The pericardium forms as a distinct structure during heart formation.

The heart is formed from diverse cell lineages that assemble into a functional unit, including the pericardium, a mesothelial sac that supports movement, homeostasis, and immune responses. However, its developmental origins remain unresolved. Here, we find the pericardium forms within the lateral plate mesoderm from dedicated mesothelial progenitors that are distinct from the classic heart field. Imaging of transgenic zebrafish reporters documents lateral plate mesoderm cells that emerge lateral of the heart field among a continuous mesothelial progenitor band. Single-cell transcriptomics and trajectories of hand2-expressing lateral plate mesoderm reveal distinct populations of mesothelial precursors, including pericardial precursors. Their mesothelial gene expression signature is conserved in mammals and carries over to post-natal development. Light sheet imaging and machine learning-supported cell tracking documents the migration of pericardial precursors from the edge of the heart field to form the pericardial cavity. Genetic perturbations reveal this process occurs independently of heart formation, with canonical Wnt/β-catenin signaling modulating pericardial cell number and tissue rigidity. We connect the pathological expression of secreted Wnt antagonists of the SFRP family found in pediatric dilated cardiomyopathy to increased pericardial stiffness in neonatal rats. Altogether, our data integrate pericardium formation as an independent process into heart morphogenesis.