Isolation and characterization of embryonic and adult epicardium and epicardium-derived cells

Epicardium-Derived Tbx18+ CDCs Transplantation Improve Heart Function in Infarcted Mice

Cardiosphere-derived cells (CDCs) constitute a cardiac stem cell pool, a promising therapeutics in treating myocardial infarction (MI). However, the cell source of CDCs remains unclear. In this study, we isolated CDCs directly from adult mouse heart epicardium named primary epicardium-derived CDCs (pECDCs), which showed a different expression profile compared with primary epicardial cells (pEpiCs). Interestingly, pECDCs highly expressed T-box transcription factor 18 (Tbx18) and showed multipotent differentiation ability in vitro. Human telomerase reverse transcriptase (hTERT) transduction could inhibit aging-induced pECDCs apoptosis and differentiation, thus keeping a better proliferation capacity. Furthermore, immortalized epicardium CDCs (iECDCs) transplantation extensively promote cardiogenesis in the infracted mouse heart. This study demonstrated epicardium-derived CDCs that may derive from Tbx18+ EpiCs, which possess the therapeutic potential to be applied to cardiac repair and regeneration and suggest a new kind of CDCs with identified origination that may be followed in the developing and injured heart.

Establishment and characterization of an immortalized epicardial cell line

Recently, the increasing significance of the epicardium in cardiac development and regeneration is beginning to be recognized. However, because of the small proportion of primary epicardial cells and the limited cell culture time, further research on the mechanism of epicardial cells is hindered. Here, we transfected simian virus 40 Large T (SV40-LT) into primary epicardial cells to establish an immortalized cell line, named EpiSV40. We further demonstrated that EpiSV40 can be easy to culture and has the proliferation, migration and differentiation capacities comparable to primary epicardial cells. EpiSV40 can serve as an ideal in vitro model for epicardial cell research, which will booster the study of the epicardium in cardiac development and heart regeneration.

Thymosin β4 released from functionalized self-assembling peptide activates epicardium and enhances repair of infarcted myocardium

The epicardium plays an important role in cardiomyogenesis during development, while it becomes quiescent in adult heart during homeostasis. This study investigates the efficiency of thymosin β4 (Tβ4) release with RPRHQGVM conjugated to the C-terminus of RADA16-I (RADA-RPR), the functionalized self-assembling peptide (SAP), to activate the epicardium and repairing the infarcted myocardium. Methods: The functionalized SAP was constituted with self-assembling motif, Tβ4-binding site, and cell adhesive ligand. Myocardial infarction (MI) models of the transgenic mice were established by ligation of the left anterior descending coronary artery. At one week after intramyocardial injection of Tβ4-conjugated SAP, the activation of the epicardium was assessed. At four weeks after implantation, the migration and differentiation of epicardium-derived cells (EPDCs) as well as angiogenesis, lymphangiogenesis and myocardial regeneration were examined. Results: We found that the designer RADA-RPR bound Tβ4 and adhered to EPDCs and that Tβ4 released from the functionalized SAP could effectively activate the epicardium and induce EPDCs to differentiate towards cardiovascular cells as well as lymphatic endothelial cells. Moreover, SAP-released Tβ4 (SAP-Tβ4) promoted proliferation of cardiomyocytes. Furthermore, angiogenesis, lymphangiogenesis and myocardial regeneration were enhanced in the MI models at 4 weeks after delivery of SAP-Tβ4 along with attenuation of adverse myocardial remodeling and significantly improved cardiac function. Conclusions: These results demonstrate that sustained release of Tβ4 from the functionalized SAP can activate the epicardium and effectively enhance the repair of infarcted myocardium. We believe the delivery of SAP-Tβ4 may be a promising strategy for MI therapy.

Novel technique for the simultaneous isolation of cardiac fibroblasts and epicardial stromal cells from the infarcted murine heart

AIMS

Myocardial infarction (MI) leads to activation of cardiac fibroblasts (aCFs) and at the same time induces the formation of epicardium-derived cells at the heart surface. To discriminate between the two cell populations, we elaborated a fast and efficient protocol for the simultaneous isolation and characterization of aCFs and epicardial stromal cells (EpiSCs) from the infarcted mouse heart.

