Aortic Hemodynamics of Spiral-Flow-Generated Mechanical Assistance.

BACKGROUND: Mechanical circulatory support devices are being increasingly used as destination therapy in end-stage heart failure patients. Although current devices have significantly improved survival rates, the resulting hemodynamics remains nonphysiological. Spiral forms of blood flow are known to exist in the large arteries (eg, aorta) and serve as a biomimetic-motivation for generating these physiologically adapted flow regimes. We aimed to study the potential benefits of generating spiral flow at the mechanical circulatory support outflow graft and the resultant flow-fields in the aorta, including recirculation zones and endothelial wall shear stress (WSS) areas. METHODS: A three-dimensional model of an outflow graft virtually anastomosed end-to-side to an image-derived aortic arch was used in computational fluid dynamic simulations. To study the impact of both spiral flow modulation (clockwise/counterclockwise helical-flow content) and the outflow graft anastomosis angle (inferiorly/superiorly directed, anteriorly/posteriorly directed), flow velocities were measured, low/high WSS were computed, and fluid streamlines were visualized. RESULTS: Increased helical-flow content reduced regions of low velocity (<5 cm/s), minimized areas exhibiting low WSS (<3 dyn/cm2), and concomitantly increased areas of high WSS (>80 dyn/cm2). The outflow graft anastomosis angle was a key determinant of aortic root washout and fluid-jet wall impingement. Despite counterclockwise spiral flow predominance in diminishing the size of recirculation/stasis zones compared to straight/clockwise flow, exceptions to this were noted with the superiorly directed and posteriorly directed graft placements. CONCLUSIONS: Spiral flow-forms better tailored to the underlying three-dimensional aortic curvature and graft angle positioning is expected to help attenuate atherogenesis, preventing vascular remodeling and minimizing plaque formation/erosion in mechanically assisted circulation.

Dual Defibrillation is Highly Variable: An Analysis of Pulse Interval Delivered in Dual Defibrillation.

Background: Dual defibrillation (DD) is a technique where two external defibrillators are applied with two different pad configurations and discharged to treat refractory ventricular fibrillation (RVF). Although commonly called dual sequential defibrillation (DSD), if the delivered electrical pulses overlap with no pulse interval, the shocks are actually dual simultaneous defibrillation (DSiD). Manual DD technique is not standardized and the effect that the method of activation has on the delivered pulse interval has never been studied. Objectives: This study measured the timing of four methods of DD and the resulting inter-shock intervals, frequency with which they were either DSiD or DSD, and frequency which the true DSDs delivered any previously reported optimum pulse interval. Methods: This was a single-blinded prospective evaluation of a convenience sample of volunteer physicians, nurses, and paramedics each performing DD in our simulation center on two types of defibrillators using four techniques: single operator-simultaneous with 2 hands (SOSI), two operators-simultaneous (TOSI), single operator-sequential with 1 hand (SOSE1), and single operator-sequential with 2 hands (SOSE2). Results: The four DD methods generated a variable set of pulse intervals depending on the technique and defibrillator employed. The pulse intervals ranged from 0 msec (i.e., overlapping waveforms or DSiD) to 1800 msec. Of all DD attempts, 85.9% met the definition of DSD, 14.1% were DSiD, and 49.4% delivered any one of the optimum pulse intervals previously described in the literature. SOSI and TOSI techniques resulted in DSD between 47.2 and 87.6% of the time, depending on the technique and defibrillator. Shocks delivered sequentially on purpose (SOSE2 and SOSE1) were always DSD but with widely variable pulse intervals. SOSI resulted in the shortest pulse intervals, SOSE1 resulted in the longest, and TOSI and SOSE2 were the least skewed. Conclusion: DD using the various methods currently employed produces a highly variable set of pulse intervals even within a single method. It is difficult to reach a conclusion about the efficacy of DD unless the delivered pulse interval is measured or the method of activation reproducibly produces a precise pulse interval.

The mechanics of spiral flow: Enhanced washout and transport.

