A tree‐like hierarchical branching structure is present in many biological systems, such as the kidney, lung, mammary gland, and blood vessels. Most of these organs form through branching ...morphogenesis, where outward growth results in smaller and smaller branches. However, the blood vasculature is unique in that it exists as two trees (arterial and venous) connected at their tips. Obtaining this organization might therefore require unique developmental mechanisms. As reviewed here, recent data indicate that arterial trees often form in reverse order. Accordingly, initial arterial endothelial cell differentiation occurs outside of arterial vessels. These pre‐artery cells then build trees by following a migratory path from smaller into larger arteries, a process guided by the forces imparted by blood flow. Thus, in comparison to other branched organs, arteries can obtain their structure through inward growth and coalescence. Here, new information on the underlying mechanisms is discussed, and how defects can lead to pathologies, such as hypoplastic arteries and arteriovenous malformations.
Many studies have shown that blood vessel sprouting resembles branching morphogenesis, where smaller branches sprout from larger stems. New work indicates that arteries can also form through inward growth of pre‐specified arterial cells and remodeling. Therefore, the mechanism building hierarchically patterned blood vessel trees might differ between arteries and veins.
Blood vascular endothelial cells (BECs) control the immune response by regulating blood flow and immune cell recruitment in lymphoid tissues. However, the diversity of BEC and their origins during ...immune angiogenesis remain unclear. Here we profile transcriptomes of BEC from peripheral lymph nodes and map phenotypes to the vasculature. We identify multiple subsets, including a medullary venous population whose gene signature predicts a selective role in myeloid cell (vs lymphocyte) recruitment to the medulla, confirmed by videomicroscopy. We define five capillary subsets, including a capillary resident precursor (CRP) that displays stem cell and migratory gene signatures, and contributes to homeostatic BEC turnover and to neogenesis of high endothelium after immunization. Cell alignments show retention of developmental programs along trajectories from CRP to mature venous and arterial populations. Our single cell atlas provides a molecular roadmap of the lymph node blood vasculature and defines subset specialization for leukocyte recruitment and vascular homeostasis.
How mechanotransduction intersects with chemical and transcriptional factors to shape organogenesis is an important question in developmental biology. This is particularly relevant to the ...cardiovascular system, which uses mechanical signals from flowing blood to stimulate cytoskeletal and transcriptional responses that form a highly efficient vascular network. Using this system, artery size and structure are tightly regulated, but the underlying mechanisms are poorly understood. Here, we demonstrate that deletion of
increased the diameter of coronary arteries during mouse embryonic development, a phenotype that followed the initiation of blood flow. At the same time, the BMP signal transducers SMAD1/5/8 were activated in developing coronary arteries. In a culture model of blood flow-induced shear stress, human coronary artery endothelial cells failed to align when either BMPs were inhibited or
was depleted. In contrast to control cells,
deficient cells did not migrate against the direction of shear stress and increased proliferation rates specifically under flow. Similar alterations were seen in coronary arteries
Thus, endothelial cells perceive the direction of blood flow and respond through SMAD signaling to regulate artery size.
Epicardial cells on the heart's surface give rise to coronary artery smooth muscle cells (caSMCs) located deep in the myocardium. However, the differentiation steps between epicardial cells and ...caSMCs are unknown as are the final maturation signals at coronary arteries. Here, we use clonal analysis and lineage tracing to show that caSMCs derive from pericytes, mural cells associated with microvessels, and that these cells are present in adults. During development following the onset of blood flow, pericytes at arterial remodeling sites upregulate Notch3 while endothelial cells express Jagged-1. Deletion of Notch3 disrupts caSMC differentiation. Our data support a model wherein epicardial-derived pericytes populate the entire coronary microvasculature, but differentiate into caSMCs at arterial remodeling zones in response to Notch signaling. Our data are the first demonstration that pericytes are progenitors for smooth muscle, and their presence in adult hearts reveals a new potential cell type for targeting during cardiovascular disease.
The establishment of distinct cellular identities was pivotal during the evolution of Metazoa, enabling the emergence of an array of specialized tissues with different functions. In most animals ...including vertebrates, cell specialization occurs in response to a combination of intrinsic (e.g., cellular ontogeny) and extrinsic (e.g., local environment) factors that drive the acquisition of unique characteristics at the single‐cell level. The first functional organ system to form in vertebrates is the cardiovascular system, which is lined by a network of endothelial cells whose organ‐specific characteristics have long been recognized. Recent genetic analyses at the single‐cell level have revealed that heterogeneity exists not only at the organ level but also between neighboring endothelial cells. Thus, how endothelial heterogeneity is established has become a key question in vascular biology. Drawing upon evidence from multiple organ systems, here we will discuss the role that lineage history may play in establishing endothelial heterogeneity.
Recent technological advances have revealed unexpected levels of heterogeneity among endothelial cells in different organs and tissues. Here we discuss the role that lineage history plays during the specification of blood and lymphatic endothelial cells, and its possible contribution to the establishment of their heterogeneity.
BACKGROUND:Endothelial cells (ECs) display considerable functional heterogeneity depending on the vessel and tissue in which they are located. Whereas these functional differences are presumably ...imprinted in the transcriptome, the pathways and networks that sustain EC heterogeneity have not been fully delineated.
