Embryonic Artery Research Articles

The evolving hematopoietic niche during development.

Mammalian hematopoietic stem cells (HSCs) emerge from the hemogenic endothelium in the major embryonic arteries. HSCs undergo a complex journey first migrating to the fetal liver (FL) and from there to the fetal bone marrow (FBM), where they mostly remain during adult life. In this process, a pool of adult HSCs is produced, which sustains lifelong hematopoiesis. Multiple cellular components support HSC maturation and expansion and modulate their response to environmental and developmental cues. While the adult HSC niche has been extensively studied over the last two decades, the niches present in the major embryonic arteries, FL, FBM and perinatal bone marrow (BM) are poorly described. Recent investigations highlight important differences among FL, FBM and adult BM niches and emphasize the important role that inflammation, microbiota and hormonal factors play regulating HSCs and their niches. We provide a review on our current understanding of these important cellular microenvironments across ontogeny. We mainly focused on mice, as the most widely used research model, and, when possible, include relevant insights from other vertebrates including birds, zebrafish, and human. Developing a comprehensive picture on these processes is critical to understand the earliest origins of childhood leukemia and to achieve multiple goals in regenerative medicine, such as mimicking HSC development in vitro to produce HSCs for broad transplantation purposes in leukemia, following chemotherapy, bone marrow failure, and in HSC-based gene therapy.

Akt is a mediator of artery specification during zebrafish development.

The dorsal aorta (DA) is the first major blood vessel to develop in the embryonic cardiovascular system. Its formation is governed by a coordinated process involving the migration, specification, and arrangement of angioblasts into arterial and venous lineages, a process conserved across species. Although vascular endothelial growth factor a (VEGF-A) is known to drive DA specification and formation, the kinases involved in this process remain ambiguous. Thus, we investigated the role of protein kinase B (Akt) in zebrafish by generating a quadruple mutant (aktΔ/Δ), in which expression and activity of all Akt genes - akt1, -2, -3a and -3b - are strongly decreased. Live imaging of developing aktΔ/Δ DA uncovers early arteriovenous malformations. Single-cell RNA-sequencing analysis of aktΔ/Δ endothelial cells corroborates the impairment of arterial, yet not venous, cell specification. Notably, endothelial specific expression of ligand-independent activation of Notch or constitutively active Akt1 were sufficient to re-establish normal arterial specification in aktΔ/Δ. The Akt loss-of-function mutant unveils that Akt kinase can act upstream of Notch in arterial endothelial cells, and is involved in proper embryonic artery specification. This sheds light on cardiovascular development, revealing a mechanism behind congenital malformations.

Morphogenetic processes in the development and evolution of the arteries of the pharyngeal arches: their relations to congenital cardiovascular malformations.

The heart and aortic arch arteries in amniotes form a double circulation, taking oxygenated blood from the heart to the body and deoxygenated blood to the lungs. These major vessels are formed in embryonic development from a series of paired and symmetrical arteries that undergo a complex remodelling process to form the asymmetric arch arteries in the adult. These embryonic arteries form in the pharyngeal arches, which are symmetrical bulges on the lateral surface of the head. The pharyngeal arches, and their associated arteries, are found in all classes of vertebrates, but the number varies, typically with the number of arches reducing through evolution. For example, jawed vertebrates have six pairs of pharyngeal arch arteries but amniotes, a clade of tetrapod vertebrates, have five pairs. This had led to the unusual numbering system attributed to each of the pharyngeal arch arteries in amniotes (1, 2, 3, 4, and 6). We, therefore, propose that these instead be given names to reflect the vessel: mandibular (1st), hyoid (2nd), carotid (3rd), aortic (4th) and pulmonary (most caudal). Aberrant arch artery formation or remodelling leads to life-threatening congenital cardiovascular malformations, such as interruption of the aortic arch, cervical origin of arteries, and vascular rings. We discuss why an alleged fifth arch artery has erroneously been used to interpret congenital cardiac lesions, which are better explained as abnormal collateral channels, or remodelling of the aortic sac.

