Figure 1. (Left) Illustration of a forward-propagating wave that increases pressure and flow. (Right) A forward-propagating wave encounters an impedance mismatch (increased vessel stiffness indicated by thicker wall) and is partially transmitted and partially reflected. The reflected wave propagates backward and causes a further increase in pressure but a decreased flow. Red arrows indicate blood flow or average blood velocity over a cross-section (larger arrow means higher flow/velocity), while black arrows indicate wave speed and direction of wave propagation. Increased pressure is indicated by increased vessel diameter. Time plots (bottom) indicate the evolution of pressure and flow at three locations along the vessel indicated with color-coded circles.
Transcranial Doppler (TCD) ultrasound is a painless test that uses sound waves to examine blood flow in your brain. Your doctor has recommended that you have this test to diagnosis a medical condition that affects blood flow to and within the brain. The test is also used to monitor the results of certain treatments, for example, the breakup of clots inside brain arteries.
During TCD, sound waves are sent through the tissues of your skull. These sound waves reflect off blood cells moving within your blood vessels, allowing the radiologist or neurologist to interpret their speed and direction. The sound waves are recorded and displayed on a computer screen.
Hematopoiesis in vertebrate embryos occurs in temporally and spatially overlapping waves in close proximity to blood vascular endothelial cells. Initially, yolk sac hematopoiesis produces primitive erythrocytes, megakaryocytes, and macrophages. Thereafter, sequential waves of definitive hematopoiesis arise from yolk sac and intraembryonic hemogenic endothelia through an endothelial-to-hematopoietic transition (EHT). During EHT, the endothelial and hematopoietic transcriptional programs are tightly co-regulated to orchestrate a shift in cell identity. In the yolk sac, EHT generates erythro-myeloid progenitors, which upon migration to the liver differentiate into fetal blood cells, including erythrocytes and tissue-resident macrophages. In the dorsal aorta, EHT produces hematopoietic stem cells, which engraft the fetal liver and then the bone marrow to sustain adult hematopoiesis. Recent studies have defined the relationship between the developing vascular and hematopoietic systems in animal models, including molecular mechanisms that drive the hemato-endothelial transcription program for EHT. Moreover, human pluripotent stem cells have enabled modeling of fetal human hematopoiesis and have begun to generate cell types of clinical interest for regenerative medicine.
Early studies identified two hematopoietic waves in the mammalian embryo: an early extra-embryonic wave in the yolk sac that produces transient blood cells and was termed primitive, and an intraembryonic wave that generates hematopoietic stem cells (HSCs) termed definitive. Subsequently, the yolk sac was shown to also produce hematopoietic cells that seed the embryo and persist into fetal and, to some extent, adult life . For this reason, a model of three hematopoietic waves (Fig. 1) is now widely accepted: (1) primitive hematopoiesis, which takes place in the yolk sac and produces short-lived blood cells; (2) pro-definitive hematopoiesis, which originates in the yolk sac but produces hematopoietic progenitors that seed the embryo to contribute blood cells until birth; (3) definitive hematopoiesis, which originates in the embryo and produces HSCs that initially seed the fetal liver and thereafter permanently colonize the bone marrow to support adult hematopoiesis. All three waves are spatiotemporally connected to blood vascular development.
During EHT, RUNX1 acts in concert with other key transcription factors such as TAL1 and GATA2. In the embryo, TAL1 is necessary for the specification of all three hematopoietic waves [60, 61], whereas in the adult, TAL1 is required for the maintenance of HSCs and hematopoietic progenitors as well as for blood lineage commitment . TAL1 action is indeed strongly context-dependent, as it forms complexes with other hematopoietic transcription factors; thus, TAL1 regulates HSC maintenance with GATA2 but directs erythroid and megakaryocytic differentiation with GATA1 [60,61,62]. GATA2 is required for both pro-definitive and definitive hematopoiesis, and mutant mouse embryos lacking GATA2 die before E11.5 due to severe anemia . By contrast, GATA2 is not strictly required for primitive hematopoiesis, when the main factors driving primitive erythropoiesis are GATA1 and TAL1 .
