Induction of vasculogenesis and hematopoiesis in vitro

Fibroblast growth factor-2 selectively stimulates angiogenesis of small vessels in arterial tree

There is a critical need for quantifiable models of angiogenesis in vivo, and in general, differential effects of angiogenic regulators on vascular morphology have not been measured. Because the potent angiogenic stimulators fibroblast growth factor (FGF)-2 (basic FGF) and vascular endothelial growth factor (VEGF) are reported to stimulate angiogenesis through distinct signaling pathways, we hypothesized that FGF-2 stimulates vascular morphology differently than does VEGF and that stimulation of angiogenesis by FGF-2 is directly correlated to FGF receptor density. FGF-2 was applied at embryonic day 7 (E7), E8, or E9 to the quail chorioallantoic membrane (CAM); subsequent response of the arterial tree was measured by the fractal dimension (D(f)), a mathematical descriptor of complex spatial patterns, and by several generational branching parameters that included vessel length density (L(v)). After application of FGF-2 at E7, arterial density increased according to D(f) as a direct function of increasing FGF-2 concentration, and FGF-2 stimulated the growth of small vessels, but not of large vessels, according to L(v) and other branching parameters. For untreated control specimens at E7, L(v) of small vessels and D(f) were 11.1+/-1.6 cm(-1) and 1.38+/-0.01, respectively; at E8, after treatment with 5 microgram FGF-2/CAM for 24 hours, L(v) of small vessels and D(f) increased respectively to 22.8+/-0.7 cm(-1) and 1. 49+/-0.02 compared with 16.3+/-0.9 cm(-1) and 1.43+/-0.02 for PBS-treated control specimens. Application of FGF-2 at E8 and E9 did not significantly increase arterial density. By immunohistochemistry, the expression of 4 high-affinity tyrosine kinase FGF receptors was significantly expressed at E7, when CAM vasculature responded strongly to FGF-2 stimulation, but FGF receptor expression decreased throughout the CAM by E8, when vascular response to FGF-2 was negligible. In conclusion, the "fingerprint" vascular pattern elicited by FGF-2 was distinct from vascular patterns induced by other angiogenic regulators that included VEGF(165), transforming growth factor-beta1, and angiostatin.

Endoderm and heart development

Since the first half of the 20th century, experimental embryologists have noted a relationship between endoderm cells and the development of cardiac tissue from mesoderm. During the past decade, the accumulation of evidence for an obligatory interaction between endoderm and mesoderm during the specification and terminal differentiation of myocardial, and more recently endocardial, cells has markedly accelerated. Moreover, the endoderm-derived molecules that may regulate these processes are being identified. It now appears that endoderm-derived growth factors regulate the formation of both myocardial and endocardial cells during specification, terminal differentiation, and perhaps morphogenesis of cells in the developing embryonic heart.

Skeletal myogenic progenitors originating from embryonic dorsal aorta coexpress endothelial and myogenic markers and contribute to postnatal muscle growth and regeneration

Skeletal muscle in vertebrates is derived from somites, epithelial structures of the paraxial mesoderm, yet many unrelated reports describe the occasional appearance of myogenic cells from tissues of nonsomite origin, suggesting either transdifferentiation or the persistence of a multipotent progenitor. Here, we show that clonable skeletal myogenic cells are present in the embryonic dorsal aorta of mouse embryos. This finding is based on a detailed clonal analysis of different tissue anlagen at various developmental stages. In vitro, these myogenic cells show the same morphology as satellite cells derived from adult skeletal muscle, and express a number of myogenic and endothelial markers. Surprisingly, the latter are also expressed by adult satellite cells. Furthermore, it is possible to clone myogenic cells from limbs of mutant c-Met-/- embryos, which lack appendicular muscles, but have a normal vascular system. Upon transplantation, aorta-derived myogenic cells participate in postnatal muscle growth and regeneration, and fuse with resident satellite cells.The potential of the vascular system to generate skeletal muscle cells may explain observations of nonsomite skeletal myogenesis and raises the possibility that a subset of satellite cells may derive from the vascular system.