Endovascular occlusion of a persistent stapedial artery prior to stapedotomy: a novel approach.

The stapedial artery is an embryonic artery that transiently supplies the cranial vasculature of the human embryo. Postnatal persistence of the stapedial artery may cause conductive hearing loss and pulsatile tinnitus due to its course through the middle ear. We describe a patient with a persistent stapedial artery (PSA) managed by endovascular coil occlusion prior to stapedotomy. A 48-year-old woman presented with left-sided conductive hearing loss and pulsatile tinnitus. Ten years earlierthe patient had undergone explorative tympanoplasty, which was aborted due to a large PSA. Digital subtraction angiography was performed to confirm the anatomy and endovascular occlusion of the proximal PSA was achieved by deployment of coils. The pulsatile tinnitus improved immediately after the procedure. The size of the artery subsequently decreased and surgery could be performed with only a minor intraoperative bleeding. Successful stapedotomy resulted in postoperative normalization of her hearing with some minor residual tinnitus. Endovascular coil occlusion of a PSA is feasible and safe in patients with favorable anatomy and facilitates middle ear surgery. It decreases the size of the artery and minimizes the risk of intraoperative bleeding in patients with a large PSA. Thefuture role of this novel technique in the management of patients with PSA-related conductive hearing loss and pulsatile tinnitus remains to be determined.

The Role of SCL Isoforms in Embryonic Hematopoiesis

Three waves of hematopoiesis occur in the mouse embryo. The primitive hematopoiesis appears as blood islands in the extra embryonic yolk sac at E7.5. The extra embryonic pro-definitive hematopoiesis launches in late E8 and the embryonic definitive one turns on at E10.5 indicated by the emergence of hemogenic endothelial cells on the inner wall of the extra embryonic arteries and the embryonic aorta. To study the roles of SCL protein isoforms in murine hematopoiesis, the SCL-large (SCL-L) isoform was selectively destroyed with the remaining SCL-small (SCL-S) isoform intact. It was demonstrated that SCL-S was specifically expressed in the hemogenic endothelial cells (HECs) and SCL-L was only detected in the dispersed cells after budding from HECs. The SCLΔ/Δ homozygous mutant embryos only survived to E10.5 with normal extra embryonic vessels and red blood cells. In wild-type mouse embryos, a layer of neatly aligned CD34+ and CD43+ cells appeared on the endothelial wall of the aorta of the E10.5 fetus. However, the cells at the same site expressed CD31 rather than CD34 and/or CD43 in the E10.5 SCLΔ/Δ embryo, indicating that only the endothelial lineage was developed. These results reveal that the SCL-S is sufficient to sustain the primitive hematopoiesis and SCL-L is necessary to launch the definitive hematopoiesis.

Proposed new classification for internal carotid artery segmental agenesis based on embryologic and angiographic correlation.

Internal carotid artery (ICA) agenesis is a rare anatomical variant that can involve different segments of the ICA. Although many authors proposed their own classifications of this variant basing on different criteria, none of these allows to include all the cases described in the literature. The aim of the authors is to propose a new classification that allows to include all cases of ICA agenesis described until now. The study is based on the review of all the cases of ICA agenesis described in the literature and of the classifications already proposed. After the analysis of these cases, we looked for the limits of each classification to elaborate a new more complete one. We found 228 cases of ICA agenesis. Among them 40 were not includible in the existing classifications. For this reason, we proposed a new classification based on the type of flow compensation and on the embryological events that determine the different variants. The flow is compensated in Type I by the Circle of Willis; in Type II by the non-regression of an embryonic artery; in Type III by the presence of an arterio-arterial anastomosis and in Type IV by external-internal carotid arteries anastomoses. After the literature review, we proposed a new comprehensive classification based on the detailed analysis of arterial embryology. Even if all the embryological details that determine this complex variant are not completely understood yet, this classification allows to include all the cases described in literature.

