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/β-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.心脏将多种细胞谱系整合为功能整体，其中包括心包（pericardium）——一种支撑心脏运动、维持稳态并参与免疫应答的间皮囊（mesothelial sac）。尽管心包发挥着至关重要的作用，但其发育起源仍因不同的模型体系而尚不明确。本研究通过在斑马鱼（zebrafish）中开展活细胞成像（live imaging）、谱系示踪（lineage tracking）及单细胞转录组学（single-cell transcriptomics）分析，发现心包起源于侧板中胚层（lateral plate mesoderm）中特化的前部间皮祖细胞，且其来源与经典心区（classic heart field）截然不同。通过对斑马鱼转基因报告基因（transgenic reporters）的成像分析，本研究记录到侧板中胚层细胞起源于经典心区的外侧，并处于连续的间皮祖细胞区域中。对表达hand2的侧板中胚层进行单细胞转录组学分析及细胞轨迹推演，结果揭示了间皮细胞与心脏前体细胞的不同亚群，其中包括与心肌细胞谱系截然不同的心包前体细胞。间皮细胞的基因表达特征在哺乳动物中保守存在，并延续至出生后发育阶段。基于光片的活细胞成像（light sheet-based live-imaging）与机器学习辅助的细胞追踪（machine learning-supported cell tracking）结果显示，在心管形成（heart tube formation）过程中，位于心区前缘的心包前体细胞会向前内侧迁移并发生融合，进而包裹胚胎心脏，形成单个心包腔（pericardial cavity）。即便心管形成受到遗传扰动，心包的形成过程仍可正常进行，表明二者的发育过程彼此解偶联。经典Wnt/β-连环蛋白（canonical Wnt/β-catenin）信号通路可调控心包细胞的数量：抑制经典Wnt信号会导致心包上皮（pericardial epithelium）细胞数量减少并出现扩张。本研究将小儿扩张型心肌病（pediatric dilated cardiomyopathy）中发现的SFRP家族分泌型Wnt拮抗剂（secreted Wnt antagonists of the SFRP family）的异常表达与心包僵硬度（pericardial stiffness）升高联系起来：在儿茶酚胺（catecholamines）水平升高的条件下，sFRP1可通过原子力显微镜（atomic force microscopy）检测的方式，使新生大鼠（neonatal rats）的心肌细胞僵硬度升高。综上，本研究的数据将心包形成作为一个独立过程整合至心脏形态发生（heart morphogenesis）中，并将心包僵硬度等心包组织特性异常与小儿心肌病（pediatric cardiomyopathies）联系起来。