Epicardial Cells: Roles in Heart Formation and Repair

Epicardial cells play a crucial role in heart development and repair, contributing to cardiac function by forming the outermost layer of the heart and influencing the development of blood vessels and connective tissue. Their ability to transition into different cell types is essential for both embryonic heart formation and the organ’s response to injury.

Understanding how these cells function provides insight into potential therapeutic strategies for heart disease. Researchers are exploring ways to harness their regenerative properties to improve cardiac repair.

Origins During Heart Development

Epicardial cells originate from the proepicardium, a transient cluster of mesothelial cells near the venous pole of the developing heart. This structure, adjacent to the septum transversum, gives rise to cells that migrate toward the myocardium, eventually enveloping the heart tube. This process begins around embryonic day 9.5 (E9.5) in mice and between Carnegie stages 10 and 12 in humans. As these cells spread across the myocardial surface, they establish the epicardium, which serves as a reservoir for progenitor cells contributing to various cardiac lineages.

Once established, the epicardium undergoes molecular and structural changes that influence heart development. Signals from the myocardium, including retinoic acid and fibroblast growth factors (FGFs), regulate epicardial adhesion, proliferation, and later differentiation. Lineage-tracing studies in murine models show that epicardial-derived cells contribute to fibroblasts, smooth muscle cells, and pericytes, all essential for coronary vasculature formation. Without proper epicardial development, coronary artery anomalies and myocardial hypoplasia can occur.

The interaction between epicardial cells and the myocardium is dynamic, with reciprocal signaling shaping both tissues. Myocardial-secreted factors such as platelet-derived growth factor (PDGF) and transforming growth factor-beta (TGF-β) influence epicardial behavior, while epicardial-derived signals, including Wnt and Hedgehog pathways, modulate myocardial proliferation and compaction. Disruptions in these networks have been linked to congenital heart defects such as hypoplastic left heart syndrome. Genetic knockout models reveal that loss of epicardial-specific transcription factors, such as Wilms’ tumor 1 (WT1) and T-box 18 (Tbx18), impairs epicardial migration and differentiation.

Epithelial-To-Mesenchymal Transition

Epicardial cells undergo epithelial-to-mesenchymal transition (EMT), allowing them to detach from the epicardial layer and migrate into the myocardium. This transformation generates epicardial-derived cells (EPDCs), which contribute to fibroblasts, smooth muscle cells, and pericytes. EMT is regulated by signaling pathways such as TGF-β, Wnt, and Notch, which orchestrate changes in adhesion, polarity, and cytoskeletal organization. A hallmark of EMT is the downregulation of epithelial markers like E-cadherin and the upregulation of mesenchymal markers such as N-cadherin and vimentin, enabling migration.

During EMT, transcription factors Snail, Slug, and Twist suppress epithelial traits while promoting mesenchymal characteristics. This molecular shift allows cells to dissociate from the epicardium and invade the subepicardial space, where they differentiate further. Murine studies show that disrupting EMT-related transcription factors results in severe coronary vasculature defects. Extracellular matrix remodeling, mediated by matrix metalloproteinases (MMPs), facilitates migration by breaking down basement membrane components.

The fate of EPDCs depends on signaling cues. Myocardial-secreted fibroblast growth factors (FGFs) promote vascular smooth muscle differentiation, while platelet-derived growth factor receptor-beta (PDGFR-β) signaling guides perivascular integration. However, excessive EMT can lead to fibrosis, with persistent TGF-β signaling driving a pro-fibrotic phenotype linked to congenital heart defects and myocardial fibrosis. Understanding how EMT is regulated remains a key focus in cardiac regeneration research.

Molecular Markers And Signaling Factors

Epicardial cells are defined by molecular markers that regulate their function. WT1 is a key transcription factor essential for epicardial proliferation and differentiation, with knockout models showing impaired migration and coronary vasculature defects. Tbx18, highly expressed in early epicardial progenitors, contributes to epicardial expansion and movement. While WT1 is more involved in EMT, Tbx18 primarily influences epicardial sheet formation.