The onset of circulation triggers a metabolic switch required for endothelial to hematopoietic transition.

SummaryHematopoietic stem cells (HSCs) emerge during development from the vascular wall of the main embryonic arteries. The onset of circulation triggers several processes that provide critical external factors for HSC generation. Nevertheless, it is not fully understood how and when the onset of circulation affects HSC emergence. Here we show that in Ncx1−/− mouse embryos devoid of circulation the HSC lineage develops until the phenotypic pro-HSC stage. However, these cells reside in an abnormal microenvironment, fail to activate the hematopoietic program downstream of Runx1, and are functionally impaired. Single-cell transcriptomics shows that during the endothelial-to-hematopoietic transition, Ncx1−/− cells fail to undergo a glycolysis to oxidative phosphorylation metabolic switch present in wild-type cells. Interestingly, experimental activation of glycolysis results in decreased intraembryonic hematopoiesis. Our results suggest that the onset of circulation triggers metabolic changes that allow HSC generation to proceed.

Fibrous Caps in Atherosclerosis Form by Notch-Dependent Mechanisms Common to Arterial Media Development.

Atheromatous fibrous caps are produced by smooth muscle cells (SMCs) that are recruited to the subendothelial space. We tested whether the recruitment mechanisms are the same as in embryonic artery development, which relies prominently on Notch signaling to form the subendothelial medial SMC layers. Notch elements were expressed in regions of fibrous cap in human and mouse plaques. To assess the causal role of Notch signaling in cap formation, we studied atherosclerosis in mice where the Notch pathway was inactivated in SMCs by conditional knockout of the essential effector transcription factor RBPJ (recombination signal-binding protein for immunoglobulin kappa J region). The recruitment of cap SMCs was significantly reduced without major effects on plaque size. Lineage tracing revealed the accumulation of SMC-derived plaque cells in the cap region was unaltered but that Notch-defective cells failed to re-acquire the SMC phenotype in the cap. Conversely, to analyze whether the loss of Notch signaling is required for SMC-derived cells to accumulate in atherogenesis, we studied atherosclerosis in mice with constitutive activation of Notch signaling in SMCs achieved by conditional expression of the Notch intracellular domain. Forced Notch signaling inhibited the ability of medial SMCs to contribute to plaque cells, including both cap SMCs and osteochondrogenic cells, and significantly reduced atherosclerosis development. Sequential loss and gain of Notch signaling is needed to build the cap SMC population. The shared mechanisms with embryonic arterial media assembly suggest that the cap forms as a neo-media that restores the connection between endothelium and subendothelial SMCs, transiently disrupted in early atherogenesis.

Stapedial Artery: From Embryology to Different Possible Adult Configurations.

The stapedial artery is an embryonic artery that represents the precursor of some orbital, dural, and maxillary branches. Although its embryologic development and transformations are very complex, it is mandatory to understand the numerous anatomic variations of the middle meningeal artery. Thus, in the first part of this review, we describe in detail the hyostapedial system development with its variants, referring also to some critical points of ICA, ophthalmic artery, trigeminal artery, and inferolateral trunk embryology. This basis will allow the understanding of the anatomic variants of the middle meningeal artery, which we address in the second part of the review.

Persistent stapedial artery in PHACE syndrome

IntroductionPHACE syndrome is characterized by posterior fossa malformations, haemangioma, arterial anomalies, coarctation of the aorta, and eye abnormalities. Case reportWe present the case of a 6-year-old girl followed since birth for PHACE syndrome and left hemifacial haemangioma, who presented with left hearing loss. Computed tomography scan showed left persistent stapedial artery (PSA). DiscussionTwo types of arterial anomalies may be observed in PHACE syndrome: persistence of embryonic arteries and anomalies of cerebral arteries. PSA can be observed in the context of PHACE syndrome. Children with PHACE syndrome require regular audiometric follow-up to detect hearing loss and avoid its consequences on speech and language development.

