleting stem cells and that immune suppression can be accomplished with monoclonal antibodies that specifically target immune-reactive cells in the periphery.

Mesenchymal stem cells modified with Akt prevent remodeling and restore performance of infarcted hearts

Transplantation of adult bone marrow-derived mesenchymal stem cells has been proposed as a strategy for cardiac repair following myocardial damage. However, poor cell viability associated with transplantation has limited the reparative capacity of these cells in vivo. In this study, we genetically engineered rat mesenchymal stem cells using ex vivo retroviral transduction to overexpress the prosurvival gene Akt1 (encoding the Akt protein). Transplantation of 5 x 10(6) cells overexpressing Akt into the ischemic rat myocardium inhibited the process of cardiac remodeling by reducing intramyocardial inflammation, collagen deposition and cardiac myocyte hypertrophy, regenerated 80-90% of lost myocardial volume, and completely normalized systolic and diastolic cardiac function. These observed effects were dose (cell number) dependent. Mesenchymal stem cells transduced with Akt1 restored fourfold greater myocardial volume than equal numbers of cells transduced with the reporter gene lacZ. Thus, mesenchymal stem cells genetically enhanced with Akt1 can repair infarcted myocardium, prevent remodeling and nearly normalize cardiac performance.

Hematopoietic cells from bone marrow have the potential to differentiate into cardiomyocytes in vitro

Recent studies have indicated that hematopoietic progenitor cells (HPCs) have the capacity to form cardiomyocytes. In the present study, we further examined the cardiac competence of HPCs by asking whether these cells by themselves can be provoked to undergo cardiac differentiation. Our data indicate that in response to growth factor treatment, HPCs from avian bone marrow (BM) can undergo cardiac differentiation, as indicated by their expression of multiple cardiac transcription factors and sarcomeric proteins. Furthermore, coculture experiments with adult mouse BM cells and embryonic heart tissue confirmed that HPCs are able to both integrate into cardiac tissue and differentiate into cardiomyocytes. In an additional set of experiments, we investigated whether other hematopoietic populations might possess cardiac potential by examining whether blood cells that normally are recruited to damaged tissue might act as a source of newly generated cardiomyocytes. Remarkably, macrophages cocultured with cardiac explants also demonstrated an ability to integrate into contractile heart tissue and undergo cardiac differentiation. Thus, our data suggest that the capacity of blood cells to transdifferentiate into cardiomyocytes is not limited to classically defined hematopoietic progenitors.

Hope for a Broken Heart?

Heated debate has surrounded the issue of whether adult stem cells can differentiate into cardiac myocytes and contribute to the function of the heart. In this issue of Cell, demonstrate stem cells in the adult rat heart that differentiate into cardiac myocytes in vitro and, when injected into the adult rat heart, can reconstitute the injured myocardium and improve function. These findings should weigh heavily in future debates about the existence of stem cells in the adult heart and their capacity for functional repair after injury.

Cardiomyocyte regeneration from circulating bone marrow cells in mice

We investigated the role of circulating bone marrow cells (BMC) in cardiomyocyte regeneration. BMC, isolated from transgenic mice expressing enhanced green fluorescent protein (GFP), were transplanted into lethally irradiated C57BL6 mice. Five weeks after bone marrow transplantation (BMT), flow cytometric analysis for GFP-positive cells confirmed reconstitution of transplanted bone marrow. Bone marrow transplant mice subsequently underwent left coronary artery ligation (myocardial infarction) or sham-operation, and were killed at 1 mo or 3 mo after operation. Infarct size was similar in bone marrow transplant mice at 1 mo (47.1 +/- 5.9%) and at 3 mo (45.3 +/- 7.8%), and echocardiography at 2 and 8 wk revealed decreasing left ventricular function. In infarcted heart, GFP-positive cells that expressed desmin and troponin T-C were identified by confocal microscopy. GFP and troponin T-C double-positive cells were predominantly in the peri-infarcted region (1 mo, 365 +/- 45 cells/50 sections; 3 mo: 458 +/- 100 cells/50 sections; p < 0.05 versus noninfarct, infarct, and sham-operated regions). Furthermore, BMC mobilization and differentiation into cardiomyocytes was found to be complete within 1 mo after myocardial infarction. These results demonstrate that circulating BMC undergo mobilization and differentiation in cardiac cells after myocardial infarction. Future studies are required to determine the molecular signaling mechanisms responsible for this phenomenon.

Liver regeneration in a retrorsine/CCl4-induced acute liver failure model: do bone marrow-derived cells contribute?

BACKGROUND/AIMS

Adult bone marrow contains progenitors capable of generating hepatocytes. Here a new liver failure model is introduced to assess whether bone marrow-derived progeny contribute to liver regeneration after acute hepatotoxic liver failure.

METHODS

Retrorsine was used to inhibit endogenous hepatocyte proliferation, before inducing acute liver failure by carbon tetrachloride. Bone marrow chimeras were generated before inducing liver failure to trace bone marrow-derived cells. Therefore, CD45 and major histocompatibility complex (MHC) class I dimorphic rat models were applied.