VEGF and vascular fusion: implications for normal and pathological vessels

The avian embryo is well suited for the study of blood vessel morphogenesis. This is especially true of investigations that focus on the de novo formation of blood vessels from mesoderm, a process referred to as vasculogenesis. To examine the cellular and molecular mechanisms regulating vasculogenesis, we developed a bioassay that employs intact avian embryos. Among the many bioactive molecules we have examined, vascular epithelial growth factor (VEGF) stands out for its ability to affect vasculogenesis. Using the whole-embryo assay, we discovered that VEGF induces a vascular malformation we refer to as hyperfusion. Our studies showed that microinjection of recombinant VEGF165 converted the normally discrete network of embryonic blood vessels into enlarged endothelial sinuses. Depending on the amount of VEGF injected and the time of postinjection incubation, the misbehavior of the primordial endothelial cells can become so exaggerated that for all practical purposes the embryo contains a single enormous vascular sinus; all normal vessels are subsumed into a composite vascular structure. This morphology is reminiscent of the abnormal vascular sinuses characteristic of certain neovascular pathologies. (J Histochem Cytochem 47:1351-1355, 1999)

Bone marrow origin of endothelial progenitor cells responsible for postnatal vasculogenesis in physiological and pathological neovascularization

Circulating endothelial progenitor cells (EPCs) have been isolated in peripheral blood of adult species. To determine the origin and role of EPCs contributing to postnatal vasculogenesis, transgenic mice constitutively expressing beta-galactosidase under the transcriptional regulation of an endothelial cell-specific promoter (Flk-1/LZ or Tie-2/LZ) were used as transplant donors. Localization of EPCs, indicated by flk-1 or tie-2/lacZ fusion transcripts, were identified in corpus luteal and endometrial neovasculature after inductive ovulation. Mouse syngeneic colon cancer cells (MCA38) were implanted subcutaneously into Flk-1/LZ/BMT (bone marrow transplantation) and Tie-2/LZ/BMT mice; tumor samples harvested at 1 week disclosed abundant flk-1/lacZ and tie-2/lacZ fusion transcripts, and sections stained with X-gal demonstrated that the neovasculature of the developing tumor frequently comprised Flk-1- or Tie-2-expressing EPCs. Cutaneous wounds examined at 4 days and 7 days after skin removal by punch biopsy disclosed EPCs incorporated into foci of neovascularization at high frequency. One week after the onset of hindlimb ischemia, lacZ-positive EPCs were identified incorporated into capillaries among skeletal myocytes. After permanent ligation of the left anterior descending coronary artery, histological samples from sites of myocardial infarction demonstrated incorporation of EPCs into foci of neovascularization at the border of the infarct. These findings indicate that postnatal neovascularization does not rely exclusively on sprouting from preexisting blood vessels (angiogenesis); instead, EPCs circulate from bone marrow to incorporate into and thus contribute to postnatal physiological and pathological neovascularization, which is consistent with postnatal vasculogenesis.

Cell-autonomous and non-autonomous requirements for the zebrafish gene cloche in hematopoiesis