Spiral/helical forms of blood flow have been observed in large arteries of the cardiovascular system, but their benefits remain underappreciated. Spiral flow has been postulated to improve near-wall washout, promoting anti-atherothrombotic conditions. This research aims to study the washout characteristics of spiral flow, specifically, its ability to increase velocity and wall shear stress (WSS) in atherothrombotic-prone regions. Using 1.2 cm diameter angled test-conduits (45°, 90°, 135°) with known recirculation/stasis regions at the bend corners, spiral flow washout potential was evaluated in terms of low velocity and low WSS. Two sub-studies were conducted: the first utilized a spiral flow-inducing device to enable qualitative analysis of washout-potential in both computational fluid dynamic (CFD) simulations and benchtop ultrasound visualization; the second used CFD to study the impact of several induced helical wavelengths on the conduit-dependent recirculation/stasis zones. Physical models of the angled conduits and spiral flow-inducer were 3D-printed to facilitate ultrasound visualization. Compared to straight flow, spiral flow generated by the flow-inducer significantly cleared the recirculation/stasis zones at the corners of the angled conduits. CFD simulations demonstrated that past a geometry-dependent threshold, increased helical content improved washout, denoted by decreased regions of low velocity and low WSS. Overall, spiral flow markedly improved washout in difficult to reach areas in the angled conduits. This has several important clinical implications: spiral flow shows great promise in reducing blood-transport-related complications and can be used to enhance the performance of future medical devices (eg grafts, mechanical circulatory support devices, hemodialysis access ports).

Circulatory Mechanotherapeutics: Moving with the Force.

PURPOSE OF REVIEW: This review describes the current state of advancements in mechanical circulatory support (MCS) devices with significantly improved hemodynamic performance and decreased adverse events. Novel considerations for future MCS designs that impart spiral flow regimes will be detailed. RECENT FINDINGS: Significant challenges in MCS device use have included size reduction, premature pump mechanical bearing failure, acquired bleeding disorders, and vascular complications related to high shear forces and jetting. Some of these problems have been improved upon, such as the use of magnetically levitated impellers and hydrodynamic bearings. The relative simplicity of continuous flow pumps has also enabled their miniaturization, portability, and reduced energy consumption. Recent studies by our group demonstrated that spiral forms of flow possess hemodynamically beneficial attributes at the MCS outflow cannula and aorta interface, reducing jet impact, organizing streamlines, and thereby improving endothelial function through wall shear stress modulation. Despite MCS design improvements, they are far from perfect. Induced spiral fluid modulation may help address the known flow-mediated disturbances in vascular mechanobiology.

Vortical flow characteristics of mechanical cavopulmonary assistance: Pre- and post-swirl dynamics.

Surgical optimization of the cavopulmonary connection and pharmacological therapy for dysfunctional Fontan physiology continue to advance, but these treatment approaches only slow the progression of decline to end-stage heart failure. The development of a mechanical cavopulmonary assist device will provide a viable therapeutic option in the bridging of patients to transplant or to stabilization. We hypothesize that rotational blood flow, delivered by an implantable axial flow blood pump, could effectively assist the venous circulation in Fontan patients by mimicking vortical blood flow patterns in the cardiovascular system. This study investigated seven new models of mechanical cavopulmonary assistance (single and dual-pump assist), created combinations of pump designs that deliver counter rotating vortical flow conditions, and analyzed pump performance, velocity streamlines, swirling strength, and energy augmentation in the cavopulmonary circuit for each support scenario. The model having an axial clockwise-oriented impeller in the inferior vena cava and an axial counterclockwise-oriented impeller rotating in the superior vena cava outperformed all of the support scenarios by enhancing the energy of the cavopulmonary circulation an average of 10.3% over the entire flow range and a maximum of 27.4% at %the higher flow rates. This research will guide the development of axial flow blood pumps for Fontan patients and demonstrated the high probability of %a cardiovascular benefit using counter rotating pumps in a dual support scenario, but found that this is dependent upon the patient-specific cavopulmonary anatomy.