RESULTS

Early after acute liver failure a multilineage inflammatory infiltrate was observed, mainly consisting of granulocytes. In long-term experiments small numbers of CD90+/CD45- cells of donor origin occurred in clusters associated with portal triads. Bone marrow cell infusion was not able to enhance liver regeneration. Cellular hypertrophy was the predominant way of liver mass regeneration in models applying retrorsine.

CONCLUSIONS

Retrorsine pretreatment did not affect sensitivity for carbon tetrachloride. A multilineage inflammatory infiltrate was observed in rats whether pretreated with retrorsine or not. Few donor cells co-expressing CD90 (THY 1) were present in recipient livers, which may resemble donor-derived hematopoietic progenitors or oval cells. No other donor cells within liver parenchyma were detected. This is in contrast to other cell infusion models of acute cell death.

Treatment of myocardial ischemia with bone marrow-derived mesenchymal stem cells overexpressing hepatocyte growth factor

Mesenchymal stem cells could differentiate into cardiomyocytes in vitro and have been shown to reconstitute the impaired myocardium in vivo. Hepatocyte growth factor, a recognized angiogenic factor and endothelial cell chemoattractant, has been applied in the treatment of myocardial ischemia. In this study, we used a ligation model of proximal left anterior descending coronary artery of rats to evaluate the effect of mesenchymal stem cells overexpressing hepatocyte growth factor in the treatment of myocardial ischemia. Bone marrow-derived mesenchymal stem cells were isolated, expanded, characterized, and infected with adenovirus carrying human hepatocyte growth factor cDNA (Ad-HGF). Mesenchymal stem cells infected by Ad-HGF released soluble HGF protein at a high level, which was maintained at least for 2 weeks. Implantation of mesenchymal stem cells overexpressing hepatocyte growth factor into left anterior descending risk areas improved the functions of impaired myocardium, including diminishing the area of ischemia, increasing the number of capillaries, and reducing collagen content. By using the sry gene as a marker, we also demonstrated that the engrafted cells or their progeny incorporated into ischemic cardiac muscle. These results showed that treatment of myocardial ischemia with bone marrow-derived mesenchymal stem cells overexpressing hepatocyte growth factor could be a novel strategy that can both restore local blood flow and regenerate lost cardiomyocytes.

Effect of stromal-cell-derived factor 1 on stem-cell homing and tissue regeneration in ischaemic cardiomyopathy

BACKGROUND

Myocardial regeneration via stem-cell mobilisation at the time of myocardial infarction is known to occur, although the mechanism for stem-cell homing to infarcted tissue subsequently and whether this approach can be used for treatment of ischaemic cardiomyopathy are unknown. We investigated these issues in a Lewis rat model (ligation of the left anterior descending artery) of ischaemic cardiomyopathy.

METHODS

We studied the effects of stem-cell mobilisation by use of granulocyte colony-stimulating factor (filgrastim) with or without transplantation of syngeneic cells. Shortening fraction and myocardial strain by tissue doppler imaging were quantified by echocardiography.

FINDINGS

Stem-cell mobilisation with filgrastim alone did not lead to engraftment of bone-marrow-derived cells. Stromal-cell-derived factor 1 (SDF-1), required for stem-cell homing to bone marrow, was upregulated immediately after myocardial infarction and downregulated within 7 days. 8 weeks after myocardial infarction, transplantation into the peri-infarct zone of syngeneic cardiac fibroblasts stably transfected to express SDF-1 induced homing of CD117-positive stem cells to injured myocardium after filgrastim administration (control vs SDF-1-expressing cardiac fibroblasts mean 7.2 [SD 3.4] vs 33.2 [6.0] cells/mm2, n=4 per group, p<0.02) resulting in greater left-ventricular mass (1.24 [0.29] vs 1.57 [0.27] g) and better cardiac function (shortening fraction 9.2 [4.9] vs 17.2 [4.2]%, n=8 per group, p<0.05).

INTERPRETATION

These findings show that SDF-1 is sufficient to induce therapeutic stem-cell homing to injured myocardium and suggest a strategy for directed stem-cell engraftment into injured tissues. Our findings also indicate that therapeutic strategies focused on stem-cell mobilisation for regeneration of myocardial tissue must be initiated within days of myocardial infarction unless signalling for stem-cell homing is re-established.

Biology of hematopoietic stem cells and progenitors: implications for clinical application

Stem cell biology is scientifically, clinically, and politically a current topic. The hematopoietic stem cell, the common ancestor of all types of blood cells, is one of the best-characterized stem cells in the body and the only stem cell that is clinically applied in the treatment of diseases such as breast cancer, leukemias, and congenital immunodeficiencies. Multicolor cell sorting enables the purification not only of hematopoietic stem cells, but also of their downstream progenitors such as common lymphoid progenitors and common myeloid progenitors. Recent genetic approaches including gene chip technology have been used to elucidate the gene expression profile of hematopoietic stem cells and other progenitors. Although the mechanisms that control self-renewal and lineage commitment of hematopoietic stem cells are still ambiguous, recent rapid advances in understanding the biological nature of hematopoietic stem and progenitor cells have broadened the potential application of these cells in the treatment of diseases.