METHODS AND RESULTS

For the isolation of aCFs and EpiSCs, infarcted hearts (50 min ischaemia/reperfusion) were digested by perfusion with a collagenase-containing medium for only 8 min, while EpiSCs were enzymatically removed from the outside by applying mild shear forces via a motor driven device. Cardiac fibroblasts (CFs) isolated from unstressed hearts served as control. Viability of isolated cells was >90%. Purity of EpiSCs was confirmed by immunofluorescence staining and qPCR of various mesenchymal markers including Wilms-tumor-protein-1. Microarray analysis of CFs, aCFs, and EpiSCs on day 5 post-MI revealed a unique gene expression pattern in the EpiSC fraction, which was enriched for epithelial markers and epithelial to mesenchymal transition-related genes. Compared to aCFs, 336 significantly altered gene entities were identified in the EpiSC fraction. qPCR analysis showed high expression of Serpinb2, Cxcl13, Adora2b, and Il10 in EpiSCs relative to CFs and aCFs. Furthermore, microarray data identified Ddah1 and Cemip to be highly up-regulated in aCFs compared to CFs. Immunostaining of the infarcted heart revealed a unique distribution of Dermokine, Aquaporin-1, Cytokeratin, Lipocalin2, and Periostin within the epicardial cell layer.

CONCLUSIONS

We describe the simultaneous isolation of viable, purified fractions of aCFs and EpiSCs from the infarcted mouse heart. In this study, several differentially expressed markers for aCFs and EpiSCs were identified, underlining the importance of cell separation to study heterogeneity of stromal cells in the healing process after MI.

Human Pluripotent Stem Cell-Derived Multipotent Vascular Progenitors of the Mesothelium Lineage Have Utility in Tissue Engineering and Repair

In this report we describe a human pluripotent stem cell-derived vascular progenitor (MesoT) cell of the mesothelium lineage. MesoT cells are multipotent and generate smooth muscle cells, endothelial cells, and pericytes and self-assemble into vessel-like networks in vitro. MesoT cells transplanted into mechanically damaged neonatal mouse heart migrate into the injured tissue and contribute to nascent coronary vessels in the repair zone. When seeded onto decellularized vascular scaffolds, MesoT cells differentiate into the major vascular lineages and self-assemble into vasculature capable of supporting peripheral blood flow following transplantation. These findings demonstrate in vivo functionality and the potential utility of MesoT cells in vascular engineering applications.

Wdpcp promotes epicardial EMT and epicardium-derived cell migration to facilitate coronary artery remodeling

During coronary vasculature development, endothelial cells enclose the embryonic heart to form the primitive coronary plexus. This structure is remodeled upon recruitment of epicardial cells that may undergo epithelial-mesenchymal transition (EMT) to enable migration and that give rise to smooth muscle cells. In mice expressing a loss-of-function mutant form of Wdpcp, a gene involved in ciliogenesis, the enclosure of the surface of the heart by the subepicardial coronary plexus was accelerated because of enhanced chemotactic responses to Shh. Coronary arteries, but not coronary veins in Wdpcp mutant mice, showed reduced smooth muscle cell coverage. In addition, Wdpcp mutant hearts had reduced expression of EMT and mesenchymal markers and had fewer epicardium-derived cells (EPDCs) that showed impaired migration. Epicardium-specific deletion of Wdpcp recapitulated the coronary artery defect of the Wdpcp mutant. Thus, Wdpcp promotes epithelial EMT and EPDC migration, processes that are required for remodeling of the coronary primitive plexus. The Wdpcp mutant mice will be a useful tool to dissect the molecular mechanisms that govern the remodeling of the primitive plexus during coronary development.

Prokineticin receptor-1-dependent paracrine and autocrine pathways control cardiac tcf21+ fibroblast progenitor cell transformation into adipocytes and vascular cells