A Novel Technique for Experimental Flow Visualization of Mechanical Valves.

The geometry of the hinge region in mechanical heart valves has been postulated to play an important role in the development of thromboembolic events (TEs). This study describes a novel technique developed to visualize washout characteristics in mechanical valve hinge areas. A dairy-based colloidal suspension (DBCS) was used as a high-contrast tracer. It was introduced directly into the hinge-containing sections of two commercially available valves mounted in laser-milled fluidic channels and subsequently washed out at several flow rates. Time-lapse images were analyzed to determine the average washout rate and generate intensity topography maps of the DBCS clearance. As flow increased, washout improved and clearance times were shorter in all cases. Significantly different washout rate time constants were observed between valves, average >40% faster clearance (p < 0.01). The topographic maps revealed that each valve had a characteristic pattern of washout. The technique proved reproducible with a maximum recorded standard error of mean (SEM) of ±3.9. Although the experimental washout dynamics have yet to be correlated with in vivo visualization studies, the methodology may help identify key flow features influencing TEs. This visualization methodology can be a useful tool to help evaluate stagnation zones in new and existing heart valve hinge designs.

Ultrasound-induced modulation of cardiac rhythm in neonatal rat ventricular cardiomyocytes.

Isolated neonatal rat ventricular cardiomyocytes were used to study the influence of ultrasound on the chronotropic response in a tissue culture model. The beat frequency of the cells, varying from 40 to 90 beats/min, was measured based upon the translocation of the nuclear membrane captured by a high-speed camera. Ultrasound pulses (frequency = 2.5 MHz) were delivered at 300-ms intervals [3.33 Hz pulse repetition frequency (PRF)], in turn corresponding to 200 pulses/min. The intensity of acoustic energy and pulse duration were made variable, 0.02-0.87 W/cm(2) and 1-5 ms, respectively. In 57 of 99 trials, there was a noted average increase in beat frequency of 25% with 8-s exposures to ultrasonic pulses. Applied ultrasound energy with a spatial peak time average acoustic intensity (Ispta) of 0.02 W/cm(2) and pulse duration of 1 ms effectively increased the contraction rate of cardiomyocytes (P < 0.05). Of the acoustic power tested, the lowest level of acoustic intensity and shortest pulse duration proved most effective at increasing the electrophysiological responsiveness and beat frequency of cardiomyocytes. Determining the optimal conditions for delivery of ultrasound will be essential to developing new models for understanding mechanoelectrical coupling (MEC) and understanding novel nonelectrical pacing modalities for clinical applications.

Mechanically stimulated contraction of engineered cardiac constructs using a microcantilever.

The beating heart undergoes cyclic mechanical and electrical activity during systole and diastole. The interaction between mechanical stimulation and propagation of the depolarization wavefront is important for understanding not just normal sinus rhythm, but also mechanically induced cardiac arrhythmia. This study presents a new platform to study mechanoelectrical coupling in a 3-D in vitro model of the myocardium. Cardiomyocytes and cardiac fibroblasts are seeded within extracellular matrix proteins and form constructs constrained by microfabricated tissue gauges that provide in situ measurement of contractile function. The microcantilever of an atomic force microscope is indented into the construct at varying magnitudes and frequencies to cause a coordinated contraction. The results indicate that changes in indentation depth and frequency do not significantly affect the magnitude of contraction, but increasing indentation frequency significantly increases the contractile velocity. Overall, this study demonstrates the validity of this platform as a means to study mechanoelectrical coupling in a 3-D setting, and to investigate the mechanism underlying mechanically stimulated contraction.

Augmentation of integrin-mediated mechanotransduction by hyaluronic acid.

