The pericardium forms as a distinct structure during heart formation
The heart integrates diverse cell lineages into a functional unit, including the pericardium, a mesothelial sac that supports heart movement, homeostasis, and immune responses. However, despite its critical roles, the developmental origins of the pericardium remain uncertain due to disparate models. Here, using live imaging, lineage tracking, and single-cell transcriptomics in zebrafish, we find the pericardium forms within the lateral plate mesoderm from dedicated anterior mesothelial progenitors and distinct from the classic heart field. Imaging of transgenic reporters in zebrafish documents lateral plate mesoderm cells that emerge lateral of the classic heart field and among a continuous mesothelial progenitor field. Single-cell transcriptomics and trajectories of hand2-expressing lateral plate mesoderm reveal distinct populations of mesothelial and cardiac precursors, including pericardial precursors that are distinct from the cardiomyocyte lineage. The mesothelial gene expression signature is conserved in mammals and carries over to postnatal development. Light sheet-based live-imaging and machine learning-supported cell tracking documents that during heart tube formation, pericardial precursors that reside at the anterior edge of the heart field migrate anteriorly and medially before fusing, enclosing the embryonic heart to form a single pericardial cavity. Pericardium formation proceeds even upon genetic disruption of heart tube formation, uncoupling the two structures. Canonical Wnt/ß-catenin signaling modulates pericardial cell number, resulting in a stretched pericardial epithelium with reduced cell number upon canonical Wnt inhibition. We connect the pathological expression of secreted Wnt antagonists of the SFRP family found in pediatric dilated cardiomyopathy to increased pericardial stiffness: sFRP1 in the presence of increased catecholamines causes cardiomyocyte stiffness in neonatal rats as measured by atomic force microscopy. Altogether, our data integrate pericardium formation as an independent process into heart morphogenesis and connect disrupted pericardial tissue properties such as pericardial stiffness to pediatric cardiomyopathies. Overall design: We sought to probe whether pericardial progenitors are a transcriptionally distinct cell population in the emerging LPM and what gene expression signature defines them. We conducted a 10xGenomics-based single-cell RNA-sequencing (scRNA-seq) analysis of zebrafish LPM cells at 10 hpf (tailbud stage, end of gastrulation) when the LPM has coalesced into its bilateral stripe architecture. We asked if this differential transcriptome signature between the myocardium and pericardium is also detectable in mammalian systems and remains in more mature hearts. We compiled cluster-defining LPM gene lists for myocardium versus mesothelium/pericardium as informed from our zebrafish scRNA-seq and prior work. We then compared the expression of these select gene lists to bulk RNA-sequencing of isolated pericardia and left ventricle myocardia from neonatal 3-week-old rats (Rattus norvegicus).