Although the brain comprises only 2% of body weight, it receives 15% of cardiac output and consumes 20% of total body oxygen delivered through its blood vasculature. The brain blood vasculature consists of a highly branched vessel network that is tailored to efficiently deliver oxygen and nutrients to each brain region. However, little is known about how the brain vasculature develops. Using in vivo long-term serial confocal imaging of zebrafish larvae, we analyze this process and find that the developing midbrain vasculature undergoes not only vessel growth but also blood flow-driven vessel pruning. We show that vessel pruning occurs preferentially at loop-shaped vessel segments via the migration of endothelial cells to adjacent unpruned segments; over time, such vessel pruning reduces the complexity of the early primitive midbrain vasculature. We also observe that pruned vessel segments exhibit a lower and more variable blood flow than do unpruned segments and that the local blocking of blood flow triggers vessel pruning. By contrast, increases in blood flow impair vessel pruning. Finally, we show that pruning events can be predicted using a haemodynamics based mathematical model of the midbrain vasculature. These findings demonstrate the existence of brain vessel pruning during development and provide novel insights into the role of haemodynamics in brain vascular refinement. The brain blood vasculature consists of a highly ramified vessel network that is tailored to meet its physiological functions. How the brain vasculature is formed has long been fascinating biologists. Here we report that the developing vasculature in the zebrafish midbrain undergoes not only angiogenesis but also extensive vessel pruning, which is driven by changes in blood flow. This pruning process shapes the initial exuberant interconnected meshwork into a simplified architecture. Using in vivo long-term serial confocal imaging of the same zebrafish larvae during 1.5–7.5 d post-fertilization, we found that the early formed midbrain vasculature consisted of many vessel loops and higher order segments. Vessel pruning occurred preferentially at loop-forming segments via a process mainly involving lateral migration of endothelial cells (ECs) from pruned to unpruned segments rather than EC apoptosis, leading to gradual reduction in the vasculature complexity with development. Compared to unpruned ones, pruned segments exhibited a low and variable blood flow, which further decreased irreversibly prior to the onset of pruning. Local blockade of blood flow with micro-bead obstruction led to vessel pruning, whereas increasing blood flow by noradrenergic elevation of heartbeat impeded the pruning process. Furthermore, the occurrence of vessel pruning could be largely predicted by haemodynamics-based numerical simulation of vasculature refinement. Thus, changes of blood flow drive vessel pruning via lateral migration of ECs, leading to the simplification of the vasculature and possibly efficient routing of blood flow in the developing brain.

Mural cells of the vessel wall, namely pericytes and vascular smooth muscle cells, are essential for vascular integrity. The developmental sources of these cells and molecular mechanisms controlling their progenitors in the heart are only partially understood. Here we show that endocardial endothelial cells are progenitors of pericytes and vascular smooth muscle cells in the murine embryonic heart. Endocardial cells undergo endothelial–mesenchymal transition and convert into primitive mesenchymal progenitors expressing the platelet-derived growth factor receptors, PDGFRα and PDGFRβ. These progenitors migrate into the myocardium, differentiate and assemble the wall of coronary vessels, which requires canonical Wnt signalling involving Frizzled4, β-catenin and endothelial cell-derived Wnt ligands. Our findings identify a novel and unexpected population of progenitors for coronary mural cells with potential relevance for heart function and disease conditions.

Radiotherapy and chemotherapy disrupt bone vasculature, but the underlying causes and mechanisms enabling vessel regeneration after bone marrow (BM) transplantation remain poorly understood. Here, we show that loss of hematopoietic cells per se, in response to irradiation and other treatments, triggers vessel dilation, permeability, and endothelial cell (EC) proliferation. We further identify a small subpopulation of Apelin-expressing (Apln⁺) ECs, representing 0.003% of BM cells, that is critical for physiological homeostasis and transplant-induced BM regeneration. Genetic ablation of Apln⁺ ECs or Apln-CreER-mediated deletion of Kitl and Vegfr2 disrupt hematopoietic stem cell (HSC) maintenance and contributions to regeneration. Consistently, the fraction of Apln⁺ ECs increases substantially after irradiation and promotes normalization of the bone vasculature in response to VEGF-A, which is provided by transplanted hematopoietic stem and progenitor cells (HSPCs). Together, these findings reveal critical functional roles for HSPCs in maintaining vascular integrity and for Apln⁺ ECs in hematopoiesis, suggesting potential targets for improving BM transplantation.

Abnormal hematopoiesis advances cardiovascular disease by generating excess inflammatory leukocytes that attack the arteries and the heart. The bone marrow niche regulates hematopoietic stem cell proliferation and hence the systemic leukocyte pool, but whether cardiovascular disease affects the hematopoietic organ’s microvasculature is unknown. Here we show that hypertension, atherosclerosis and myocardial infarction (MI) instigate endothelial dysfunction, leakage, vascular fibrosis and angiogenesis in the bone marrow, altogether leading to overproduction of inflammatory myeloid cells and systemic leukocytosis. Limiting angiogenesis with endothelial deletion of Vegfr2 (encoding vascular endothelial growth factor (VEGF) receptor 2) curbed emergency hematopoiesis after MI. We noted that bone marrow endothelial cells assumed inflammatory transcriptional phenotypes in all examined stages of cardiovascular disease. Endothelial deletion of Il6 or Vcan (encoding versican), genes shown to be highly expressed in mice with atherosclerosis or MI, reduced hematopoiesis and systemic myeloid cell numbers in these conditions. Our findings establish that cardiovascular disease remodels the vascular bone marrow niche, stimulating hematopoiesis and production of inflammatory leukocytes.