Changes in tissue and organ stiffness occur during development and are frequently symptoms of disease. Many cell types respond to the stiffness of substrates and neighboring cells in vitro and most cell types increase adherent area on stiffer substrates that are coated with ligands for integrins or cadherins. In vivo cells engage their extracellular matrix (ECM) by multiple mechanosensitive adhesion complexes and other surface receptors that potentially modify the mechanical signals transduced at the cell/ECM interface. Here we show that hyaluronic acid (also called hyaluronan or HA), a soft polymeric glycosaminoglycan matrix component prominent in embryonic tissue and upregulated during multiple pathologic states, augments or overrides mechanical signaling by some classes of integrins to produce a cellular phenotype otherwise observed only on very rigid substrates. The spread morphology of cells on soft HA-fibronectin coated substrates, characterized by formation of large actin bundles resembling stress fibers and large focal adhesions resembles that of cells on rigid substrates, but is activated by different signals and does not require or cause activation of the transcriptional regulator YAP. The fact that HA production is tightly regulated during development and injury and frequently upregulated in cancers characterized by uncontrolled growth and cell movement suggests that the interaction of signaling between HA receptors and specific integrins might be an important element in mechanical control of development and homeostasis.

α-Catenin localization and sarcomere self-organization on N-cadherin adhesive patterns are myocyte contractility driven.

The N-cadherin (N-cad) complex plays a crucial role in cardiac cell structure and function. Cadherins are adhesion proteins linking adjacent cardiac cells and, like integrin adhesions, are sensitive to force transmission. Forces through these adhesions are capable of eliciting structural and functional changes in myocytes. Compared to integrins, the mechanisms of force transduction through cadherins are less explored. α-catenin is a major component of the cadherin-catenin complex, thought to provide a link to the cell actin cytoskeleton. Using N-cad micropatterned substrates in an adhesion constrainment model, the results from this study show that α-catenin localizes to regions of highest internal stress in myocytes. This localization suggests that α-catenin acts as an adaptor protein associated with the cadherin mechanosensory apparatus, which is distinct from mechanosensing through integrins. Myosin inhibition in cells bound by integrins to fibronectin-coated patterns disrupts myofibiril organization, whereas on N-cad coated patterns, myosin inhibition leads to better organized myofibrils. This result indicates that the two adhesion systems provide independent mechanisms for regulating myocyte structural organization.

Reprogramming cardiomyocyte mechanosensing by crosstalk between integrins and hyaluronic acid receptors.

The elastic modulus of bioengineered materials has a strong influence on the phenotype of many cells including cardiomyocytes. On polyacrylamide (PAA) gels that are laminated with ligands for integrins, cardiac myocytes develop well organized sarcomeres only when cultured on substrates with elastic moduli in the range 10 kPa-30 kPa, near those of the healthy tissue. On stiffer substrates (>60 kPa) approximating the damaged heart, myocytes form stress fiber-like filament bundles but lack organized sarcomeres or an elongated shape. On soft (<1 kPa) PAA gels myocytes exhibit disorganized actin networks and sarcomeres. However, when the polyacrylamide matrix is replaced by hyaluronic acid (HA) as the gel network to which integrin ligands are attached, robust development of functional neonatal rat ventricular myocytes occurs on gels with elastic moduli of 200 Pa, a stiffness far below that of the neonatal heart and on which myocytes would be amorphous and dysfunctional when cultured on polyacrylamide-based gels. The HA matrix by itself is not adhesive for myocytes, and the myocyte phenotype depends on the type of integrin ligand that is incorporated within the HA gel, with fibronectin, gelatin, or fibrinogen being more effective than collagen I. These results show that HA alters the integrin-dependent stiffness response of cells in vitro and suggests that expression of HA within the extracellular matrix (ECM) in vivo might similarly alter the response of cells that bind the ECM through integrins. The integration of HA with integrin-specific ECM signaling proteins provides a rationale for engineering a new class of soft hybrid hydrogels that can be used in therapeutic strategies to reverse the remodeling of the injured myocardium.

Intercellular and extracellular mechanotransduction in cardiac myocytes.