Circulating vascular progenitor cells do not contribute to compensatory lung growth

The biological principles that underlie the induction and process of alveolization in the lung as well as the maintenance of the complex lung tissue structure are one of the major obstacles in pulmonary medicine today. Bone marrow-derived cells have been shown to participate in angiogenesis, vascular repair, and remodeling of various organs. We addressed this phenomenon in the lung vasculature of mice in a model of regenerative lung growth. C57BL/6 mice were transplanted with bone marrow from one of three different reporter gene-transgenic strains. flk-1+/lacZ mice, tie-2/lacZ transgenic mice (both exhibiting endothelial cell-specific reporter gene expression), and ubiquitously enhanced green fluorescent protein (eGFP)-expressing mice served as marrow donors. After hematopoietic recovery, compensatory lung growth was induced by unilateral pneumonectomy and led to complete restoration of initial lung volume and surface area. The lungs were consecutively investigated for bone marrow-derived vascular cells by lacZ staining and immunohistochemistry for phenotype identification of vascular cells. lacZ- or eGFP-expressing bone marrow-derived endothelial cells could not be found in microvascular regions of alveolar septa. Single eGFP-positive endothelial cells were detected in pulmonary arteries at very low frequencies, whereas no eGFP-positive vascular smooth muscle cells were observed. In conclusion, we demonstrate in a model of lung growth and alveolization in adult mice the absence of significant bone marrow-derived progenitor cell contribution to the concomitant vascular growth and remodeling processes.

Practical Modeling Concepts for Connective Tissue Stem Cell and Progenitor Compartment Kinetics

Stem cell activation and development is central to skeletal development, maintenance, and repair, as it is for all tissues. However, an integrated model of stem cell proliferation, differentiation, and transit between functional compartments has yet to evolve. In this paper, the authors review current concepts in stem cell biology and progenitor cell growth and differentiation kinetics in the context of bone formation. A cell-based modeling strategy is developed and offered as a tool for conceptual and quantitative exploration of the key kinetic variables and possible organizational hierarchies in bone tissue development and remodeling, as well as in tissue engineering strategies for bone repair.

Systemic delivery of bone marrow-derived mesenchymal stem cells to the infarcted myocardium: feasibility, cell migration, and body distribution

BACKGROUND

Systemic delivery of bone marrow-derived mesenchymal stem cells (BM-MSCs) is an attractive approach for myocardial repair. We aimed to test this strategy in a rat model after myocardial infarction (MI).

METHODS AND RESULTS

BM-MSCs were obtained from rat bone marrow, expanded in vitro to a purity of >50%, and labeled with 99mTc exametazime, fluorescent dye, LacZ marker gene, or bromodeoxyuridine. Rats were subjected to MI by transient coronary artery occlusion or to sham MI. 99mTc-labeled cells (4x10(6)) were transfused into the left ventricular cavity of MI rats either at 2 or 10 to 14 days after MI and were compared with sham-MI rats or MI rats treated with intravenous infusion. Gamma camera imaging and isolated organ counting 4 hours after intravenous infusion revealed uptake of the 99mTc-labeled cells mainly in the lungs, with significantly smaller amounts in the liver, heart, and spleen. Delivery by left ventricular cavity infusion resulted in drastically lower lung uptake, better uptake in the heart, and specifically higher uptake in infarcted compared with sham-MI hearts. Histological examination at 1 week after infusion identified labeled cells either in the infarcted or border zone but not in remote viable myocardium or sham-MI hearts. Labeled cells were also identified in the lung, liver, spleen, and bone marrow.

CONCLUSIONS

Systemic intravenous delivery of BM-MSCs to rats after MI, although feasible, is limited by entrapment of the donor cells in the lungs. Direct left ventricular cavity infusion enhances migration and colonization of the cells preferentially to the ischemic myocardium.

Characterization of mesenchymal stem cells isolated from murine bone marrow by negative selection

Mesenchymal stem cells (MSCs) are typically enriched from bone marrow via isolation of the plastic adherent, fibroblastoid cell fraction. However, plastic adherent cultures elaborated from murine bone marrow are an admixture of fibroblastoid and hematopoietic cell types. Here we report a reliable method based on immunodepletion to fractionate fibroblastoid cells from hematopoietic cells within plastic adherent murine marrow cultures. The immunodepleted cells expressed the antigens Sca-1, CD29, CD44, CD81, CD106, and the stem cell marker nucleostemin (NST) but not CD11b, CD31, CD34, CD45, CD48, CD90, CD117, CD135, or the transcription factor Oct-4. They were also capable of differentiating into adipocytes, chondrocytes, and osteoblasts in vitro as well as osteoblasts/osteocytes in vivo. Therefore, immunodepletion yields a cell population devoid of hematopoietic and endothelial cells that is phenotypically and functionally equivalent to MSCs. The immunodepleted cells exhibited a population doubling time of approximately 5-7 days in culture. Poor growth was due to the dramatic down regulation of many genes involved in cell proliferation and cell cycle progression as a result of immunodepletion. Exposure of immunodepleted cells to fibroblast growth factor 2 (FGF2) but not insulin-like growth factor (IGF), murine stem cell factor, or leukemia inhibitory factor (LIF) significantly increased their growth rate. Moreover, 82% of the transcripts down regulated by immunodepletion remain unaltered in the presence of FGF2. Exposure to the later also reversibly inhibited the ability of the immunodepleted cells to differentiate into adipocytes, chondrocytes, and osteoblasts in vitro. Therefore, FGF2 appears to function as a mitogen and self-maintenance factor for murine MSCs enriched from bone marrow by negative selection.

