Myocardial repair: from salvage to tissue reconstruction

A Microenvironment-responsive small-molecule probe and application in quick acute myocardial infarction imaging

Acute myocardial infarction (AMI) patients are at an elevated risk for life-threatening myocardial ischemia/reperfusion injury. Early-stage nonradioactive and noninvasive diagnosis of AMI is imperative for the subsequent disease treatment, yet it presents substantial challenges. After AMI, the myocardium typically exhibits elevated levels of peroxynitrite (ONOO-), constituting a distinct microenvironmental feature. In this context, the near-infrared imaging probe (BBEB) is employed to precisely delineate the boundaries of AMI lesions with a high level of sensitivity and specificity by monitoring endogenous ONOO-. This probe allows for the early detection of myocardial damage at cellular and animal levels, providing exceptional temporal and spatial resolution. Notably, BBEB enables visualization of ONOO- level alterations during AMI treatment incorporating antioxidant drugs. Overall, BBEB can rapidly and accurately visualize myocardial injury, particularly in the early stages, and can further facilitate antioxidant drug screening.

Recent Advances in Cardiac Tissue Engineering for the Management of Myocardium Infarction

Myocardium Infarction (MI) is one of the foremost cardiovascular diseases (CVDs) causing death worldwide, and its case numbers are expected to continuously increase in the coming years. Pharmacological interventions have not been at the forefront in ameliorating MI-related morbidity and mortality. Stem cell-based tissue engineering approaches have been extensively explored for their regenerative potential in the infarcted myocardium. Recent studies on microfluidic devices employing stem cells under laboratory set-up have revealed meticulous events pertaining to the pathophysiology of MI occurring at the infarcted site. This discovery also underpins the appropriate conditions in the niche for differentiating stem cells into mature cardiomyocyte-like cells and leads to engineering of the scaffold via mimicking of native cardiac physiological conditions. However, the mode of stem cell-loaded engineered scaffolds delivered to the site of infarction is still a challenging mission, and yet to be translated to the clinical setting. In this review, we have elucidated the various strategies developed using a hydrogel-based system both as encapsulated stem cells and as biocompatible patches loaded with cells and applied at the site of infarction.

Cardiac cAMP-PKA Signaling Compartmentalization in Myocardial Infarction

Under physiological conditions, cAMP signaling plays a key role in the regulation of cardiac function. Activation of this intracellular signaling pathway mirrors cardiomyocyte adaptation to various extracellular stimuli. Extracellular ligand binding to seven-transmembrane receptors (also known as GPCRs) with G proteins and adenylyl cyclases (ACs) modulate the intracellular cAMP content. Subsequently, this second messenger triggers activation of specific intracellular downstream effectors that ensure a proper cellular response. Therefore, it is essential for the cell to keep the cAMP signaling highly regulated in space and time. The temporal regulation depends on the activity of ACs and phosphodiesterases. By scaffolding key components of the cAMP signaling machinery, A-kinase anchoring proteins (AKAPs) coordinate both the spatial and temporal regulation. Myocardial infarction is one of the major causes of death in industrialized countries and is characterized by a prolonged cardiac ischemia. This leads to irreversible cardiomyocyte death and impairs cardiac function. Regardless of its causes, a chronic activation of cardiac cAMP signaling is established to compensate this loss. While this adaptation is primarily beneficial for contractile function, it turns out, in the long run, to be deleterious. This review compiles current knowledge about cardiac cAMP compartmentalization under physiological conditions and post-myocardial infarction when it appears to be profoundly impaired.

Elastin-inspired supramolecular hydrogels: a multifaceted extracellular matrix protein in biomedical engineering

The phenomenal advancement in regenerative medicines has led to the development of bioinspired materials to fabricate a biomimetic artificial extracellular matrix (ECM) to support cellular survival, proliferation, and differentiation. Researchers have diligently developed protein polymers consisting of functional sequences of amino acids evolved in nature. Nowadays, certain repetitive bioinspired polymers are treated as an alternative to synthetic polymers due to their unique properties like biodegradability, easy scale-up, biocompatibility, and non-covalent molecular associations which imparts tunable supramolecular architecture to these materials. In this direction, elastin has been identified as a potential scaffold that renders extensibility and elasticity to the tissues. Elastin-like polypeptides (ELPs) are artificial repetitive polymers that exhibit lower critical solution temperature (LCST) behavior in a particular environment than synthetic polymers and hence have gained extensive interest in the fabrication of stimuli-responsive biomaterials. This review discusses in detail the unique structural aspects of the elastin and its soluble precursor, tropoelastin. Furthermore, the versatility of elastin-like peptides is discussed through numerous examples that bolster the significance of elastin in the field of regenerative medicines such as wound care, cardiac tissue engineering, ocular disorders, bone tissue regeneration, etc. Finally, the review highlights the importance of exploring short elastin-mimetic peptides to recapitulate the structural and functional aspects of elastin for advanced healthcare applications.