Adult cardiomyocytes are terminally differentiated with minimal replicative capacity. Therefore, long-term preservation or enhancement of cardiac function depends on structural adaptation. Myocytes interact with the extracellular matrix, fibroblasts, and vascular cells and with each other (end to end; side to side). We review the current understanding of the mechanical determinants and environmental sensing systems that modulate and regulate myocyte molecular machinery and its structural organization. We feature the design and application of engineered cellular microenvironments to demonstrate the ability of cardiac cells to remodel their cytoskeletal organization and shape, including sarcomere/myofibrillar architectural topography. Cell shape-dependent functions result from complex mechanical interactions between the cytoskeleton architecture and external conditions, be they cell-cell or cell-extracellular matrix (ECM) adhesion contact-mediated. This mechanobiological perspective forms the basis for viewing the cardiomyocyte as a mechanostructural anisotropic continuum, exhibiting constant mechanosensory-driven self-regulated adjustment of the cytoskeleton through tight interplay between its force generation activity and concurrent cytoarchitectural remodeling. The unifying framework guiding this perspective is the observation that these emerging events and properties are initiated by and respond to cytoskeletal reorganization, regulated by cell-cell and cell-ECM adhesion and its corresponding (mutually interactive) signaling machinery. It is important for future studies to elucidate how cross talk between these mechanical signals is coordinated to control myocyte structure and function. Ultimately, understanding how the highly interactive mechanical signaling can give rise to phenotypic changes is critical for targeting the underlying pathways that contribute to cardiac remodeling associated with various forms of dilated and hypertrophic myopathies, myocardial infarction, heart failure, and reverse remodeling.

Cardiac myocyte remodeling mediated by N-cadherin-dependent mechanosensing.

Cell-to-cell adhesions are crucial in maintaining the structural and functional integrity of cardiac cells. Little is known about the mechanosensitivity and mechanotransduction of cell-to-cell interactions. Most studies of cardiac mechanotransduction and myofibrillogenesis have focused on cell-extracellular matrix (ECM)-specific interactions. This study assesses the direct role of intercellular adhesion, specifically that of N-cadherin-mediated mechanotransduction, on the morphology and internal organization of neonatal ventricular cardiac myocytes. The results show that cadherin-mediated cell attachments are capable of eliciting a cytoskeletal network response similar to that of integrin-mediated force response and transmission, affecting myofibrillar organization, myocyte shape, and cortical stiffness. Traction forces mediated by N-cadherin were shown to be comparable to those sustained by ECM. The directional changes in predicted traction forces as a function of imposed loads (gel stiffness) provide the added evidence that N-cadherin is a mechanoresponsive adhesion receptor. Strikingly, the mechanical sensitivity response (gain) in terms of the measured cell-spread area as a function of imposed load (adhesive substrate rigidity) was consistently higher for N-cadherin-coated surfaces compared with ECM protein-coated surfaces. In addition, the cytoskeletal architecture of myocytes on an N-cadherin adhesive microenvironment was characteristically different from that on an ECM environment, suggesting that the two mechanotransductive cell adhesion systems may play both independent and complementary roles in myocyte cytoskeletal spatial organization. These results indicate that cell-to-cell-mediated force perception and transmission are involved in the organization and development of cardiac structure and function.

Pacing-induced cardiac gap junction remodeling: modulation of connexin43 phosphorylation state.