Neuronal Stem Cells Biology and Plasticity

Recent studies have suggested that stem cells are able to cross primordial tissue barriers. Their ability to respond to unrelated microenvironmental signals strongly suggest that they have greater potential than previously imagined especially for their future clinical use for the regeneration of tissues or even perhaps organ systems. In particular there is an intriguing reciprocal relationship between the hematopoietic and neuronal stem cell systems. Both stem cell pools appear to respond to microenvironmental signals that are not developmental related. These intriguing observations have led to a number of studies that have attempted to explore this apparent phenomenon of plasticity associated with both hematopoietic and neuronal stem cell populations.

Molecular biology of hematopoietic stem cells

Human CD34+ hematopoietic stem and progenitor cells are capable of maintaining a life-long supply of the entire spectrum of blood cells dependent on systemic needs. Recent studies suggest that hematopoietic stem cells are, beyond their hematopoietic potential, able to differentiate into nonhematopoietic cell types, which could open novel avenues in the field of cellular therapy. Here, we concentrate on the molecular biology underlying basic features of hematopoietic stem cells. Immunofluorescence analyses, culture assays, and transplantation models permit an extensive immunological as well as functional characterization of human hematopoietic stem and progenitor cells. New methods such as cDNA array technology have demonstrated that distinct gene expression patterns of transcription factors and cell cycle genes molecularly control self-renewal, differentiation, and proliferation. Furthermore, several adhesion molecules have been shown to play an important role in the regulation of hematopoiesis and stem cell trafficking. Progress has also been made in elucidating molecular mechanisms of stem cell aging that limit replicative potential. Finally, more recent data provide the first molecular basis for a better understanding of transdifferentiation and developmental plasticity of hematopoietic stem cells. These findings could be helpful for non-hematopoietic cell therapeutic approaches.

Participation of bone marrow derived cells in cutaneous wound healing

Bone marrow has long been known to be a source of stem cells capable of regeneration of the hematopoeitic system. Recent reports, however, have indicated that bone marrow might also contain early stem cells that can differentiate into other organ tissues such as skin. While these studies have illustrated that bone marrow stem cells could find their way to the skin, they have not addressed the dynamics of how bone marrow stem cells might participate in the homeostatis and regeneration of skin. In this report we followed green fluorescent protein (GFP) labeled bone marrow transplanted into non-GFP mice in order to determine the participation of bone marrow stem cells in cutaneous wounds. Our results indicate that there are a significant number of bone marrow cells that traffic through both wounded and non-wounded skin. Wounding stimulated the engraftment of bone marrow cells to the skin and induced bone marrow derived cells to incorporate into and differentiate into non-hematopoietic skin structures. This report thus illustrates that bone marrow might be a valuable source of stem cells for the skin and possibly other organs. Wounding could be a stimulus for bone marrow derived stem cells to travel to organs and aid in the regeneration of damaged tissue.

Bone-marrow-derived cells contribute to glomerular endothelial repair in experimental glomerulonephritis

Glomerular endothelial injury plays an important role in the pathogenesis of renal diseases and is centrally involved in renal disease progression. Glomerular endothelial repair may help maintain renal function. We examined whether bone-marrow (BM)-derived cells contribute to glomerular repair. A rat allogenic BM transplant model was used to allow tracing of BM-derived cells using a donor major histocompatibility complex class-I specific mAb. In glomeruli of chimeric rats we identified a small number of donor-BM-derived endothelial and mesangial cells, which increased in a time-dependent manner. Induction of anti-Thy-1.1-glomerulonephritis (transient mesangial and secondary glomerular endothelial injury) caused a significant, more than fourfold increase in the number of BM-derived glomerular endothelial cells at day 7 after anti-Thy-1.1 injection compared to chimeric rats without glomerular injury. The level of BM-derived endothelial cells remained high at day 28. We also observed a more than sevenfold increase in the number of BM-derived mesangial cells at day 28. BM-derived endothelial and mesangial cells were fully integrated in the glomerular structure. Our data show that BM-derived cells participate in glomerular endothelial and mesangial cell turnover and contribute to microvascular repair. These findings provide novel insights into the pathogenesis of renal disease and suggest a potential role for stem cell therapy.