Cardiac fat tissue volume and vascular dysfunction are strongly associated, accounting for overall body mass. Despite its pathophysiological significance, the origin and autocrine/paracrine pathways that regulate cardiac fat tissue and vascular network formation are unclear. We hypothesize that adipocytes and vasculogenic cells in adult mice hearts may share a common cardiac cells that could transform into adipocytes or vascular lineages, depending on the paracrine and autocrine stimuli. In this study utilizing transgenic mice overexpressing prokineticin receptor (PKR1) in cardiomyocytes, and tcf21ERT-cre^TM-derived cardiac fibroblast progenitor (CFP)-specific PKR1 knockout mice (PKR1^tcf−/−), as well as FACS-isolated CFPs, we showed that adipogenesis and vasculogenesis share a common CFPs originating from the tcf21⁺ epithelial lineage. We found that prokineticin-2 is a cardiomyocyte secretome that controls CFP transformation into adipocytes and vasculogenic cells in vivo and in vitro. Upon HFD exposure, PKR1^tcf−/− mice displayed excessive fat deposition in the atrioventricular groove, perivascular area, and pericardium, which was accompanied by an impaired vascular network and cardiac dysfunction. This study contributes to the cardio-obesity field by demonstrating that PKR1 via autocrine/paracrine pathways controls CFP–vasculogenic- and CFP-adipocyte-transformation in adult heart. Our study may open up new possibilities for the treatment of metabolic cardiac diseases and atherosclerosis.

CDC42 is required for epicardial and pro-epicardial development by mediating FGF receptor trafficking to the plasma membrane

The epicardium contributes to multiple cardiac lineages and is essential for cardiac development and regeneration. However, the mechanism of epicardium formation is unclear. This study aimed to establish the cellular and molecular mechanisms underlying the dissociation of pro-epicardial cells (PECs) from the pro-epicardium (PE) and their subsequent translocation to the heart to form the epicardium. We used lineage tracing, conditional deletion, mosaic analysis and ligand stimulation in mice to determine that both villous protrusions and floating cysts contribute to PEC translocation to myocardium in a CDC42-dependent manner. We resolved a controversy by demonstrating that physical contact of the PE with the myocardium constitutes a third mechanism for PEC translocation to myocardium, and observed a fourth mechanism in which PECs migrate along the surface of the inflow tract to reach the ventricles. Epicardial-specific Cdc42 deletion disrupted epicardium formation, and Cdc42 null PECs proliferated less, lost polarity and failed to form villous protrusions and floating cysts. FGF signaling promotes epicardium formation in vivo, and biochemical studies demonstrated that CDC42 is involved in the trafficking of FGF receptors to the cell membrane to regulate epicardium formation.

Highlighted article: During epicardial formation in mice, four different mechanisms of pro-epicardial cell translocation to the myocardium can be identified, with CDC42 playing a key role.

Biological characteristics of embryonic epicardial cells in vitro correlate with embryonic day

The epicardial cell (EpiC) culture system plays an important role in investigating the specific mechanisms and signaling molecules that are involved in the development of EpiCs. From this early formation until adulthood, EpiCs undergo dynamic changes in the expression of embryonic genes that correlate with changes in the embryonic EpiC properties. The differences of embryonic EpiC properties may affect the related results of experiments in which EpiC culture system is used; however, these differences have not been explored. Therefore, in this study we examined the differences in the biological characteristics of EpiCs on different embryonic days in vitro EpiCs were isolated from embryonic ventricle explants on embryonic day (E) 11.5, E13.5, and E15.5. The differences in the migration, proliferation and differentiation were studied in EpiCs of different embryonic day by scratch assay, cell cycle analysis and platelet derived growth factor-bb (PDGF-BB) treatment. The results showed that EpiCs were successfully cultured from E11.5, E13.5, and E15.5 embryonic ventricle explants. The time windows of E11.5, E13.5, and E15.5 EpiC isolation out of the explants were different. The migration abilities of E11.5, E13.5, and E15.5 EpiCs decreased during embryonic development. Smooth muscle cell differentiation potential of early stage EpiCs was better than that of the later stage EpiCs. Although the proliferation ability of E11.5 EpiCs was significantly weaker than those of E13.5 and E15.5 EpiCs, the proliferation abilities of E13.5 and E15.5 EpiCs did not differ. These results suggest that the biological characteristics of EpiCs correlate with the timing of embryonic development, and different embryonic stage of ventricle should be properly chosen for culturing EpiCs depending on the purposes of the specific experiments.

Human fetal and adult epicardial-derived cells: a novel model to study their activation

Background

The epicardium, a cell layer covering the heart, plays an important role during cardiogenesis providing cardiovascular cell types and instructive signals, but becomes quiescent during adulthood. Upon cardiac injury the epicardium is activated, which includes induction of a developmental gene program, epithelial-to-mesenchymal transition (EMT) and migration. However, the response of the adult epicardium is suboptimal compared to the active contribution of the fetal epicardium to heart development. To understand the therapeutic value of epicardial-derived cells (EPDCs), a direct comparison of fetal and adult sources is paramount. Such analysis has been hampered by the lack of appropriate culture systems.