A novel dressing seeded with embryonic artery CD133+ cells and loaded with the Sirt1 agonist SRT1720 accelerates the healing of diabetic ischemic ulcers

Refractory ischemic ulcers that occur in patients with diabetes present a major clinical challenge. Embryonic artery cluster of differentiation 133+ cells (EACCs) may promote the healing of diabetic ulcers; however, the high glucose environment in the diabetic ulcers decreases the survival rate of transplanted EACCs and inhibit their biological function. Furthermore, microcirculation in diabetic ischemic ulcers is impaired, which inhibits the beneficial effect of EACCs. In the current study, the Sirt1 agonist SRT1720 was selected as a therapeutic drug and loaded into a dressing composed of PLGA, collagen and silk (PCSS) formed using electrospinning technology. EACCs were seeded onto the PCSS dressing and this was used to treat diabetic ulcers. The results indicated that SRT1720 promotes the proliferation of EACCs, enhances the secretion of vascular endothelial growth factor A, interluekin 8 and basic fibroblast growth factor, and inhibits the secretion of tumor necrosis factor α. Furthermore, SRT1720 promoted the paracrine function of EACCs and promoted the proliferation and migration of human umbilical vein endothelial cells. PCSS induced the steady release of SRT1720 over a 15-day period and PCSS seeded with EACCs (PCSS-EACCs) were transplanted into the diabetic ischemic ulcers of mice with diabetes. The results of these experiments indicated that angiogenesis and the healing of diabetic ischemic ulcers was significantly improved following the transplantation of PCSS-EACCs. Therefore, PCSS-EACCs may be a novel and effective treatment for diabetic ischemic ulcers.

Localization of transient immature hematopoietic cells to two distinct, potential niches in the developing mouse placenta

Previous studies have shown that human and mouse placentas have hematopoietic potential during mid-gestation. In this investigation, we used histological and immunohistological approaches to visualize hematopoietic cells in mouse placenta between 9.5 and 12.5 days of gestation (gd), identifying their topography and niche. Putative hematopoietic foci were present on 10.5 and 11.5 gd but not 9.5 or 12.5 gd and was restricted to the placental labyrinth. Two major niches each with distinctive hematopoietic cell clusters were present. One type of hematopoietic cell cluster involved the chorioallantoic vasculature and fetal vessels near the chorionic plate. These clusters resembled the hematopoietic stem cells produced by large embryonic arteries such as aorta that persist in postnatal marrow. The other type of hematopoietic cell cluster identified was at the opposite side of labyrinth next to the junctional zone and was composed of erythropoietic foci. Our results suggest that mouse placenta not only produces hematopoietic stem/progenitor cells but also a second wave of primitive erythrocytes that may support a rapid, mid-pregnancy, fetal growth trajectory. Our data also point to a close relationships in the origins of hematopoietic and endothelial cells within placenta.

Clinical study of the relationship between Willis arterial loop variation and intracranial aneurysms

Objective To explore the relationship between the changes of the cerebral arterial circle of Willis and the occurrence of intracranial aneurysms in the diagnosis of digital angiography. Methods From December 2014 to January 2014, 144 cases of intracranial aneurysms were treated in the First Affiliated Hospital of Luzhou Medical University. According to the location of the aneurysm of all patients, the patients were divided into the anterior communicating artery and the posterior communicating artery aneurysms. The relationship between two groups of patients was compared by the sex, age, and the length of the intracranial aneurysms. The relationship was analyzed between the Willis ring and the intracranial aneurysms. The sensitivity and specificity were compared with the surgical results. Results A total of 144 cases of intracranial aneurysms were detected by digital angiography, 68 patients in anterior communicating artery aneurysms, and posterior communicating artery aneurysm in 76 cases. There were no significant differences in sex distribution, age distribution, and length of tumor in the patients with anterior communicating artery aneurysms and the patients with a group of patients with a posterior communicating artery aneurysm. The anterior communicating artery aneurysm group, A1 segment of slender or absence rate was up to 79.41%, and communicating artery aneurysm group A1 segment of slender or absence rate was only 52.63%, two groups were compared with statistical significance (P 0.05). Conclusions Digital angiography can be more accurate in the diagnosis of intracranial aneurysms. Anterior communicating artery aneurysm occurrence is closely related to A1 segment of slender, and absent and bilateral embryo brain artery. Communicating artery aneurysms is more closely related to unilateral embryonic brain artery. Key words: Circle of willis/PA/RA; Tomography, X-ray computed; Intracranial aneurysm/PA/RA