Vertebrate embryonic hematopoiesis is a complex process that involves a number of cellular interactions, notably those occurring between endothelial and blood cells. The zebrafish cloche mutation affects both the hematopoietic and endothelial lineages from an early stage (Stainier, D. Y. R., Weinstein, B. M., Detrich, H. W. R., Zon, L. I. and Fishman, M. C. (1995) Development 121, 3141–3150). cloche mutants lack endocardium, as well as head and trunk endothelium, and nearly all blood cells. Cell transplantation studies have revealed that the endocardial defect in cloche is cell-autonomous: wild-type cells can form endocardium in mutant hosts, but mutant cells never contribute to the endocardium in wild-type or mutant hosts. In this paper, we analyze the cell-autonomy of the blood defect in cloche. The blood cell deficiency in cloche mutants could be an indirect effect of the endothelial defects. Alternatively, cloche could be required cell-autonomously in the blood cells themselves. To distinguish between these possibilities, we cotransplanted wild-type and mutant cells into a single wild-type host in order to compare their respective hematopoietic capacity. We found that transplanted wild-type cells were much more likely than mutant cells to contribute to circulating blood in a wild-type host. Furthermore, in the few cases where both wild-type and mutant donors contributed to blood in a wild-type host, the number of blood cells derived from the wild-type donor was always much greater than the number of blood cells derived from the mutant donor. These data indicate that cloche is required cell-autonomously in blood cells for their differentiation and/or proliferation. When we assessed early expression of the erythropoietic gene gata-1 in transplant recipients, we found that mutant blastomeres were as likely as wild-type blastomeres to give rise to gata-1-expressing cells in a wild-type host. Together, these two sets of data argue that cloche is not required cell-autonomously for the differentiation of red blood cells, as assayed by gata-1 expression, but rather for their proliferation and/or survival, as assayed by their contribution to circulating blood. In addition, we found that transplanted wild-type cells were less likely to express gata-1 in a mutant environment than in a wild-type one, suggesting that cloche also acts non-autonomously in red blood cell differentiation. This non-autonomous function of cloche in red blood cell differentiation may reflect its cell-autonomous requirement in the endothelial lineage. Thus, cloche appears to be required in erythropoiesis cell non-autonomously at a step prior to gata-1 expression, and cell-autonomously subsequently.

Defective angiogenesis in mice lacking endoglin

Endoglin is a transforming growth factor-beta (TGF-beta) binding protein expressed on the surface of endothelial cells. Loss-of-function mutations in the human endoglin gene ENG cause hereditary hemorrhagic telangiectasia (HHT1), a disease characterized by vascular malformations. Here it is shown that by gestational day 11.5, mice lacking endoglin die from defective vascular development. However, in contrast to mice lacking TGF-beta, vasculogenesis was unaffected. Loss of endoglin caused poor vascular smooth muscle development and arrested endothelial remodeling. These results demonstrate that endoglin is essential for angiogenesis and suggest a pathogenic mechanism for HHT1.

Elucidating the origins of the vascular system: a fate map of the vascular endothelial and red blood cell lineages in Xenopus laevis

Required to supply nutrients and oxygen to the growing embryo, the vascular system is the first functional organ system to develop during vertebrate embryogenesis. Although there has been substantial progress in identifying the genetic cascade regulating vascular development, the initial stages of vasculogenesis, namely, the origin of vascular endothelial cells within the early embryo, remain unclear. To address this issue we constructed a fate map for specific vascular structures, including the aortic arches, endocardium, dorsal aorta, cardinal veins, and lateral abdominal veins, as well as for the red blood cells at the 16-cell stage and the 32-cell stage of Xenopus laevis. Using genetic markers to identify these cell types, our results suggest that vascular endothelial cells can arise from virtually every blastomere of the 16-cell-stage and the 32-cell-stage embryo, with different blastomeres preferentially, though not exclusively, giving rise to specific vascular structures. Similarly, but more surprisingly, every blastomere in the 16-cell-stage embryo and all but those in the most animal tier of the 32-cell-stage embryo serve as progenitors for red blood cells. Taken together, our results suggest that during normal development, both dorsal and ventral blastomeres contribute significantly to the vascular endothelial and red blood cell lineages.