Methods

Human fetal and adult EPDCs were isolated from cardiac specimens obtained after informed consent. EPDCs were cultured in the presence of an inhibitor of the TGFβ receptor ALK5. EMT was induced by stimulation with 1 ng/ml TGFβ. PCR, immunofluorescent staining, scratch assay, tube formation assay and RT²-PCR for human EMT genes were performed to functionally characterize and compare fetal and adult EPDCs.

Results

In this study, a novel protocol is presented that allows efficient isolation of human EPDCs from fetal and adult heart tissue. In vitro, EPDCs maintain epithelial characteristics and undergo EMT upon TGFβ stimulation. Although similar in several aspects, we observed important differences between fetal and adult EPDCs. Fetal and adult cells display equal migration abilities in their epithelial state. However, while TGFβ stimulation enhanced adult EPDC migration, it resulted in a reduced migration in fetal EPDCs. Matrigel assays revealed the ability of adult EPDCs to form tube-like structures, which was absent in fetal cells. Furthermore, we observed that fetal cells progress through EMT faster and undergo spontaneous EMT when TGFβ signaling is not suppressed, indicating that fetal EPDCs more rapidly respond to environmental changes.

Conclusions

Our data suggest that fetal and adult EPDCs are in a different state of activation and that their phenotypic plasticity is determined by this activation state. This culture system allows us to establish the cues that determine epicardial activation, behavior, and plasticity and thereby optimize the adult response post-injury.

Electronic supplementary material

The online version of this article (doi:10.1186/s13287-016-0434-9) contains supplementary material, which is available to authorized users.

Prokineticin receptor-1 signaling promotes Epicardial to Mesenchymal Transition during heart development

The epicardium plays an essential role in coronary artery formation and myocardial development. However, signals controlling the developing epicardium and epicardial-mesenchymal transition (EMT) in the normal and diseased adult heart are studied less rigorously. Here we investigated the role of angiogenic hormone, prokineticin-2 and its receptor PKR1 in the epicardium of developing and adult heart. Genetic ablation of PKR1 in epicardium leads to partial embryonic and postnatal lethality with abnormal heart development. Cardiac developmental defects are manifested in the adult stage as ischemic cardiomyopathy with systolic dysfunction. We discovered that PKR1 regulates epicardial-mesenchymal transition (EMT) for epicardial-derived progenitor cell (EPDC), formation. This event affects at least three consequential steps during heart development: (i) EPDC and cardiomyocyte proliferation involved in thickening of an outer compact ventricular chamber wall, (ii) rhythmicity, (iii) formation of coronary circulation. In isolated embryonic EPDCs, overexpression or activation of PKR1 alters cell morphology and EMT markers via activating Akt signaling. Lack of PKR1 signal in epicardium leads to defective heart development and underlies the origin of congenital heart disease in adult mice. Our mice provide genetic models for congenital dysfunction of the heart and should facilitate studies of both pathogenesis and therapy of cardiac disorders in humans.

Prokineticin receptor 1 is required for mesenchymal-epithelial transition in kidney development

Identification of factors regulating renal development is important to understand the pathogenesis of congenital kidney diseases. Little is known about the molecular mechanism of renal development and functions triggered by the angiogenic hormone prokineticin-2 and its receptor, PKR1. Utilizing the Gata5 (G5)-Cre and Wilms tumor 1 (Wt1)(GFP)cre transgenic lines, we generated mutant mice with targeted PKR1 gene disruptions in nephron progenitors. These mutant mice exhibited partial embryonic and postnatal lethality. Kidney developmental defects in PKR(G5-/-) mice are manifested in the adult stage as renal atrophy with glomerular defects, nephropathy, and uremia. PKR1(Wt1-/-) embryos exhibit hypoplastic kidneys with premature glomeruli and necrotic nephrons as a result of impaired proliferation and increased apoptosis in Wt1(+) renal mesenchymal cells. PKR1 regulates renal mesenchymal-epithelial transition (MET) that is involved in formation of renal progenitors, regulating glomerulogenesis toward forming nephrons during kidney development. In the isolated embryonic Wt1(+) renal cells, overexpression or activation of PKR1 promotes MET defined by the transition from elongated cell to octagonal cell morphology, and alteration of the expression of MET markers via activating NFATc3 signaling. Together, these results establish PKR1 via NFATc3 as a crucial modifier of MET processing to the development of nephron. Our study should facilitate new therapeutic opportunities in human renal disorders.-Arora, H., Boulberdaa, M., Qureshi, R., Bitirim, V., Messadeq, N., Dolle, P., Nebigil, C. G. Prokineticin receptor 1 is required for mesenchymal-epithelial transition in kidney development.