Human pulmonary chimerism after hematopoietic stem cell transplantation

Many of the body's tissues once thought to be only locally regenerative may, in fact, be actively replaced by circulating stem cells after hematopoietic stem cell transplantation. Localization of donor-derived cells ("chimerism") has recently been shown to occur in the lungs of mice after either hematopoietic stem cell transplantation or infusion of cultured marrow. To determine whether tissues of the human lung might be similarly derived from extrapulmonary sources, we examined lung specimens from a retrospective cohort of female allogeneic hematopoietic stem cell transplant recipients who received stem cells from male donors. Tissue samples from three such patients who had undergone diagnostic lung biopsy or autopsy were examined. Slides were stained by immunohistochemistry for cytokeratin (epithelium) and platelet endothelial cell adhesion molecule, CD31 (PECAM) (endothelium) and were imaged and then examined by fluorescent in situ hybridization analysis to identify male cells. The resulting overlapping in situ hybridization and immunohistochemistry images were examined for the presence and, if present, cell type of donor cells in the lung. We found significant rates of epithelial (2.5-8.0%) and endothelial (37.5-42.3%) chimerism. These results suggest that significant chimerism of the human lung may follow hematopoietic stem cell transplantation and that adult human stem cells could potentially play a therapeutic role in treatment of the damaged lung.

Teaching cells new tricks

The direct conversion of one differentiated cell type into another--a process referred to as transdifferentiation--would be beneficial for producing isogenic (patient's own) cells to replace sick or damaged cells or tissue. Adult stem cells display a broader differentiation potential than anticipated and might contribute to tissues other than those in which they reside. As such, they could be worthy therapeutic agents. Recent advances in transdifferentiation involve nuclear transplantation, manipulation of cell culture conditions, induction of ectopic gene expression and uptake of molecules from cellular extracts. These approaches open the doors to new avenues for engineering isogenic replacement cells. To avoid unpredictable tissue transformation, nuclear reprogramming requires controlled and heritable epigenetic modifications. Considerable efforts remain to unravel the molecular processes underlying nuclear reprogramming and evaluate stable of the changes in reprogrammed cells.

Adult cardiac stem cells--where do we go from here?

A potential treatment for cardiovascular disease involves the transplantation of a patient's bone marrow stem cells into the heart of that same patient. In order to maximize the potential benefits to select patient populations, the continued clinical development of this technology will require a comprehensive understanding of the role(s) of the transplanted cells in the repair of damaged heart tissue as well as an understanding of which types of cardiac injury can be repaired by this approach. The widespread application of cardiovascular stem cell therapies, however, will likely be based on pharmacological approaches to enhance the capacity of endogenous bone marrow stem cells to provide for the replacement of cardiac muscle and vascular cells after myocardial injury.

Bone marrow and bone marrow derived mononuclear stem cells therapy for the chronically ischemic myocardium

Bone marrow stem cells have been shown to differentiate into various phenotypes including cardiomyocytes, vascular endothelial cells and smooth muscle. Bone marrow stem cells are mobilized and home in to areas of injured myocardium where they are involved in tissue repair. In addition, bone marrow secretes multiple growth factors, which are essential for angiogenesis and arteriogenesis. In some patients, these processes are not enough to avert clinical symptoms of ischemic disease. Therefore, in vivo administration of an adequate number of stem cells would be a significant therapeutic advance. Unfractionated bone marrow derived mononuclear stem cells, which contain both hematopoietic and nonhematopoietic cells may be more appropriate for cell therapy. Studies in animal models suggest that implantation of different types of stem cells improve angiogenesis and arteriogenesis, tissue perfusion as well as left ventricular function. Several unanswered questions remain. For example, the optimal delivery approach, dosage and timing of the administration of cell therapy as well as durability of improvements need to be studied. Early clinical studies have demonstrated safety and feasibility of various cell therapies in ischemic disease. Randomized, double blind and placebo-controlled clinical trials need to be completed to determine the effectiveness of stem cell.

Cardiomyocyte-mediated contact programs human mesenchymal stem cells to express cardiogenic phenotype

BACKGROUND

Intercellular crosstalk and cellular plasticity are key factors in embryogenesis and organogenesis. The microenvironment plays a critical role in directing the progression of stem cells into differentiated cells. We hypothesized that intercellular interaction between adult human mesenchymal stem cells and adult human cardiomyocytes would induce stem cells to acquire the phenotypical characteristics of cardiomyocytes, and we tested the role that direct cell-to-cell contact plays in directing this differentiation process. Human mesenchymal stem cells were cultured in the presence of human cardiomyocytes ("coculture") or in the presence of media conditioned by separate cultures of human cardiomyocytes ("conditioned media").

METHODS

Human cardiomyocytes were labeled with chloromethyl derivatives of fluorescein diacetate. In the coculture experiments, human mesenchymal stem cells and human cardiomyocytes were mixed at a 1:1 ratio in smooth muscle 2 media and seeded at a cell density of 10,000 cells/cm(2). Cells were cocultured in an incubator at 37 degrees C for 48 hours. Subsequently, fluorescence-activated cell sorting was used to extract the differentiating human mesenchymal stem cells. In the conditioned media experiments, human mesenchymal stem cells were incubated in media previously conditioned by cardiomyocytes, in the presence and absence of serum (+/-serum). The conditioned media was changed 3 times, at intervals of 48 hours. Total RNA was isolated and reverse transcriptase-polymerase chain reaction was performed for expression of contractile proteins and cardiac specific genes. Immunostaining against myosin heavy chain, beta-actin troponin-T, and troponin-I was performed.