Instantaneous In Vivo Imaging of Acute Myocardial Infarct by NIR-II Luminescent Nanodots

Fast and precise localization of ischemic tissues in the myocardium after an acute infarct is required by clinicians as the first step toward accurate and efficient treatment. Nowadays, diagnosis of a heart attack at early times is based on biochemical blood analysis (detection of cardiac enzymes) or by ultrasound-assisted imaging. Alternative approaches are investigated to overcome the limitations of these classical techniques (time-consuming procedures or low spatial resolution). As occurs in many other fields of biomedicine, cardiological preclinical imaging can also benefit from the fast development of nanotechnology. Indeed, bio-functionalized near-infrared-emitting nanoparticles are herein used for in vivo imaging of the heart after an acute myocardial infarct. Taking advantage of the superior acquisition speed of near-infrared fluorescence imaging, and of the efficient selective targeting of the near-infrared-emitting nanoparticles, in vivo images of the infarcted heart are obtained only a few minutes after the acute infarction event. This work opens an avenue toward cost-effective, fast, and accurate in vivo imaging of the ischemic myocardium after an acute infarct.

Elastin-Based Materials: Promising Candidates for Cardiac Tissue Regeneration

Stroke and cardiovascular episodes are still some of the most common diseases worldwide, causing millions of deaths and costing billions of Euros to healthcare systems. The use of new biomaterials with enhanced biological and physical properties has opened the door to new approaches in cardiovascular applications. Elastin-based materials are biomaterials with some of the most promising properties. Indeed, these biomaterials have started to yield good results in cardiovascular and angiogenesis applications. In this review, we explore the latest trends in elastin-derived materials for cardiac regeneration and the different possibilities that are being explored by researchers to regenerate an infarcted muscle and restore its normal function. Elastin-based materials can be processed in different manners to create injectable systems or hydrogel scaffolds that can be applied by simple injection or as patches to cover the damaged area and regenerate it. Such materials have been applied to directly regenerate the damaged cardiac muscle and to create complex structures, such as heart valves or new bio-stents that could help to restore the normal function of the heart or to minimize damage after a stroke. We will discuss the possibilities that elastin-based materials offer in cardiac tissue engineering, either alone or in combination with other biomaterials, in order to illustrate the wide range of options that are being explored. Moreover, although tremendous advances have been achieved with such elastin-based materials, there is still room for new approaches that could trigger advances in cardiac tissue regeneration.

Stem cell therapy of myocardial infarction: a promising opportunity in bioengineering

Myocardial infarction (MI) is a life-threatening disease resulting from irreversible death of cardiomyocytes (CMs) and weakening of the heart blood-pumping function. Stem cell-based therapies have been studied for MI treatment over the last two decades with promising outcome. In this review, we critically summarize the past work in this field to elucidate the advantages and disadvantages of treating MI using pluripotent stem cells (PSCs) including both embryonic stem cells (ESCs) and induced pluripotent stem cells (iPSCs), adult stem cells, and cardiac progenitor cells. The main advantage of the latter is their cytokine production capability to modulate immune responses and control the progression of healing. However, human adult stem cells have very limited (if not 'no') capacity to differentiate into functional CMs in vitro or in vivo. In contrast, PSCs can be differentiated into functional CMs although the protocols for the cardiac differentiation of PSCs are mainly for adherent cells under 2D culture. Derivation of PSC-CMs in 3D, allowing for large-scale production of CMs via modulation of the Wnt/β-catenin signal pathway with defined chemicals and medium, may be desired for clinical translation. Furthermore, the technology of purification and maturation of the PSC-CMs may need further improvements to eliminate teratoma formation after in vivo implantation of the PSC-CMs for treating MI. In addition, in vitro derived PSC-CMs may have mechanical and electrical mismatch with the patient's cardiac tissue, which causes arrhythmia. This supports the use of PSC-derived cells committed to cardiac lineage without beating for implantation to treat MI. In this case, the PSC derived cells may utilize the mechanical, electrical, and chemical cues in the heart to further differentiate into mature/functional CMs in situ. Another major challenge facing stem cell therapy of MI is the low retention/survival of stem cells or their derivatives (e.g., PSC-CMs) in the heart for MI treatment after injection in vivo. This may be resolved by using biomaterials to engineer stem cells for reduced immunogenicity, immobilization of the cells in the heart, and increased integration with the host cardiac tissue. Biomaterials have also been applied in the derivation of CMs in vitro to increase the efficiency and maturation of differentiation. Collectively, a lot has been learned from the past failure of simply injecting intact stem cells or their derivatives in vivo for treating MI, and bioengineering stem cells with biomaterials is expected to be a valuable strategy for advancing stem cell therapy towards its widespread application for treating MI in the clinic.