Untypical percheron artery infarct: a case report

Percheron artery infarct(PAI) is uncommon clinically. One case had an acute onset of coma, impaired left limb activities, verbal incoherence, drinking cough and vertical gaze palsy after regaining consciousness. Magnetic resonance imaging(MRI) showed symmetrical high signal intensity on diffusion-weighted imaging(DWI) in bilateral paramedian thalami and a distinct pattern of V-shaped midbrain hyperintensity on DWI. Magnetic resonance angiography(MRA) showed left-sided embryonal posterior cerebral artery(PCA) from ipsilateral internal carotid artery(ICA). Systemic treatment yielded a poor prognosis. Key words: Basilar artery; Brain infarction

느린 성장력을 보이는 구치후방 협점막에 생긴 지속성 두께의 동맥혈

Caliber persistent artery (CPA) is a vascular anomaly presenting as a bluish and pulsatile artery in the subepithelial tissue. Although the incidence of CPA was debated, many CPAs occurred in the perioral and facial tissues at which the embryonal strapedial artery networks were distributed. The present study demonstrated a case of CPA occurred in the retromolar buccal mucosa in a 37 years old male. The lesion showed many pinkish granular spots, but was asymptomatic except biting irritation during mastication. It had slowly increased in size up to 20 × 25 mm for 3 years, and recently became hemorrhagic due to the biting injury between left upper and lower second molars. With the fear of oral cancer an incisional biopsy was performed, and followed by histological and immunohistochemical study. Histologically the lesion showed many tortuous artery localized at the submucosa area, and the arterial wall was thick and its lumen was narrowed and shrunken. In the immunochemistry α-SMA was positive for thick smooth muscle layer of artery and arterioles, TGase 2 was weakly positive for the luminal surface of arterial intima, and bFGF was consistently positive for the perivascular fibrous tissue. But PCNA, VEGF, CD31, CMG2, TGF-β1, HSP-70, and 14-3-3 were almost negative for the vascular tissue. Therefore, it was presumed that the lesion was not actively proliferative nor degenerative but still retained its cellular stability and slow growing potential. It was finally diagnosed as CPA differentially from arterio-venous malformation, hemangioma, lymphangioma, and squamous cell carcinoma. The retromolar buccal mucosa CPA is first reported in this study and may present usual clinical findings depending on its size and location. This asymptomatic lesion could be severely hemorrhagic by minor biting injury, therefore, precise differential diagnosis should be made through biopsy, and careful therapy be followed.

Notch1 Is Pan-Endothelial at the Onset of Flow and Regulated by Flow

Arteriovenous differentiation is a key event during vascular development and hemodynamic forces play an important role. Arteriovenous gene expression is present before the onset of flow, however it remains plastic and flow can alter arteriovenous identity. Notch signaling is especially important in the genetic determination of arteriovenous identity. Nevertheless, the effect of the onset of circulation on Notch expression and signaling has not been studied. The aim of this study is therefore to investigate the interaction of Notch1 signaling and hemodynamic forces during early vascular development. We find that the onset of Notch1 expression coincides with the onset of flow, and that expression is pan-endothelial at the onset of circulation in mouse embryos and only becomes arterial-specific after remodeling has occurred. When we ablate flow in the early embryo, endothelial cells fail to express Notch1. We show that low and disturbed flow patterns upregulate Notch1 expression in endothelial cells in vitro, but that higher shear stress levels do not (≥10 dynes/cm2). Using siRNA, we knocked down Notch1 to investigate the role of Notch1 in mechanotransduction. When we applied shear stress levels similar to those found in embryonic arteries, we found an upregulation of Klf2, Dll1, Dll4, Jag1, Hey1, Nrp1 and CoupTFII but that only Dll4, Hey1, Nrp1 and EphB4 required Notch1 for flow-induced expression. Our results therefore indicate that Notch1 can modulate mechanotransduction but is not a critical mediator of the process since many genes mechanotransduce normally in the absence of Notch1, including genes involved in arteriovenous differentiation.