RESULTS

Fluorescence-activated cell sorting analysis identified 66% of the human mesenchymal stem cells in the G1 phase. Differentiated hMSCs from the coculture experiments expressed myosin heavy chain, beta-actin, and troponin-T by reverse transcriptase-polymerase chain reaction. Immunostaining was also positive against myosin heavy chain and troponin-T. In contrast, only beta-actin expression was observed in the human mesenchymal stem cells incubated with conditioned media +/- serum.

CONCLUSION

In addition to soluble signaling molecules, direct cell-to-cell contact is obligatory in relaying the external cues of the microenvironment controlling the differentiation of adult stem cells to cardiomyocytes. These data indicate that human mesenchymal stem cells are plastic and can be reprogrammed into a cardiomyogenic lineage that may be used in cell-based therapy for treating heart failure.

Stem cells and cardiovascular disease

Several recent discoveries have shifted the paradigm that there is no potential for myocardial regeneration and have fueled enthusiasm for a new frontier in the treatment of cardiovascular disease-stem cells. Fundamental to this emerging field is the cumulative evidence that adult bone marrow stem cells can differentiate into a wide variety of cell types, including cardiac myocytes and endothelial cells. This phenomenon has been termed stem cell plasticity and is the basis for the explosive recent interest in stem cell-based therapies. Directed to cardiovascular disease, stem cell therapy holds the promise of replacing lost heart muscle and enhancing cardiovascular revascularization. Early evidence of the feasibility of stem cell therapy for cardiovascular disease came from a series of animal experiments demonstrating that adult stem cells could become cardiac muscle cells (myogenesis) and participate in the formation of new blood vessels (angiogenesis and vasculogenesis) in the heart after myocardial infarction. These findings have been rapidly translated to ongoing human trials, but many questions remain. This review focuses on the use of adult bone marrow-derived stem cells for the treatment of ischemic cardiovascular disease and will contrast how far we have come in a short time with how far we still need to go before stem cell therapy becomes routine in cardiovascular medicine.

Hemangioblastic characteristics of fetal bone marrow-derived Flk1(+)CD31(-)CD34(-) cells

OBJECTIVE

To investigate whether Flk1(+)CD31(-)CD34(-) cells isolated from fetal bone marrow (BM) have characteristics of hemangioblasts, i.e., progenitors of endothelial and hematopoietic cells.

MATERIALS AND METHODS

Mononuclear cells from fetal BM were negatively sorted by CD45, GlyA, and CD34 micromagnetic beads, then cultured to form cell colonies. A single colony was harvested. Culture-expanded cells were seeded on ECM gel or semisolid media supplemented with endothelial and hematopoietic growth factors, respectively. Immunochemistry staining and RT-PCR were performed for cell characterization.

RESULTS

99% of cells from the single colony maintained Flk1(+) and CD31/CD34(-) during passaging. On ECM gel, Flk1(+)CD31(-)CD34(-) cells could grow into vascular structure that was positive for CD31 and vWF. There were round CD34(+) cells around the vascular structure. When angiogenesis inhibitor suramin was added before tube formation, formation of vascular structure was blocked. Additionally, Flk1(+)CD31(-)CD34(-) cells cultured on hematopoietic condition could differentiate into hematopoietic cells which expressed GATA-1, 2, and gamma, beta globin gene. After being replated in methylcellulose medium, they formed typical erythroid colonies.

CONCLUSIONS

Flk1(+)CD31(-)CD34(-) cells derived from fetal BM could differentiate into endothelial and hematopoietic cells. The results suggested that these Flk1(+)CD31(-)CD34(-) cells after embryo stage bear characteristics of hemangioblast and may have potential application for the hematopoietic and vascular diseases.

Endothelial progenitor cells: precursors for angiogenesis

The presence of endothelial progenitor cells has been demonstrated in the bone marrow and systemic circulation of adults, thus raising the possibility of a novel strategy to induce therapeutic angiogenesis for ischemic arterial disease. Successful incorporation into sites of actively occurring angiogenesis in numerous animal models has accelerated the enthusiasm for exploiting their therapeutic capacity in humans and has led to the recent use of putative endothelial precursor cells in phase I feasibility and safety studies. However, key biological issues remain ill defined. The relative contribution of these cells to postnatal physiological and pathological neovascularization has not been fully characterized. Furthermore, the molecular phenotype of the putative endothelial progenitor cell and the processes leading to their mobilization from the bone marrow and homing to sites of angiogenesis have yet to be elucidated. This review addresses these fundamental issues that warrant further basic investigation before the full therapeutic potential of these cells can be achieved within appropriate target patients.

Autologous stem cells for functional myocardial repair

Recent experimental studies based on innovative hypothesis utilizing cell therapy for the damaged myocardium are recently becoming increasingly promising. The naturally occurring myocardial reparative process is apparently complex and relatively inefficient. It consists of up-regulation of progenitor cell release from the bone marrow after myocardial infarction, homing of these cells to the injured tissue, and differentiation of these progenitor cells into vascular cells and cardiomyocytes within the infarcted tissue. Accordingly, there are two main strategies to regenerate myocardium with autologous stem cells: (1) Extracting stem cells from the bone marrow and injecting these cells into the damaged area, (2) Increasing the efficiency of the naturally occurring reparative process by increasing the mobilization of bone marrow-derived stem cells after myocardial infarction. This review summarizes the growing field of autologous stem cell utilization over the past decade and outlines scientific and clinical hurdles that need to be overcome before this therapy can fully reach its clinical potential.