Angiogenesis defects and mesenchymal apoptosis in mice lacking SMAD5

The transforming growth factor-beta (TGF-beta) signals are mediated by a family of at least nine SMAD proteins, of which SMAD5 is thought to relay signals of the bone morphogenetic protein (BMP) pathway. To investigate the role of SMAD5 during vertebrate development and tumorigenesis, we disrupted the Smad5 gene by homologous recombination. We showed that Smad5 was expressed predominantly in mesenchyme and somites during embryogenesis, and in many tissues of the adult. Mice homozygous for the mutation died between days 10.5 and 11.5 of gestation due to defects in angiogenesis. The mutant yolk sacs lacked normal vasculature and had irregularly distributed blood cells, although they contained hematopoietic precursors capable of erythroid differentiation. Smad5 mutant embryos had enlarged blood vessels surrounded by decreased numbers of vascular smooth muscle cells, suffered massive apoptosis of mesenchymal cells, and were unable to direct angiogenesis in vitro. These data suggest that SMAD5 may regulate endothelium-mesenchyme interactions during angiogenesis and that it is essential for mesenchymal survival.

[Vascular endothelial factor (VEGF): therapeutic angiogenesis and vasculogenesis in the treatment of cardiovascular disease]

The formation of new blood vessel is essential for a variety of physiological processes like embryogenesis and the female reproduction as well as pathological processes like tumor growth, wound healing and neovascularization of ischemic tissue. Vasculogenesis and angiogenesis are the mechanisms responsible for the development of the blood vessels. While angiogenesis refers to the formation of capillaries from pre-existing vessels in the embryo and adult organism, vasculogenesis, the development of new blood vessels from in situ differentiating endothelial cells, has been previously considered restricted to embryogenesis. Recent investigations, however, show the existence of endothelial progenitor cells (EPCs) in the peripheral blood of the adult and their participation in ongoing neovascularization. Molecular and cell-biological experiments suggest that different cytokines and growth factors have a stimulatory effect on these bone-marrow derived EPCs. Results with GM-CSF (granulocyte macrophage-colony stimulating factor) and VEGF (vascular endothelial growth factor) open a new insight into the clinical use of cytokines and in particular the use of growth factors in gene therapy. The administration via protein or plasmid-DNA for neovascularization seems to enhance both pathways, angiogenesis and vasculogenesis.

Differentiation of hemangioblasts from embryonic mesothelial cells? A model on the origin of the vertebrate cardiovascular system

The existence of the hemangioblast, a common progenitor of the endothelial and hematopoietic cell lineages, was proposed at the beginning of the century. Although recent findings seem to confirm its existence, it is still unknown when and how the hemangioblasts differentiate. We propose a hypothesis about the origin of hemangioblasts from the embryonic splanchnic mesothelium. The model is based on observations collected from the literature and from our own studies. These observations include: (1) the extensive population of the splanchnic mesoderm by mesothelial-derived cells coinciding with the emergence of the endothelial and hematopoietic progenitors; (2) the transient localization of cytokeratin, the main mesothelial intermediate filament protein, in some embryonic vessels and endothelial progenitors; (3) the possible origin of cardiac vessels from epicardial-derived cells; (4) the origin of endocardial cells from the splanchnic mesoderm when this mesoderm is an epithelium; (5) the evidence that mesothelial cells migrate to the hemogenic areas of the dorsal aorta. (6) Biochemical and antigenic similarities between mesothelial and endothelial cells. We suggest that the endothelium-lined vascular system arose as a specialization of the phylogenetically older coelomic cavities. The origin of the hematopoietic cells might be related to the differentiation, reported in some invertebrates, of coelomocytes from the coelomic epithelium. Some types of coelomocytes react against microbial invasion and other types transport respiratory pigments. We propose that this phylogenetic origin is recapitulated in the vertebrate ontogeny and explains the differentiation of endothelial and blood cells from a common mesothelial-derived progenitor.