Alginate for cardiac regeneration: From seaweed to clinical trials

Heart failure is a growing endemic in the aging Western population with a prevalence of over 20 million people worldwide¹. Existing heart failure therapies are unable to reverse heart failure and do not address its fundamental cause, the loss of cardiomyocytes². In order to induce myocardial regeneration for the myocardium and the heart valve, facilitate self-repair, improve tissue salvage, reduce or reverse the adverse-remodeling and ultimately achieve long-term functional stabilization and improvement in the heart function, novel strategies for therapeutic regeneration are being developed which are aiming to compensate for the insufficient and low intrinsic regenerative ability of the adult heart³. Similarly, valve replacement with mechanical or biological substitutes meets numerous hurdles. New approaches using multicellular approaches and new material are extensively studied. Most of those strategies depend on biomaterials that help to achieve functional integrated vasculogenesis and myogenesis in the heart/tissue. Especially for failed heart valve function a number of therapeutic approaches are common from corrective intervention to complete replacement⁴. However the complexity of the heart valve tissue and its high physical exposure has led to a variety of approaches, however therapeutic regeneration needs to be established. Beside other approaches alginate has been identified as one building block to achieve therapeutic regeneration.

Alginate is a versatile and adaptable biomaterial that has found numerous biomedical applications which include wound healing, drug delivery and tissue engineering. Due to its biologically favorable properties including the ease of gelation and its biocompatibility, alginate-based hydrogels have been considered a particularly attractive material for the application in cardiac regeneration and valve replacement techniques. Here, we review current applications of alginate in cardiac regeneration as well as perspectives for the alginate-dependent, cardiac regeneration strategies.

Alginate biomaterial for the treatment of myocardial infarction: Progress, translational strategies, and clinical outlook: From ocean algae to patient bedside

Alginate biomaterial is widely utilized for tissue engineering and regeneration due to its biocompatibility, non-thrombogenic nature, mild and physical gelation process, and the resemblance of its hydrogel matrix texture and stiffness to that of the extracellular matrix. In this review, we describe the versatile biomedical applications of alginate, from its use as a supporting cardiac implant in patients after acute myocardial infarction (MI) to its employment as a vehicle for stem cell delivery and for the controlled delivery and presentation of multiple combinations of bioactive molecules and regenerative factors into the heart. Preclinical and first-in-man clinical trials are described in details, showing the therapeutic potential of injectable acellular alginate implants to inhibit the damaging processes after MI, leading to myocardial repair and tissue reconstruction.

Concise review: reprogramming strategies for cardiovascular regenerative medicine: from induced pluripotent stem cells to direct reprogramming

Myocardial cell-replacement therapies are emerging as novel therapeutic paradigms for myocardial repair but are hampered by the lack of sources of autologous human cardiomyocytes. The recent advances in stem cell biology and in transcription factor-based reprogramming strategies may provide exciting solutions to this problem. In the current review, we describe the different reprogramming strategies that can give rise to cardiomyocytes for regenerative medicine purposes. Initially, we describe induced pluripotent stem cell technology, a method by which adult somatic cells can be reprogrammed to yield pluripotent stem cells that could later be coaxed ex vivo to differentiate into cardiomyocytes. The generated induced pluripotent stem cell-derived cardiomyocytes could then be used for myocardial cell transplantation and tissue engineering strategies. We also describe the more recent direct reprogramming approaches that aim to directly convert the phenotype of one mature cell type (fibroblast) to another (cardiomyocyte) without going through a pluripotent intermediate cell type. The advantages and shortcomings of each strategy for cardiac regeneration are discussed, along with the hurdles that need to be overcome on the road to clinical translation.