Cardiac tissue engineering for replacement therapy

Cell therapy is a new concept to repair diseased organs. For patients with myocardial infarction, heart failure, and congenital heart diseases cell based therapies might represent a potential cure. The field can be subdivided into two principally different approaches: (1) Implantation of isolated cells and (2) implantation of in vitro engineered tissue constructs. This review will focus on the latter approach. Cardiac tissue engineering comprises the fields of material sciences and cell biology. In general, scaffold materials such as gelatin, collagen, alginate, or synthetic polymers and cardiac cells are utilized to reconstitute tissue-like constructs in vitro. Ideally, these constructs display properties of native myocardium such as coherent contractions, low diastolic tension, and syncytial propagation of action potentials. To be applicable for surgical repair of diseased myocardium engineered tissue constructs should have the propensity to integrate and remain contractile in vivo. Size and mechanical properties of engineered constructs are critical for surgical repair of large tissue defects. Successful application of tissue engineering in men will depend on the utilization of an autologous or non-immunogeneic cell source and scaffold material to avoid life long immunosuppression. This review will give an overview of recent approaches in cardiac tissue engineering and its first applications in vivo. We will discuss materials and cell sources for cardiac tissue engineering. Further, principle obstacles will be addressed. Cardiac tissue engineering for replacement therapy has an intriguing perspective, but is in its early days. Its true value remains to be thoroughly evaluated.

Side population cells and Bcrp1 expression in lung

Side population (SP) cells are a rare subset of cells found in various tissues that are highly enriched for stem cell activity. SP cells can be isolated by dual-wavelength flow cytometry because of their capacity to efflux Hoechst dye, a process mediated by the ATP-binding cassette transporter breast cancer resistance protein (Bcrp) 1. By performing flow cytometry of enzymedigested mouse lung stained with Hoechst dye, we found that SP cells comprise 0.03-0.07% of total lung cells and are evenly distributed in proximal and distal lung regions. By RT-PCR, we found that lung SP cells express hepatocyte nuclear factor-3beta, but not thyroid transcription factor-1. Surface marker analysis revealed lung SP cells to be stem cell antigen 1 positive, Bcrp1 positive, lineage marker negative, and heterogeneous at the CD45 locus. As expected, we did not detect lung SP cells in Bcrp1-deficient animals. We, therefore, employed nonisotopic in situ hybridization and immunostaining for Bcrp1 as a strategy to localize these cells in vivo. Expression was observed in distinct lung cell types: bronchial and vascular smooth muscle cells and round cells within the distal air space. We confirmed the expression of Bcrp1 in primary bronchial smooth muscle cell cultures (BSMC) and in lavaged distal airway cells, but neither possessed the capacity to efflux Hoechst dye. In BSMC, Bcrp1 was localized to an intracellular compartment, suggesting that the molecular site of Bcrp1 expression regulates SP phenotype.

Hepatic oval cells have the side population phenotype defined by expression of ATP-binding cassette transporter ABCG2/BCRP1

Organ-specific stem cells can be identified by the side population (SP) phenotype, which is defined by the property to effectively exclude the Hoechst 33342 dye. The ATP-binding cassette transporter ABCG2/BCRP1 mediates the SP phenotype. Because hepatic oval cells possess several characteristics of stem cells, we examined whether they have the SP phenotype using the 2-acetylaminofluorene/partial hepatectomy (PH) model. Fluorescence-activated cell sorting analysis showed that a population of non-parenchymal cells containing oval cells, prepared on day 7 after PH, carried a significant number of SP cells, whereas that of non-parenchymal cells without oval cells, prepared on day 0 after PH, did not. Northern blot analysis using total liver RNA obtained on various days after PH showed that the expression of ABCG2/BCRP1 mRNA increased after PH, reaching the highest level on day 7, and then gradually decreased. This pattern of changes in the ABCG2/BCRP1 mRNA level was well correlated to that in the number of oval cells. Furthermore, in situ hybridization revealed that oval cells were the sites of expression of ABCG2/BCRP1 mRNA. These results indicate that oval cells have the SP phenotype defined by expression of ABCG2/BCRP1, suggesting that oval cells may represent stem cells in the liver.

Multistep nature of microvascular recruitment of ex vivo-expanded embryonic endothelial progenitor cells during tumor angiogenesis

Tissue neovascularization involves recruitment of circulating endothelial progenitor cells that originate in the bone marrow. Here, we show that a class of embryonic endothelial progenitor cells (Tie-2+, c-Kit+, Sca-1+, and Flk-1-/low), which were isolated at E7.5 of mouse development at the onset of vasculogenesis, retain their ability to contribute to tumor angiogenesis in the adult. Using intravital fluorescence videomicroscopy, we further defined the multistep process of embryonic endothelial progenitor cell (eEPC) homing and incorporation. Circulating eEPCs are specifically arrested in "hot spots" within the tumor microvasculature, extravasate into the interstitium, form multicellular clusters, and incorporate into functional vascular networks. Expression analysis and in vivo blocking experiments provide evidence that the initial cell arrest of eEPC homing is mediated by E- and P-selectin and P-selectin glycoprotein ligand 1. This paper provides the first in vivo insights into the mechanisms of endothelial progenitor cell recruitment and, thus, indicates novel ways to interfere with pathological neovascularization.