Progressive maturation toward hematopoietic stem cells in the mouse embryo aorta

AbstractClusters of cells attached to the endothelium of the main embryonic arteries were first observed a century ago. Present in most vertebrate species, such clusters, or intraaortic hematopoietic clusters (IAHCs), derive from specialized hemogenic endothelial cells and contain the first few hematopoietic stem cells (HSCs) generated during embryonic development. However, some discrepancies remained concerning the spatio-temporal appearance and the numbers of IAHCs and HSCs. Therefore, the exact cell composition and function of IAHCs remain unclear to date. We show here that IAHCs contain pre-HSCs (or HSC precursors) that can mature into HSCs in vivo (as shown by the successful long-term multilineage reconstitution of primary neonates and secondary adult recipients). Such IAHC pre-HSCs could contribute to the HSC pool increase observed at midgestation. The novel insights in pre-HSC to HSC transition represent an important step toward generating transplantable HSCs in vitro that are needed for autologous HSC transplantation therapies.

Unilateral triplication of superior cerebellar artery associated with fetal posterior cerebral artery: Case report

Introduction: Superior cerebellar artery is the most constant among the cerebellar arteries and is almost always present, sometimes it can be double, and more rarely, triple. Methods: We report the case of a 72-yr male patient who came to hospital in whom cerebral angiography was performed because of subarachnoid hemorrhage. Results: cerebral angiography showed the unusual presence of an embryonic right posterior cerebral artery, associated with triplication of the right superior cerebellar artery. Conclusion: The tripling of the SCA is seen in only 7% of angiographic abnormalities and rarely is associated with posterior communicating artery aneurysms and embryonic posterior cerebral artery, as in our case.

EMP Emergence from Hemogenic Endothelium in the Mammalian Yolk Sac Is Independent of Flow and Arterial Identity, but Is Regulated By Canonical Wnt Signaling