A mathematical model for analyzing the elasticity, viscosity, and failure of soft tissue: comparison of native and decellularized porcine cardiac extracellular matrix for tissue engineering

The clinical success of tissue-engineered constructs commonly requires mechanical properties that closely mimic those of the human tissue. Determining the viscoelastic properties of such biomaterials and the factors governing their failure profiles, however, has proven challenging, although collecting extensive data regarding their tensile behavior is straightforward. The easily calculated Young's modulus remains the most reported mechanical measure, regardless of its limitations, even though single-relaxation-time (SRT) models can provide much more information, which remain scarce due to a lack of manageable tools for implementing these models. We developed an easy-to-use algorithm for applying the Zener SRT model and determining the elastic moduli, viscosity, and failure profiles of materials under different mechanical tests in a user-independent manner. The algorithm was validated on the data resulting from tensile tests on native and decellularized porcine cardiac tissue, previously suggested as a promising scaffold material for cardiac tissue engineering. This analysis yields new and more accurate measurements such as the elastic moduli and viscosity, the model's relaxation time, and information on the factors governing the materials' failure profiles. These measurements indicate that the viscoelasticity and strength of the decellularized acellular extracellular matrix (ECM) are similar to those of native tissue, although its elasticity and apparent viscosity are higher. Nonetheless, reseeding and culturing the ECM with mesenchymal stem cells was shown to partially restore the mechanical properties lost after decellularization. We propose this algorithm as a platform for soft-tissue analysis that can provide comparable and unbiased measures for characterizing viscoelastic biomaterials commonly used in tissue engineering.

Feasibility of intracoronary GLP-1 eluting CellBead™ infusion in acute myocardial infarction

Cell therapy is a field of growing interest in the prevention of post acute myocardial infarction (AMI) heart failure. Stem cell retention upon local delivery to the heart, however, is still unsatisfactory. CellBeads were recently developed as a potential solution to this problem. CellBeads are 170-μm alginate microspheres that contain mesenchymal stem cells (MSCs) genetically modified to express glucagon-like peptide-1 (GLP-1) supplementary to inherent paracrine factors. GLP-1 is an incretin hormone that has both antiapoptotic and cardioprotective effects. Transplanting CellBeads in the post-AMI heart might induce cardiomyocyte salvage and ultimately abrogate adverse cardiac remodeling. We aimed to investigate the feasibility of intracoronary infusion of CellBeads in a large animal model of AMI. Four pigs were used in a pilot study to assess the maximal safe dose of CellBeads. In the remaining 21 animals, an AMI was induced by balloon occlusion of the left circumflex coronary artery for 90 min. During reperfusion, 60,000 CellBeads (n = 11), control beads (n = 4), or lactated Ringers' (n = 6) were infused. Animals were sacrificed after 2 or 7 days, and the hearts were excised for histological analyses. Intracoronary infusion did not permanently affect coronary flow in any of the groups. Histological analysis revealed CellBeads containing viable MSCs up to 7 days. Viability and activity of the MSCs was confirmed by qPCR analysis that showed expression of recombinant GLP-1 and human genes after 2 and 7 days. CellBeads reduced inflammatory infiltration by 29% (p = 0.001). In addition, they decreased the extent of apoptosis by 25% (p = 0.001) after 2 days. We show that intracoronary infusion of 5 million encapsulated MSCs is safe and feasible. Also, several parameters indicate that the cells have paracrine effects, suggesting a potential therapeutic benefit of this new approach.

Nanoparticles targeting the infarcted heart

We report a nanoparticulate system capable of targeting the heart after myocardial infarction (MI). Targeting is based on overexpression of angiotensin II type 1 (AT1) receptor in the infarcted heart. Liposomes 142 nm in diameter were conjugated with a ligand specific to AT1. The nanoparticles were able to specifically target cardiac cells in vitro, and in the infarcted heart after intravenous injection in vivo. This system may be useful for delivering therapeutic agents specifically to the infarcted heart.