Vezf1: A Zn finger transcription factor restricted to endothelial cells and their precursors

Using retroviral entrapment vectors, we identified a novel mouse gene whose expression is restricted to vascular endothelial cells and their precursors in the yolk sac blood islands. A 3.68-kb cDNA corresponding to the endogenous transcript was isolated using genomic DNA flanking the entrapment vector insertion as a probe. We have named this gene Vezf1 for vascular endothelial zinc finger 1. Vezf1 encodes a protein with a predicted molecular mass of 56 kDa and that contains six putative zinc finger domains and shows high homology to a previously identified human gene, DB1, that is believed to be involved in regulating expression of cytokine genes such as interleukin-3. In situ hybridization analysis revealed the onset of expression in advanced primitive streak-stage embryos being located in the extraembryonic mesodermal component of the visceral yolk sac and in the anteriormost mesoderm of the embryo proper. During head-fold and somite stages, expression was restricted to vascular endothelial cells that arise during both vasculogenesis and angiogenesis. Vezf1-related sequences were found to be highly conserved among higher vertebrate species that have acquired extraembryonic yolk sac membranes during evolution. The Vezf1 locus mapped to the proximal part of mouse chromosome 2, a region which has homology to human chromosome 9q. Vezf1 expression correlates temporally and spatially with the early differentiation of angioblasts into the endothelial cell lineage and the proliferation of endothelial cells of the embryonic vascular system. Thus, Vezf1 may play an important role in the endothelial lineage determination and may have an additional role during later stages of embryonic vasculogenesis and angiogenesis.

Maturation of Embryonic Stem Cells Into Endothelial Cells in an In Vitro Model of Vasculogenesis

A primitive vascular plexus is formed through coordinated regulation of differentiation, proliferation, migration, and cell-cell adhesion of endothelial cell (EC) progenitors. In this study, a culture system was devised to investigate the behavior of purified EC progenitors in vitro. Because Flk-1+ cells derived from ES cells did not initially express other EC markers, they were sorted and used as EC progenitors. Their in vitro differentiation into ECs, via vascular endothelial-cadherin (VE-cadherin)+ platelet-endothelial cell adhesion molecule-1 (PECAM-1)+ CD34−to VE-cadherin+ PECAM-1+CD34+ stage, occurred without exogenous factors, whereas their proliferation, particularly at low cell density, required OP9 feeder cells. On OP9 feeder layer, EC progenitors gave rise to sheet-like clusters of Flk-1+ cells, with VE-cadherin concentrated at the cell-cell junction. The growth was suppressed by Flt-1-IgG1 chimeric protein and dependent on vascular endothelial growth factor (VEGF) but not placenta growth factor (PIGF). Further addition of VEGF resulted in cell dispersion, indicating the role of VEGF in the migration of ECs as well as their proliferation. Cell-cell adhesion of ECs in this culture system was mediated by VE-cadherin. Thus, the culture system described here is useful in dissecting the cellular events of EC progenitors that occur during vasculogenesis and in investigating the molecular mechanisms underlying these processes.

Manipulation of the angiopoietic/hemangiopoietic commitment in the avian embryo

The hypothesis that the endothelial and hemopoietic lineages have a common ontogenic origin is currently being revived. We have shown previously by means of quail/chick transplantations that two subsets of the mesoderm give rise to endothelial precursors: a dorsal one, the somite, produces pure angioblasts (angiopoietic potential), while a ventral one, the splanchnopleural mesoderm, gives rise to progenitors with a dual endothelial and hemopoietic potential (hemangiopoietic potential). To investigate the cellular and molecular controls of the angiopoietic/hemangiopoietic potential, we devised an in vivo assay based on the polarized homing of hemopoietic cell precursors to the floor of the aorta detectable in the quail/chick model. In the present work, quail mesoderm was grafted, after various pretreatments, onto the splanchnopleure of a chick host; the homing pattern and nature of graft-derived QH1(+) cells were analyzed thereafter. We report that transient contact with endoderm or ectoderm could change the behavior of cells derived from treated mesoderm, and that the effect of these germ layers could be mimicked by treatment with several growth factors VEGF, bFGF, TGFbeta1, EGF and TGF(α), known to be involved in endothelial commitment and proliferation, and/or hemopoietic processes. The endoderm induced a hemangiopoietic potential in the associated mesoderm. Indeed, the association of somatopleural mesoderm with endoderm promoted the ‘ventral homing’ and the production of hemopoietic cells from mesoderm not normally endowed with this potential. The hemangiopoietic induction by endoderm could be mimicked by VEGF, bFGF and TGFbeta1. In contrast, contact with ectoderm or EGF/TGF(α) treatments totally abrogated the hemangiopoietic capacity of the splanchnopleural mesoderm, which produced pure angioblasts with no ‘ventral homing’ behaviour. We postulate that two gradients, one positive and one negative, modulate the angiopoietic/hemangiopoietic potential of the mesoderm.