Hematopoietic stem cells (HSCs) emerge from arterial vessels of the mouse embryo through a Runx1-dependent process of endothelial-to-hematopoietic transition beginning at embryonic day 10.5 (E10.5). This arterial endothelial-to-hematopoietic transition is known to require embryonic circulation as well as beta-catenin signaling within the endothelial precursor, known as hemogenic endothelium. However, embryonic survival is dependent on the earlier emergence of a robust wave of yolk sac-derived definitive erythro-myeloid progenitors (EMPs), which have unilineage as well as multilineage potential, including high-proliferative potential colony forming cell (HPP-CFC) potential (Palis et al., PNAS, 2001). Like HSCs, EMP specification is dependent on Runx1, suggesting that they also emerge from a hemogenic endothelial precursor. However, the spatial localization of EMPs in the yolk sac and the mechanisms governing their emergence are not well understood. To visualize emerging EMPs in the yolk sac, we performed whole-mount immunohistochemistry for Kit, which we have demonstrated to contain nearly all EMP potential at E9.5. Kit+ cells coexpress Runx1 and CD31, and a subset have a polygonal/endothelial morphology, appear integrated into the vascular network, and are associated with rounded Kit+ cells in clusters, features consistent with an endothelial-to-hematopoietic transition. However, unlike HSCs, which emerge from major embryonic arteries, clusters of EMPs are located in larger and smaller caliber vessels in branches of both the arterial and venous vasculature, which is spatially organized within the yolk sac. To determine if EMP emergence from the vasculature is dependent on embryonic blood flow, which is required for HSC emergence, we analyzed the yolk sacs of Ncx1-null embryos, which fail to initiate heart contractions and subsequently lack embryonic circulation. Despite the lack of vascular remodeling in these circulation-deficient yolk sacs, Ncx1-null EMPs displayed normal cluster morphology, including both polygonal and rounded kit+ cells, indicating the endothelial-to-hematopoietic transition can occur without the mechanical influence of blood flow.To address whether EMP formation is responsive to other developmental signals, we utilized a yolk sac explant culture to evaluate the propensity of hemogenic endothelial cells to commit to hematopoiesis ex vivo. Culture of intact E8.5 yolk sacs for 48 hours with the canonical Wnt ligand Wnt3a resulted in an increase in both day 6-7 colony forming cells and day 13-14 HPP-CFC when compared with control yolk sacs. Preliminary treatment with Dkk1 alone did not adversely affect colony-forming activity when compared with untreated yolk sacs, and potentiation of endogenous canonical Wnt signaling with HLY78 did not augment colony production, suggesting that low levels of endogenous Wnt ligands are produced ex vivo. Despite the positive effect of Wnt3a on whole yolk sacs, treatment of isolated E9.5 Kit+CD41+CD16/32+ EMPs with Wnt3a did not increase colony formation, suggesting that Wnt signaling augments progenitor production at, or prior to, the hemogenic endothelial stage. Preliminary results utilizing imaging flow cytometry demonstrated increased beta-catenin intensity within the nuclear region in E9.5 Kit+VE-Cadherin/AA4.1+ endothelium following Wnt3a treatment, suggesting that hemogenic endothelial cells in the yolk sac are Wnt responsive. Consistent with this finding, in vitro Wnt3a treatment on primary E8.5-9.5 VE-Cadherin/AA4.1+CD16/32- endothelial cells resulted in upregulation of the beta-catenin target gene Axin2. To address whether Wnt signaling is endogenously active in vivo, we analyzed E8.5-E9 yolk sacs of BAT-gal reporter mice (Maretto et al., PNAS, 2003), and visualized a subset of cells with endothelial morphology expressing LacZ. Taken together, these data support the concept that EMPs, like HSCs, emerge from hemogenic endothelium. Surprisingly, this earlier endothelial-to-hematopoietic transition in the yolk sac is not dependent on blood flow or an arterial identity. However, similar to HSC emergence, EMP emergence from hemogenic endothelium is positively regulated by canonical Wnt signaling. These data highlight the presence of spatially, temporally, and functionally heterogeneous populations of hemogenic endothelium in the mammalian conceptus. DisclosuresNo relevant conflicts of interest to declare.

Hemogenic endothelium generates mesoangioblasts that contribute to several mesodermal lineages in vivo.

The embryonic endothelium is a known source of hematopoietic stem cells. Moreover, vessel-associated progenitors/stem cells with multilineage mesodermal differentiation potential, such as the 'embryonic mesoangioblasts', originate in vitro from the endothelium. Using a genetic lineage tracing approach, we show that early extra-embryonic endothelium generates, in a narrow time-window and prior to the hemogenic endothelium in the major embryonic arteries, hematopoietic cells that migrate to the embryo proper, and are subsequently found within the mesenchyme. A subpopulation of these cells, distinct from embryonic macrophages, co-expresses mesenchymal and hematopoietic markers. In addition, hemogenic endothelium-derived cells contribute to skeletal and smooth muscle, and to other mesodermal cells in vivo, and display features of embryonic mesoangioblasts in vitro. Therefore, we provide new insights on the distinctive characteristics of the extra-embryonic and embryonic hemogenic endothelium, and we identify the putative in vivo counterpart of embryonic mesoangioblasts, suggesting their identity and developmental ontogeny.