Myocardial regeneration: Roles of stem cells and hydrogels

Heart failure remains the leading cause of morbidity and mortality. Recently, it was reported that the adult heart has intrinsic regenerative capabilities, prompting a great wave of research into applying cell-based therapies, especially with skeletal myoblasts and bone marrow-derived cells, to regenerate heart tissues. While the mechanism of action for the observed beneficial effects of bone marrow-derived cells remains unclear, new cell candidates are emerging, including embryonic stem (ES) and introduced pluripotent stem (iPS) cells, as well as cardiac stem cells (CSCs) from adult hearts. However, the very low engraftment efficiency and survival of implanted cells prevent cell therapy from turning into a clinical reality. Injectable hydrogel biomaterials based on hydrophilic, biocompatible polymers and peptides have great potential for addressing many of these issues by serving as cell/drug delivery vehicles and as a platform for cardiac tissue engineering. In this review, we will discuss the application of stem cells and hydrogels in myocardial regeneration.

Bioengineering the infarcted heart by applying bio-inspired materials

Induction of cardiac muscle regeneration following myocardial infarction (MI) represents a major challenge in cardiovascular therapy, as the current clinical approaches are limited in their ability to regenerate a new muscle tissue and to replace infarcted myocardium. Here, we describe the conception of two strategies based on bio-inspired materials, aimed at myocardial repair after MI. In the first strategy, alginate biomaterial was designed with affinity-binding moieties, enabling the binding of heparin-binding proteins and their controlled presentation and release. The combined features of this unique alginate hydrogel, as a temporary extracellular matrix replacement and a depot for bio-molecules such as insulin-like growth factor-1 and hepatocyte growth factor, led to improvements in cardiac structure and function, as demonstrated by the biomaterial's abilities to thicken the scar and prevent left-ventricular remodeling and dilatation. Endogenous regeneration occurring at the infarct as manifested by the enhanced angiogenesis, cardiomyocyte proliferation, and appearance of cardiac-related stem cells is likely to have contributed to this. In the second strategy, phosphatidylserine (PS)-presenting liposomes were developed to mimic apoptotic cells bodies, specifically their capability of immunomodulating activated macrophages into anti-inflammatory state. In a rat model of acute MI, targeting of PS-presenting liposomes to infarct macrophages after injection via the femoral vein was demonstrated by magnetic resonance imaging. The treatment promoted angiogenesis, the preservation of small scars, and prevention of ventricular dilatation and remodeling. Collectively, the two bio-inspired material-based strategies presented herein represent unique and clinical accessible approaches for myocardial infarct repair.

The effect of immobilized RGD peptide in alginate scaffolds on cardiac tissue engineering

Cardiac tissue engineering aims to regenerate damaged myocardial tissues by applying heart patches created in vitro. The present study was undertaken to explore the possible role of matrix-attached RGD peptide in the engineering of cardiac tissue within macroporous scaffolds. Neonatal rat cardiac cells were seeded into RGD-immobilized or unmodified alginate scaffolds. The immobilized RGD peptide promoted cell adherence to the matrix, prevented cell apoptosis and accelerated cardiac tissue regeneration. Within 6 days, the cardiomyocytes reorganized their myofibrils and reconstructed myofibers composed of multiple cardiomyocytes in a typical myofiber bundle. The nonmyocyte cell population, mainly cardiofibroblasts, benefited greatly from adhering to the RGD-alginate matrix and consequently supported the cardiomyocytes. They often surrounded bundles of cardiac myofibers in a manner similar to that of native cardiac tissue. The benefits of culturing the cardiac cells in RGD-immobilized alginate scaffolds were further substantiated by Western blotting, revealing that the relative expression levels of α-actinin, N-cadherin and connexin-43 were better maintained in cells cultured within these scaffolds. Collectively, the immobilization of RGD peptide into macroporous alginate scaffolds proved to be a key parameter in cardiac tissue engineering, contributing to the formation of functional cardiac muscle tissue and to a better preservation of the regenerated tissue in culture.

Effects of granulocyte-colony-stimulating factor on progenitor cell mobilization and heart perfusion and function in normal mice

BACKGROUND AIMS

Mobilization of stem cells and progenitor cells from the bone marrow (BM) into the peripheral blood (PB) by granulocyte-colony-stimulating factor (G-CSF) is being investigated for cardiac regeneration in ischemic heart disease. However, hematopoietic (HPC), mesenchymal (MPC) and endothelial (EPC) progenitor mobilization have not been optimized and the effect of G-CSF on myocardial perfusion and cardiac function in a normal heart has never been studied.

METHODS

Normal mice were injected daily for 1-10 days with subcutaneous recombinant human G-CSF. PB and BM were evaluated for HPC and EPC by flow cytometry and HPC and MPC by hematopoietic (CFU-GM) and mesenchymal (CFU-F) colony assays. Echocardiography, microSPECT imaging, cardiac catheterization and immunohistochemistry were performed in mice treated for 10 days.