BACKGROUND: Altered myocardial distribution of gap junctions and intercellular coupling have been implicated in nonuniform conduction of the depolarization wave and repolarization asynchrony in the mammalian heart. We tested the hypothesis that short-term cardiac pacing is associated with structural remodeling of gap junctions and their altered spatial distribution in cardiac myocytes in the immediate vicinity of the pacing site. MATERIALS AND METHODS: Isolated adult male rat hearts (n = 8) were perfused using a Langendorff apparatus. A multimicroelectrode array pacing catheter was positioned in the endocardial apical region of the right ventricle. Pacing (330 bpm; stimulus: 1.5 V, 5 milliseconds) was applied for 3 hours. Immunoblotting and immunohistochemical assays [using serine specific (Ser368) anti-connexin43 and anti-phosphoserine antibody] were used to determine the phosphorylation state of connexin43 (Cx43) and to determine its spatial distribution. RESULTS: Pacing was associated with a consistent, increased dephosphorylation state of Cx43 at the pacing site when compared to remote regions. In control hearts, Cx43 manifested a predominantly phosphorylated state; Western blotting analysis showed that dephosphorylated Cx43 was more abundant (1.5 +/- 0.33-fold) in the paced hearts than in controls (P < 0.02). Global cardiac function parameters, such as developed left ventricular pressure and oxygen demand index (rate-pressure product), did not differ significantly in paced hearts compared with controls (P > 0.05). CONCLUSIONS: A relatively short period of cardiac asynchronous pacing is associated with remodeling of gap junctions as manifested in the altered phosphorylation state of their constituent Cx43. This effect is confined to the myocardial tissue surrounding the pacing electrodes and does not alter global cardiac mechanics and energetics. These results, considered together with the known involvement of Ser368 in the gating of Cx43 and the putative role of Cx43 in the intercellular conductance, suggest that pacing-induced localized gap junctional remodeling could contribute to the creation of a reentrant substrate.

Anti- and pro-arrhythmic effects of cardiac resynchronization therapy: point of view.

Cardiac resynchronization therapy (CRT) in patients with heart failure and bundle branch block (BBB) improves regional muscle mechanics and mechanical pump function of the heart. In addition, modulation of wall motion timing and contraction can exert an antiarrhythmic effect, reducing the potential of sudden cardiac death. This effect of CRT could also be attributed to the improvement in excitation-contraction coupling, mechanical synchronization, and improved myocardial perfusion. However, it can be hypothesized that the BBB results in a concealed reentry, in which a delayed depolarization wave re-enters during phase two of the action potential. This concealed phase 2 reentry can lead to early after depolarizations and cardiac arrhythmias. By synchronizing the two ventricles, CRT eliminates the reentry substrate and the resulting arrhythmias. This hypothesis and the potential arrhythmogenic effects of CRT are discussed with regard to ventricular remodeling and mechano-electrical feedback in this setting.

Comparison of eight prosthetic aortic valves in a cadaver model.

OBJECTIVES: Proper valve selection is critical to ensure appropriate valve replacement for patients, because implantation of a small valve might place the patient at risk for persistent gradients. Labeled valve size is not the same as millimeter measure of prosthetic valve diameters or the annulus into which it will fit. Studies that use the labeled valve size in lieu of actual measured diameter in millimeters to compare different valves might be misleading. Using human cadaver hearts, we sized the aortic annulus with 8 commonly used prosthetic aortic valve sizers and compared the valves using geometric orifice area. This novel method for comparing prosthetic valves allowed us to evaluate multiple valves for implantation into the same annulus. METHODS: Aortic annular area was determined in 66 cadavers. Valve sizers for 8 prosthetic valves were used to determine the appropriate valve for aortic valve replacement. Regression analyses were performed to compare the relationship between geometric orifice area and aortic annular area. RESULTS: Tissue valves had a larger orifice area for any annular size but were not different at small sizes. Supra-annular valves were larger than intra-annular valves for the small annulus, but this relationship was not uniform with increasing annular size. CONCLUSIONS: Labeled valve size relates unpredictably to annular size and orifice area. No advantage in geometric orifice area could be demonstrated between these tissue valves at small annular sizes. Valves with the steepest slope on regression analysis might provide a larger benefit with upsizing with respect to geometric orifice area.

Using heart rate variability to stratify risk of obstetric patients undergoing spinal anesthesia.

In this study, we evaluated whether point correlation dimension (PD2), a measure of heart rate variability, can predict hypotension accompanying spinal anesthesia for cesarean delivery. After the administration of spinal anesthesia with bupivacaine, hypotension was defined as systolic blood pressure

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.