Biomechanical forces are emerging as critical regulators of embryogenesis, particularly in the developing cardiovascular system 1,2 . After initiation of the heartbeat in vertebrates, cells lining the ventral aspect of the dorsal aorta, the placental vessels, and the umbilical and vitelline arteries initiate expression of the transcription factor Runx1 (refs 3-5), a master regulator of haematopoiesis, and give rise to haematopoietic cells 4 . It remains unknown whether the biomechanical forces imposed on the vascular wall at this developmental stage act as a determinant of haematopoietic potential 6 . Here, using mouse embryonic stem cells differentiated in vitro, we show that fluid shear stress increases the expression of Runx1 in CD41 + c-Kit + haematopoietic progenitor cells 7 ,concomitantly augmenting their haematopoietic colony-forming potential. Moreover, we find that shear stress increases haematopoietic colony-forming potential and expression of haematopoietic markers in the paraaortic splanchnopleura/aorta-gonads-mesonephros of mouse embryos and that abrogation of nitric oxide, a mediator of shear-stress-induced signalling 8 , compromises haematopoietic potential in vitro and in vivo. Collectively, these data reveal a critical role for biomechanical forces in haematopoietic development.In the mouse, the first haemogenic areas appear in the yolk sac starting at day 7.5 of development (E7.5) 9 . After the establishment of circulation and the onset of vascular flow at day 8.5, additional haemogenic sites appear between day 9 and 10.5 as Runx1 + regions within

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Mechanical properties of cells have been shown to have a significant role in disease as in many instances cell stiffness changes when a cell is no longer healthy. We present a high throughput microfluidics based approach that exploits the connection between travel time of a cell through a narrow passage and cell stiffness. The system resolves both cell travel time and relative cell diameter while retaining information on the cell level. We show that stiffer cells have longer transit times than less stiff ones and that cell size significantly influences travel times. Experiments with untreated HeLa cells and cells made compliant with Latrunculin A and Cytochalasin B further demonstrate that travel time is influenced by cell stiffness, with the compliant cells having faster transit time.

+ cells, suggesting that the latter population is not derived from BM progenitors, thus disproving our initial hypothesis. To confirm these results and to eliminate potential confounding effects associated with radiation-induced damage to the heart, we repeated the experiment by performing limited BM ablation through focused irradiation of the femurs. This altered irradiation strategy did not qualitatively alter the results and confirmed that most myocardial B cells are BM derived (Supplemental Figure 1, A-C; supplemental material available online with this article; https://doi.org/10.1172/jci.insight.134700DS1).Myocardial B cells recirculate between the heart, blood, and spleen. The investigation of the origin of myocardial B cells described above did not explain the existence of a large pool of myocardial B cells. Therefore, we formulated a different hypothesis. Since several populations of resident lymphocytes have been described (14), and the heart harbors resident leukocyte populations (15), we hypothesized that myocardial B cells might represent a population of tissue-resident B lymphocytes. To test this hypothesis, we conjoined CD45.1 and CD45.2 mice via parabiosis and analyzed myocardial B cell chimerism 3 weeks after surgery. The results indicated that both CD19 + CD11b + and CD19 + CD11bcells displayed approximately 50% chimerism after 3 weeks of parabiosis ( Figure 1C), suggesting that the vast majority of myocardial B cells have immediate access to the circulation and freely recirculate between conjoined animals. To confirm this result and investigate the trafficking dynamics of myocardial B cells, we performed heterotopic heart transplant. We transplanted the heart of a CD45.2 mouse into the abdomen of a CD45.1 mouse so that both hearts were connected to the vasculature and perfused at the same time. Untransplanted hearts from CD45.1 and CD45.2 mice displayed clear populations of CD45.1 + B cells and CD45.2 + B cells, respectively ( Figure 1D). The transplant recipient mice were sacrificed 4 days after surgery, and the recipient and transplanted hearts were harvested and analyzed. Virtually all donor-derived B cells in the transplanted heart were replaced by recipient-derived B cells ( Figure 1D). Only a very small population of donor-derived cells remained in the transplanted heart ( Figure 1D). A small population of donor-derived cells was detected in the endogenous heart of the recipient mouse ( Figure 1D), whereas the vast majority of cells transplanted with the donor heart were not observed in the myocardial tissue of the recipient mouse. This result confirmed that the vast majority of myocardial B cells are not tissue-resident cells and also suggested that myocardial B cells can migrate from the heart to other organs.To identify where myocardial B cells traffic after leaving the heart, we analyzed the spleen and blood of mice that received the heart transplant. The results showed that, similarly to the myocardium, the spleen and blood of recipient mice also contained a small but clearly detecte...