RESULTS

HPC and CFU-GM in PB peaked after 2 days, CFU-F after 4 days and EPC after 3 days. Thereafter, while HPC temporally decreased before showing a second peak, EPC remained detectable only at low levels. In BM, hematopoietic stem cells (HSC) and CFU-GM did not increase much overall but peaked twice on days 2 and 7. EPC (peak on day 7) production increased in the BM, but CFU-F formation declined considerably after day 2. G-CSF enhanced myocardial perfusion and vascularization but impaired hemodynamic performance of the heart through apparently increased ventricular wall rigidity.

CONCLUSIONS

G-CSF induces the mobilization of HPC, EPC and CFU-F progenitors in PB according to very different patterns, and has a significant impact on perfusion and function of the normal heart.

The effects of controlled HGF delivery from an affinity-binding alginate biomaterial on angiogenesis and blood perfusion in a hindlimb ischemia model

Enhancing tissue self-repair through the use of active acellular biomaterials is one of the main goals of regenerative medicine. We now describe the features of an injectable alginate biomaterial designed to affinity-bind heparin-binding proteins and release them at a rate reflected by their association constant to alginate-sulfate. The interactions of hepatocyte growth factor (HGF) with alginate-sulfate resulted in factor protection from proteolysis, as shown by mass spectroscopy analysis after trypsin digestion. When the HGF/alginate-sulfate bioconjugate was incorporated into alginate hydrogel, HGF release was sustained by a factor of 3, as compared to the release rate from non-modified hydrogel. The released factor retained activity, as shown by its induction of ERK1/2 activation and affording cytoprotection in rat neonatal cardiomyocyte cultures. In vivo, an injectable form of the affinity-binding alginate system extended by 10-fold, as compared to a saline-treated group, retention of HGF in myocardial tissue when delivered immediately after myocardial infarction. In a severe murine hindlimb ischemia model, HGF delivery from the affinity-binding system improved tissue blood perfusion and induced mature blood vessel network formation. The therapeutic efficacy of the affinity-binding system, as well as its ease of delivery by injection, provides a proof-of-concept for the potential use of this bioactive biomaterial strategy in cardiovascular repair.

Genesis of myocardial repair with cardiac progenitor cells and tissue engineering

BACKGROUND

There is mounting evidence to suggest that the heart has regenerative potential in the event of myocardial injury. Recent studies have shown that a resident population of cardiac progenitor cells (CPCs) in the heart contains both vasculogenic and myogenic lineages. CPCs are able to migrate to the site of injury in the heart for participation in the healing process. The resident CPCs in the heart may also be activated through outside pharmacological intervention to promote their participation in the intrinsic repair process. In the light of these characteristics, CPCs provide a logical source for the heart cell therapy. During the regenerative cardiac process, stem cell niches (a specialised environment surrounding stem cells) provide crucial support needed for their maintenance.

DISCUSSION

Compromised niche function may lead to the selection of stem cells that no longer depend on self-renewal factors produced by its environment. The objective of stem cell transplantation associated with tissue-engineered approaches is to create a new modality in the treatment of heart failure. The use of efficient scaffolds will aid to re-establish a favourable microenvironment for stem cell survival, multiplication, differentiation and function. Cardiac tissue engineering using natural and/or synthetic materials in this regard provides a novel possibility in cardiovascular therapeutics.

Cell-based therapeutics from an economic perspective: primed for a commercial success or a research sinkhole?

Despite widespread hype and significant investment through the late 1980s and 1990s, cell-based therapeutics have largely failed from both a clinical and financial perspective. While the early pioneers were able to create clinically efficacious products, small margins coupled with small initial indications made it impossible to produce a reasonable return on the huge initial investments that had been made to support widespread research activities. Even as US FDA clearance opened up larger markets, investor interest waned, and the crown jewels of cell-based therapeutics went bankrupt or were rescued by corporate bailout. Despite the hard lessons learned from these pioneering companies, many of today's regenerative medicine companies are supporting nearly identical strategies. It remains to be seen whether or not our proposed tenets for investment and commercialization strategy yield an economic success or whether the original model can produce a return on investment sufficient to justify the large up-front investments. Irrespective of which approach yields a success, it is critically important that more of the second-generation products establish profitability if the field is to enjoy continued investment from both public and private sectors.