Forming the early heart
The heart is the first organ to form during development and is critical for the survival of the embryo. The precise molecular identities of the various cell types that make up the heart during these early stages remain poorly defined. Tyser et al. used a combination of transcriptomic, imaging, and genetic lineage–labeling approaches to profile the molecular identity and precise locations of cells involved in the formation of the mouse embryonic heart. This approach allowed them to identify the earliest known progenitor of the epicardium, the outermost layer of the heart, which is an important source of signals and cells during cardiac development and injury.
Science, this issue p. eabb2986
The vertebrate heart is composed of diverse cell types, all essential for normal cardiac function. In the mouse, the earliest mesodermal progenitors of cardiomyocytes, formed during gastrulation, migrate rostrally from the primitive streak to form the cardiac crescent and initiate contractile activity. The cardiac crescent subsequently undergoes remodeling to form the linear heart tube. There are at least two distinct groups of mesodermal cardiac progenitors, the first and second heart fields (FHF and SHF, respectively), defined broadly on the basis of marker genes expressed in different but overlapping regions of the early embryo. Cells from outside these heart fields can also contribute to the heart. One such structure, the proepicardium, gives rise to the epicardium, the outermost layer of cells of the vertebrate heart. The epicardium provides important paracrine signals and can also generate several cardiac cell types, including cardiomyocytes, vascular smooth muscle, and fibroblasts.
Our current understanding of when and how different cardiac cell types arise during early development is limited. Single-cell transcriptomics offers a powerful approach to characterize the various cell types of the embryonic heart and generate hypotheses about their origin and fate. We therefore combined single-cell RNA sequencing with high-resolution volume imaging and time-lapse microscopy to precisely characterize the cells of the mouse embryonic heart at single-cell resolution. This powerful combinatorial approach provides a unified transcriptional and anatomical definition of cardiac progenitor types and their differentiation trajectories toward cardiomyocytes.
We used manual microdissection to isolate the cardiac region of mouse embryos, from early cardiac crescent to linear heart tube stages, and performed single-cell RNA sequencing. This enabled us to transcriptionally define the cardiac progenitor populations in this region, including FHF and SHF. As a user-friendly community resource, we created a web interface to investigate these data, accessible at https://marionilab.cruk.cam.ac.uk/heartAtlas/. To identify the anatomical locations of these cell populations at single-cell resolution, we used whole-mount immunohistochemistry, multiplexed fluorescence in situ hybridization, and high-resolution volume imaging of combinations of markers. This comprehensive imaging analysis revealed the discrete locations of these transcriptional clusters and highlighted the spatially ordered maturation of cardiomyocytes. It also identified a population of progenitor cells located rostral to the cardiac crescent, at the confluence of the embryonic and extraembryonic mesoderm, which we call the juxta-cardiac field (JCF). Using single-cell resolution time-lapse imaging and genetic lineage labeling, we established that the JCF can contribute to both cardiomyocytes and the proepicardium. The JCF therefore represents the earliest known progenitors of the epicardium, the outermost layer of the vertebrate heart.
This study provides a detailed characterization of the transcriptional states and anatomical locations of cardiac progenitors, as well as their transition states during differentiation toward cardiomyocytes, thereby representing a valuable community resource. Furthermore, it provides fresh insights into the formation of the heart. By identifying the juxta-cardiac field, our work widens the cardiac progenitor region and identifies the earliest progenitors of the proepicardium. This study will contribute to a better understanding of the origin of congenital cardiac defects and provide basic insights for informing the development of regenerative approaches to treat heart disease.
The mammalian heart is derived from multiple cell lineages; however, our understanding of when and how the diverse cardiac cell types arise is limited. We mapped the origin of the embryonic mouse heart at single-cell resolution using a combination of transcriptomic, imaging, and genetic lineage labeling approaches. This mapping provided a transcriptional and anatomic definition of cardiac progenitor types. Furthermore, it revealed a cardiac progenitor pool that is anatomically and transcriptionally distinct from currently known cardiac progenitors. Besides contributing to cardiomyocytes, these cells also represent the earliest progenitor of the epicardium, a source of trophic factors and cells during cardiac development and injury. This study provides detailed insights into the formation of early cardiac cell types, with particular relevance to the development of cell-based cardiac regenerative therapies.