METHODS:To investigate the transcriptomic basis of EC specificity, we analyzed single-cell RNA sequencing data from tissue-specific mouse ECs generated by the Tabula Muris consortium. We used a number of bioinformatics tools to uncover markers and sources of EC heterogeneity from single-cell RNA sequencing data.
RESULTS:We found a strong correlation between tissue-specific EC transcriptomic measurements generated by either single-cell RNA sequencing or bulk RNA sequencing, thus validating the approach. Using a graph-based clustering algorithm, we found that certain tissue-specific ECs cluster strongly by tissue (eg, liver, brain), whereas others (ie, adipose, heart) have considerable transcriptomic overlap with ECs from other tissues. We identified novel markers of tissue-specific ECs and signaling pathways that may be involved in maintaining their identity. Sex was a considerable source of heterogeneity in the endothelial transcriptome and we discovered Lars2 to be a gene that is highly enriched in ECs from male mice. We found that markers of heart and lung ECs in mice were conserved in human fetal heart and lung ECs. We identified potential angiocrine interactions between tissue-specific ECs and other cell types by analyzing ligand and receptor expression patterns.
CONCLUSIONS:We used single-cell RNA sequencing data generated by the Tabula Muris consortium to uncover transcriptional networks that maintain tissue-specific EC identity and to identify novel angiocrine and functional relationships between tissue-specific ECs.
Coronary arteries bring blood flow to the heart muscle. Understanding the developmental program of the coronary arteries provides insights into the treatment of coronary artery diseases. Multiple ...sources have been described as contributing to coronary arteries including the proepicardium, sinus venosus （SV）, and endocardium. However, the developmental origins of coronary vessels are still under intense study. We have produced a new genetic tool for studying coronary development, an AplnCreER mouse line, which expresses an inducible Cre recombinase specifically in developing coronary vessels. Quantitative analysis of coronary development and timed induction of AplnCreER fate tracing showed that the progenies of subepicardial endothelial cells （ECs） both invade the compact myocardium to form coronary arteries and remain on the surface to produce veins. We found that these subepicardial ECs are the major sources of intramyocardial coronary vessels in the developing heart. In vitro explant assays indicate that the majority of these subepicardial ECs arise from endocardium of the SV and atrium, but not from ventricular endocardium. Clonal analysis of Apln-positive cells indicates that a single subepicardial EC contributes equally to both coronary arteries and veins. Collectively, these data suggested that subepieardial ECs are the major source of intramyocardial coronary arteries in the ventricle wall, and that coronary arteries and veins have a common origin in the developing heart.
The cardiac endothelium influences ventricular chamber development by coordinating trabeculation and compaction. However, the endothelial-specific molecular mechanisms mediating this coordination are ...not fully understood. Here, we identify the Sox7 transcription factor as a critical cue instructing cardiac endothelium identity during ventricular chamber development. Endothelial-specific loss of Sox7 function in mice results in cardiac ventricular defects similar to non-compaction cardiomyopathy, with a change in the proportions of trabecular and compact cardiomyocytes in the mutant hearts. This phenotype is paralleled by abnormal coronary artery formation. Loss of Sox7 function disrupts the transcriptional regulation of the Notch pathway and connexins 37 and 40, which govern coronary arterial specification. Upon Sox7 endothelial-specific deletion, single-nuclei transcriptomics analysis identifies the depletion of a subset of Sox9/Gpc3-positive endocardial progenitor cells and an increase in erythro-myeloid cell lineages. Fate mapping analysis reveals that a subset of Sox7-null endothelial cells transdifferentiate into hematopoietic but not cardiomyocyte lineages. Our findings determine that Sox7 maintains cardiac endothelial cell identity, which is crucial to the cellular cross-talk that drives ventricular compaction and coronary artery development.
Most cell fate trajectories during development follow a diverging, tree-like branching pattern, but the opposite can occur when distinct progenitors contribute to the same cell type. During this ...convergent differentiation, it is unknown if cells 'remember' their origins transcriptionally or whether this influences cell behavior. Most coronary blood vessels of the heart develop from two different progenitor sources-the endocardium (Endo) and sinus venosus (SV)-but whether transcriptional or functional differences related to origin are retained is unknown. We addressed this by combining lineage tracing with single-cell RNA sequencing (scRNAseq) in embryonic and adult mouse hearts. Shortly after coronary development begins, capillary endothelial cells (ECs) transcriptionally segregated into two states that retained progenitor-specific gene expression. Later in development, when the coronary vasculature is well established but still remodeling, capillary ECs again segregated into two populations, but transcriptional differences were primarily related to tissue localization rather than lineage. Specifically, ECs in the heart septum expressed genes indicative of increased local hypoxia and decreased blood flow. Adult capillary ECs were more homogeneous with respect to both lineage and location. In agreement, SV- and Endo-derived ECs in adult hearts displayed similar responses to injury. Finally, scRNAseq of developing human coronary vessels indicated that the human heart followed similar principles. Thus, over the course of development, transcriptional heterogeneity in coronary ECs is first influenced by lineage, then by location, until heterogeneity declines in the homeostatic adult heart. These results highlight the plasticity of ECs during development, and the validity of the mouse as a model for human coronary development.
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