Inhibition of let-7 augments the recruitment of epicardial cells and improves cardiac function after myocardial infarction

Heart failure due to myocardial infarction is a major cause of mortality. The microRNA (miR) family let-7 is expressed during embryonic development and is up-regulated in differentiated cells. The aim of this study was to study the role of let-7 after acute myocardial infarction (AMI). We designed an antimiR to inhibit the highest expressed members of the let-7 family, let-7 a, b and c. Administration at day 0 and day 2 after AMI resulted in sustained knockdown of let-7 after 28days. Let-7 inhibition prevented deterioration of cardiac functions compared to control treatment which was especially due to improvements in the infarcted, apical cardiac segments. We observed higher contents of fibrosis in the border zone as well as increased numbers of cells positive for TCF21, which is also expressed in epicardial cells. Markers were augmented after let-7 inhibition and let-7 blocked EMT in epicardial cells in vitro. Lineage tracing in TCF21(iCre/+):R26R(tdT) mice showed abundant tomato positive cells in the infarct and border zone. In conclusion, let-7 inhibition resulted in functional benefits due to an increase in recruitment of epicardial cells and EMT.

Epicardial Outgrowth Culture Assay and Ex Vivo Assessment of Epicardial-derived Cell Migration

A single layer of epicardial cells lines the heart, providing paracrine factors that stimulate cardiomyocyte proliferation and directly contributing cardiovascular progenitors during development and disease. While a number of factors have been implicated in epicardium-derived cell (EPDC) mobilization, the mechanisms governing their subsequent migration and differentiation are poorly understood. Here, we present in vitro and ex vivo strategies to study EPDC motility and differentiation. First, we describe a method of obtaining primary epicardial cells by outgrowth culture from the embryonic mouse heart. We also introduce a detailed protocol to assess three-dimensional migration of labeled EPDC in an organ culture system. We provide evidence using these techniques that genetic deletion of myocardin-related transcription factors in the epicardium attenuates EPDC migration. This approach serves as a platform to evaluate candidate modifiers of EPDC biology and could be used to develop genetic or chemical screens to identify novel regulators of EPDC mobilization that might be useful for cardiac repair.

Single epicardial cell transcriptome sequencing identifies Caveolin 1 as an essential factor in zebrafish heart regeneration

In contrast to mammals, adult zebrafish have a high capacity to regenerate damaged or lost myocardium through proliferation of cardiomyocytes spared from damage. The epicardial sheet covering the heart is activated by injury and aids muscle regeneration through paracrine effects and as a multipotent cell source, and has received recent attention as a target in cardiac repair strategies. Although it is recognized that epicardium is required for muscle regeneration and itself has high regenerative potential, the extent of cellular heterogeneity within epicardial tissue is largely unexplored. Here, we performed transcriptome analysis on dozens of epicardial lineage cells purified from zebrafish harboring a transgenic reporter for the pan-epicardial gene tcf21. Hierarchical clustering analysis suggested the presence of at least three epicardial cell subsets defined by expression signatures. We validated many new pan-epicardial and epicardial markers by alternative expression assays. Additionally, we explored the function of the scaffolding protein and main component of caveolae, caveolin 1 (cav1), which was present in each epicardial subset. In BAC transgenic zebrafish, cav1 regulatory sequences drove strong expression in ostensibly all epicardial cells and in coronary vascular endothelial cells. Moreover, cav1 mutant zebrafish generated by genome editing showed grossly normal heart development and adult cardiac anatomy, but displayed profound defects in injury-induced cardiomyocyte proliferation and heart regeneration. Our study defines a new platform for the discovery of epicardial lineage markers, genetic tools, and mechanisms of heart regeneration.