Establishment of lacZ-transgenic rats: a tool for regenerative research in myocardium

Animals transgenic (Tg) for reporter genes would be useful to following a given cell lineage during differentiation and regeneration processes. Here, we established a beta-galactosidase (lacZ) Tg rat to use as a tool for regenerative research. Strong lacZ expression was observed in the skeletal muscles, myocardium, pancreas, and skin obtained from these lacZ-Tg rats, and moderate lacZ expression was observed in the liver, spleen, kidney, and cartilage. In contrast, brain, vessels, lung, adrenal gland, small intestine, blood leukocytes, bone marrow (BM) cells, and peripheral blood cells showed no lacZ expression. To test whether this lacZ-Tg rat could be used for regenerative research in myocardium, we induced myocardial injury after a lacZ-Tg BM transplant (BMT) into wild-type rats. The results show that lacZ-positive cardiomyocytes were found in the peri-infarct and uninjured myocardium in the BMT recipient rats. These findings suggest that lacZ-Tg rats are useful tool for regenerative research in the myocardium.

Direct cell-cell interaction of cardiomyocytes is key for bone marrow stromal cells to go into cardiac lineage in vitro

OBJECTIVES

Cardiac environmental factors are thought to be powerful inducers in cardiomyogenic differentiation. In this study we simulated the cardiac environment using coculture and evaluated the cardiomyogenic differentiation in bone marrow stromal cells.

METHODS

In group 1 only bone marrow stromal cells derived from transgenic mice expressing green fluorescent protein (GFP-BMCs) were cultured (n = 5). In group 2 cardiomyocytes from neonatal rats were grown on inserts, which we applied to culture dishes seeded with GFP-BMCs (n = 5). In group 3 GFP-BMCs were cocultured with cardiomyocytes on the same dishes (n = 5). We cultured these cells for 7 days and evaluated the synchronous contraction and the cardiomyogenic differentiation of GFP-BMCs by means of immunostaining.

RESULTS

In groups 1 and 2 GFP-BMCs protein did not show any myogenic phenotypes for 7 days. In contrast, in group 3 some GFP-BMCs were incorporated in parallel with cardiomyocytes and revealed myotube-like formation on day 1. On day 2, some GFP-BMCs started to contract synchronously with cardiomyocytes. Myosin heavy chain-positive GFP-BMCs were recognized in 2.49% +/- 0.87% of the total GFP-BMCs on day 5 (P <.0001). Cardiac-specific troponin I-positive GFP-BMCs were in 1.86% +/- 0.53% of the total cells on day 5 (P <.0001). Atrial natriuretic peptide was also seen in GFP-BMCs, and connexin 43 was detected between GFP-BMCs and cardiomyocytes.

CONCLUSIONS

Direct cell-cell interaction with cardiomyocytes was important for bone marrow stromal cells to differentiate into cardiomyocytes. This coculture was useful for simulating the cardiac environment in vitro for the research of cell transplantation in the heart.

Stem cell antigen-1 expression in the pulmonary vascular endothelium

Although the function of the cell surface protein stem cell antigen-1 (Sca-1) has not been identified, expression of this molecule is a characteristic of bone marrow-derived hematopoietic stem cell populations. Expression of Sca-1, however, is not restricted to hematopoietic tissue. By RT-PCR and Western analysis, we found that Sca-1 is expressed in the adult mouse lung. Sca-1 immunohistochemistry revealed a linear staining pattern on the endothelial surface of large and small pulmonary arteries and veins and alveolar capillaries. Expression of Sca-1 in the pulmonary endothelium was confirmed by dual fluorescent microscopy on lung sections and by fluorescence-activated cell sorting analysis of digested lung tissue; each of these methods showed colocalization with the endothelial marker platelet/endothelial cell adhesion molecule-1. In the kidney, Sca-1 expression was also noted in large vessels, but, in contrast to the lung, was not observed in capillaries. Overall, our data indicate that Sca-1 expression helps define the surface phenotype of endothelial cells throughout the pulmonary vasculature.

Heart development: molecular insights into cardiac specification and early morphogenesis

The heart develops from two bilateral heart fields that are formed during early gastrulation. In recent years, signaling pathways that specify cardiac mesoderm have been extensively analyzed. In addition, a battery of transcription factors that regulate different aspects of cardiac morphogenesis and cytodifferentiation have been identified and characterized in model organisms. At the anterior pole, a secondary heart field is formed, which in its molecular make-up, appears to be similar to the primary heart field. The cardiac outflow tract and the right ventricle to a large extent are derivatives of this anterior heart field. Cardiac mesoderm receives positional information by which it is patterned along the three body axes. The molecular control of left-right axis development has received particular attention, and the underlying regulatory network begins to emerge. Cardiac chamber development involves the activation of a transcription program that is different from the one present in the primary heart field and regulates cardiac morphogenesis in a region-specific manner. This review also attempts to identify areas in which additional research is needed to fully understand early cardiac development.