Downstream of extrinsic signals, cell cycle regulation has emerged as a key player in orchestrating hemogenic specification and EHT. In the yolk sac, retinoic acid-dependent notch activation mediates cell cycle arrest to create permissive conditions for endothelial cells to become hemogenic . In the AGM, the anatomical position of emerging progenitors within hematopoietic clusters correlates with progressive cell cycle activation, whereby slowly cycling cells are frequently found at the base of the cluster in association with the underlying endothelium, while rapidly cycling cells are located at apical positions within the cluster . Most HSCs in the fetal liver are actively cycling, possibly to expand the stem cell pool and rapidly produce blood cells; by contrast, HSCs adopt a quiescent phenotype later during development and upon seeding the bone marrow .
Induction of distinct hematopoietic waves using hPSCs. Modulating key signalling pathways early during stepwise hPSC differentiation enables the production of primitive versus (pro-)definitive hematopoietic cells (only key steps shared between various different protocols are shown). FGF and BMP induce hPSC differentiation towards mesoderm. When combined with activin activation and WNT inhibition, mesodermal cells differentiate further into primitive hematopoietic cells. Instead, WNT activation with activin inhibition induces mesodermal cells to differentiate further into endothelial cells, including hemogenic endothelial cells that express RUNX1 and undergo EHT to produce (pro-)definitive hematopoietic cells
Understanding the molecular mechanisms that drive each of these distinct stages in vivo is fundamental to recapitulate the progression of developmental hematopoiesis in vitro. Early studies with hPSCs showed that the FGF and activin/nodal signalling pathways are master gatekeepers of pluripotency [86, 98], but that they also regulate germ layer specification [95, 96]. This dual role of FGF and activin/nodal signalling depends on crosstalk with other key signalling pathways, such as the BMP and WNT pathways. In particular, the functional interaction between these and other pathways serves to recreate signals that in vivo convey the position of cells within the embryo along the anteroposterior axis, where multiple morphogenetic gradients of agonists and inhibitors evoke position-dependent fate decisions [97, 99]. Thus, manipulating the relative activation levels of core signalling pathways such as FGF, activin/nodal, BMP, and WNT allows proper germ layer specification and subsequent germ layer patterning in hPSC cultures [95, 96, 100,101,102] (Fig. 3). Despite these advances, recapitulating the three distinct hematopoietic waves using hPSC differentiation methods remains a considerable challenge.
Several protocols have achieved hPSC differentiation into hemogenic endothelial cells capable of undergoing EHT [14, 16, 31, 42,43,44, 87, 94] (Fig. 3). Although early culture methods produced a mixture of primitive and definitive blood cells, we now have protocols that impair the production of primitive progenitors and generate endothelial cells that undergo EHT to produce more mature (pro-)definitive hematopoietic progenitors . In these protocols, the mesoderm stage of a core hPSC differentiation protocol is manipulated to produce either primitive or definitive hematopoietic progenitors [15, 42] (Fig. 3). Specifically, the inhibition of the WNT canonical pathway, when combined with activin induction, enriches for hematopoietic progenitors of the primitive wave; conversely, the induction of WNT signalling, in the absence of activin induction, enriches for hematopoietic progenitors of the definitive wave (Fig. 3). To develop these methods, functional properties like T lymphocyte potential and production of erythrocytes expressing fetal-type hemoglobin have been used as hallmarks of definitive hematopoiesis [15, 42]. Yet, despite excluding primitive progenitors, these criteria are not sufficient to discriminate between the pro-definitive and definitive hematopoietic lineages [20, 24, 33, 34]. Therefore, distinguishing molecular markers for the two waves of hematopoietic progenitors are needed. Recent mouse work proposed HLF as a marker that distinguishes HSCs from EMPs, at least in mouse , and its usefulness as a marker for human hematopoiesis should therefore be investigated. 2b1af7f3a8