Maturation of Embryonic Stem Cells Into Endothelial Cells in an In Vitro Model of Vasculogenesis

A primitive vascular plexus is formed through coordinated regulation of differentiation, proliferation, migration, and cell-cell adhesion of endothelial cell (EC) progenitors. In this study, a culture system was devised to investigate the behavior of purified EC progenitors in vitro. Because Flk-1+ cells derived from ES cells did not initially express other EC markers, they were sorted and used as EC progenitors. Their in vitro differentiation into ECs, via vascular endothelial-cadherin (VE-cadherin)+ platelet-endothelial cell adhesion molecule-1 (PECAM-1)+ CD34−to VE-cadherin+ PECAM-1+CD34+ stage, occurred without exogenous factors, whereas their proliferation, particularly at low cell density, required OP9 feeder cells. On OP9 feeder layer, EC progenitors gave rise to sheet-like clusters of Flk-1+ cells, with VE-cadherin concentrated at the cell-cell junction. The growth was suppressed by Flt-1-IgG1 chimeric protein and dependent on vascular endothelial growth factor (VEGF) but not placenta growth factor (PIGF). Further addition of VEGF resulted in cell dispersion, indicating the role of VEGF in the migration of ECs as well as their proliferation. Cell-cell adhesion of ECs in this culture system was mediated by VE-cadherin. Thus, the culture system described here is useful in dissecting the cellular events of EC progenitors that occur during vasculogenesis and in investigating the molecular mechanisms underlying these processes.

Segregation of the embryonic vascular and hemopoietic systems

The origin of endothelial cells and their subsequent assembly into the primary vascular system have been mostly analyzed in the avian embryo. Following the discovery of specific growth factors and their cognate receptors, the molecular mechanisms underlying these processes have been unraveled in both birds and mammals. In particular, experimental studies of the angiogenic vascular endothelial growth factor (VEGF) and its receptors, carried out in both vertebrate classes, have provided significant insight into the developmental biology of endothelial cells. The VEGF receptor VEGFR2 is the earliest marker known to be expressed by endothelial precursor cells of avian and mouse embryos. Based on the localization of VEGFR2+ cells in the avian embryo and on clonal culture experiments, two types of endothelial precursor cells can be distinguished from gastrulation stages onward: posterior mesodermal VEGFR2+ hemangioblasts, which have the capacity to differentiate into endothelial and hemopoietic cells, and anterior VEGFR2+ angioblasts, which can only give rise to endothelial cells.Key words: hemangioblast, endothelial cell, hemopoietic cell, embryo.

Development and differentiation of endothelium

Vascular endothelial cells play an important role in tissue homeostasis, fibrinolysis, and coagulation; blood-tissue exchange, vasotonus regulation, blood cell activation, and migration; and the vascularization of tissues. The formation of new blood vessels comprises two distinct steps: vasculogenesis, the in situ assembly of capillaries, and angiogenesis, the sprouting of capillaries from preexisting ones. Vascular endothelial growth factor (VEGF) is essential for vasculogenesis and angiogenesis. Its expression is high in the embryonic brain and kidney when angiogenesis occurs and low in the adult brain when angiogenesis is absent. In the kidney, VEGF expression remains high in glomerular podocytes even in the adult. VEGF receptors 1 and 2 (fit-1 and flk-1) are endothelial-specific receptor tyrosine kinases. Similar to the ligand, expression of these receptors is high during brain and kidney angiogenesis, low in adult brain endothelium, but high in adult glomerular endothelium. Because VEGF is also a vascular permeability factor, the expression in the adult correlates with the low permeability of blood-brain barrier endothelium and the high permeability of fenestrated glomerular endothelium. Although fenestrae formation can be induced in vitro by VEGF and a basal lamina-type extracellular matrix, blood-brain barrier characteristics seem to require the presence of still unknown brain-derived factors.