Epicardial regeneration is guided by cardiac outflow tract and Hedgehog signalling

In response to cardiac damage, a mesothelial tissue layer enveloping the heart called the epicardium is activated to proliferate and accumulate at the injury site. Recent studies have implicated the epicardium in multiple aspects of cardiac repair: a source of paracrine signals for cardiomyocyte survival or proliferation; a supply of perivascular cells and possibly other cell types like cardiomyocytes; and, a mediator of inflammation^1-9. Yet, the biology and dynamism of the adult epicardium is poorly understood. Here, we created a transgenic line to ablate this cell population in adult zebrafish. We find that genetic depletion of epicardium after myocardial loss inhibits cardiomyocyte proliferation and delays muscle regeneration. The epicardium vigorously regenerates after its ablation, through proliferation and migration of spared epicardial cells as a sheet to cover the exposed ventricular surface in a wave from the chamber base toward its apex. By reconstituting epicardial regeneration ex vivo, we show that extirpation of the bulbous arteriosus (BA), a distinct, smooth muscle-rich tissue structure that distributes outflow from the ventricle, prevents epicardial regeneration. Conversely, experimental repositioning of the BA by tissue recombination initiates epicardial regeneration and can govern its direction. Hedgehog (Hh) ligand is expressed in the BA, and treatment with Hh signaling antagonist arrests epicardial regeneration and blunts the epicardial response to muscle injury. Transplantation of Shh-soaked beads at the ventricular base stimulates epicardial regeneration after BA removal, indicating that Hh signaling can substitute for the BA influence. Thus, the ventricular epicardium has pronounced regenerative capacity, regulated by the neighboring cardiac outflow tract and Hh signaling. These findings extend our understanding of tissue interactions during regeneration and have implications for mobilizing epicardial cells for therapeutic heart repair.

Re-activated adult epicardial progenitor cells are a heterogeneous population molecularly distinct from their embryonic counterparts

Cardiovascular disease remains the major cause of mortality, and cardiac cell therapy has recently emerged as a paradigm for heart repair. The epicardium is a layer of mesothelial cells covering the heart that during development contributes to different cardiovascular lineages, including cardiomyocytes, but which becomes quiescent after birth. We previously revealed that the peptide thymosin beta 4 (Tβ4) can reactivate adult epicardium-derived cells (EPDCs) after myocardial infarction (MI), to proliferate, and differentiate into cardiovascular derivatives. The aim of this study was to provide a lineage characterization of the adult EPDCs relative to the embryonic epicardial lineage and to determine prospective cell fate biases within the activated adult population during cardiovascular repair. Wt1(GFPCre/+) mice were primed with Tβ4 and MI induced by ligation of the left anterior descending coronary artery. Adult WT1(+) GFP(+) EPDCs were fluorescence-activated cell sorted (FACS) at 2, 4, and 7 days after MI. Embryonic WT1(+) GFP(+) EPDCs were isolated from embryonic hearts (E12.5) by FACS, and sorted cells were characterized by real-time quantitative reverse transcriptase-polymerase chain reaction (RT-qPCR) and immunostaining. Adult WT1(+) GFP(+) EPDCs were highly heterogeneous, expressing cardiac progenitor and mesenchymal stem markers. Based on the expression of stem cell antigen-1 (Sca-1), CD44, and CD90, we identified different subpopulations of EPDCs of varying cardiovascular potential, according to marker gene profiles, with a molecular phenotype distinct from the source embryonic epicardial cells at E12.5. Thus, adult WT1(+) GFP(+) cells are a heterogeneous population that when activated can restore an embryonic gene programme, but do not revert entirely to adopt an embryonic phenotype. Potential biases in cardiovascular cell fate suggest that discrete subpopulations of EPDCs might be clinically relevant for regenerative therapy.

Isolation and culture of mouse proepicardium using serum-free conditions

The proepicardium (PE) is an embryonic tissue that gives rise to multipotent vascular progenitors. Most notably the PE gives rise to the epicardium, cardiac fibroblasts, myocardium, and coronary vessels including both vascular smooth muscle and vascular endothelium. Much attention has been given to epicardial-derived cells that show the capacity to differentiate into a wide variety of vascular progenitors including cardiomyocytes. However, it is the PE itself that possesses the greatest potential as a source of multipotent vascular progenitors. We show here a simple method to manually isolate mouse PE at the ninth day of mouse embryonic development and culture highly pure PE tissue in serum-free conditions. This PE culture method allows for the ex vivo analysis of specific growth factors on PE and epicardial development with greater efficiency and precision than existing epicardial culture methods.