Several signaling pathways govern epicardial function. Retinoic acid (RA) signaling is one of the earliest to activate, regulating adhesion and migration. FGFs direct EPDC differentiation, particularly toward vascular smooth muscle identity. Wnt and Hedgehog (Hh) pathways establish reciprocal communication between the epicardium and myocardium, with Wnt signaling promoting epicardial expansion and EMT, while Hedgehog signaling regulates myocardial proliferation. Disruptions in these pathways contribute to congenital heart defects.

Role In Cardiac Tissue Integrity

The epicardium plays a structural role in maintaining cardiac integrity. It contributes to the extracellular matrix (ECM), secreting collagen types I and III, fibronectin, and laminins, which regulate myocardial stiffness and prevent excessive deformation. Dysfunction in ECM composition can lead to conditions such as dilated cardiomyopathy.

Epicardial cells also support coronary vasculature development, ensuring oxygen and nutrient delivery. EPDCs differentiate into vascular smooth muscle cells and pericytes, reinforcing vessel stability. Murine lineage-tracing models show that impaired epicardial signaling reduces coronary artery branching, leading to inadequate perfusion and increased susceptibility to myocardial stress.

Activation Following Cardiac Injury

In the adult heart, epicardial cells are typically quiescent but reactivate following cardiac injury. This reactivation involves the re-expression of WT1 and Tbx18 and the initiation of EMT. After myocardial infarction, epicardial cells proliferate and migrate into the injured region, contributing to tissue remodeling. While differentiation into fibroblasts and vascular smooth muscle cells stabilizes damaged areas, excessive activation can lead to fibrosis and impaired function.

Modulating epicardial activation could improve cardiac regeneration. Experimental models show that stimulating pathways like Wnt and Hippo-YAP enhances vascular support while limiting fibrosis. Small-molecule inhibitors targeting TGF-β reduce post-injury fibrosis, while strategies promoting epicardial-derived angiogenesis improve myocardial recovery. Understanding these mechanisms offers potential therapeutic interventions for restoring cardiac function after ischemic events.

Laboratory Techniques For Culturing

Studying epicardial cells in vitro requires specialized culture techniques to preserve their physiological characteristics. These methods range from traditional monolayer cultures to advanced three-dimensional (3D) systems that better mimic the cardiac microenvironment. Co-culturing with other cardiac cell types enhances experimental relevance by replicating in vivo signaling interactions.

Monolayer Culture Methods

Monolayer cultures provide a straightforward platform for expanding epicardial cells and analyzing their properties. Cells are typically isolated from embryonic or adult heart tissue and plated on coated surfaces that support adhesion and proliferation. Serum-containing media supplemented with growth factors like FGF and RA maintains epicardial identity, while low-serum conditions can induce EMT, allowing researchers to study differentiation pathways.

Three-Dimensional Culture Systems

Three-dimensional culture systems incorporate biomimetic scaffolds or hydrogels to replicate the epicardium’s structural complexity. These models allow epicardial cells to form layered structures, enabling more accurate assessments of migration, EMT, and ECM deposition. Hydrogels derived from decellularized cardiac ECM have been used to study epicardial contributions to fibrosis and angiogenesis. Advanced techniques, such as organoids or bioprinted constructs, enhance investigations into epicardial function for regenerative medicine applications.

Co-Culture With Other Cardiac Cell Types

Co-culturing epicardial cells with cardiomyocytes, endothelial cells, or fibroblasts recreates essential signaling networks for heart development and repair. These models help researchers examine how epicardial-derived factors influence myocardial proliferation, vascular formation, and fibrotic responses. Studies show that epicardial cells secrete paracrine signals like Sonic Hedgehog (Shh) and VEGF, which enhance cardiomyocyte survival and promote angiogenesis. Integrating multiple cell types into co-culture systems provides deeper insights into cardiac homeostasis and potential therapeutic targets for heart regeneration.