Characterization of novel nuclear targeting and apoptosis-inducing domains in FAS associated factor 1

FAS associated factor 1 (FAF1) has been described as an unusual protein that binds to the intracellular portion of the apoptosis signal transducing receptor FAS/Apo-1 and potentiates apoptosis in L-cells. By means of mRNA differential display we have identified the avian homologue (qFAF) as a fibroblast growth factor-inducible gene in pluripotent cells from E0 quail embryos during mesoderm induction in vitro. Later during embryonic development, qFAF expression is ubiquitous. We confirm that qFAF is associated with FAS, and show that it is phosphorylated on serine residues and localized to the nucleus. By in vitro mutagenesis we have delimited a novel nuclear targeting domain to a short 35 amino acid α-helical region in the amino-terminal half of the protein. The nuclear function of qFAF remains unclear. However, a probably dominant negative deletion mutant of qFAF causes apoptosis of transfected cells. This function resides in the carboxy-terminal domain of qFAF which shares remarkable sequence homologies with a putative ubiquitin conjugating enzyme from Caenorhabditis elegans. Our data indicate a complex function for FAF, which may be executed during FAS signalling and/or in the ubiquitination pathway, and may be essential for cell differentiation and survival.

Neuronal defects and delayed wound healing in mice lacking fibroblast growth factor 2

Basic fibroblast growth factor (FGF2) is a wide-spectrum mitogenic, angiogenic, and neurotrophic factor that is expressed at low levels in many tissues and cell types and reaches high concentrations in brain and pituitary. FGF2 has been implicated in a multitude of physiological and pathological processes, including limb development, angiogenesis, wound healing, and tumor growth, but its physiological role is still unclear. To determine the function of FGF2 in vivo, we have generated FGF2 knockout mice, lacking all three FGF2 isoforms, by homologous recombination in embryonic stem cells. FGF2(-/-) mice are viable, fertile and phenotypically indistinguishable from FGF2(+/+) littermates by gross examination. However, abnormalities in the cytoarchitecture of the neocortex, most pronounced in the frontal motor-sensory area, can be detected by histological and immunohistochemical methods. A significant reduction in neuronal density is observed in most layers of the motor cortex in the FGF2(-/-) mice, with layer V being the most affected. Cell density is normal in other regions of the brain such as the striatum and the hippocampus. In addition, the healing of excisional skin wounds is delayed in mice lacking FGF2. These results indicate that FGF2, although not essential for embryonic development, plays a specific role in cortical neurogenesis and skin wound healing in mice, which, in spite of the apparent redundancy of FGF signaling, cannot be carried out by other FGF family members.

Targeted disruption of fibroblast growth factor (FGF) receptor 2 suggests a role for FGF signaling in pregastrulation mammalian development

We disrupted the fibroblast growth factor (FGF) receptor 2 (FGFR2) gene by introducing a neo cassette into the IIIc ligand binding exon and by deleting a genomic DNA fragment encoding its transmembrane domain and part of its kinase I domain. A recessive embryonic lethal mutation was obtained. Preimplantation development was normal until the blastocyst stage. Homozygous mutant embryos died a few hours after implantation at a random position in the uterine crypt, with collapsed yolk cavity. Mutant blastocysts hatched, adhered, and formed a layer of trophoblast giant cells in vitro, but after prolonged culture, the growth of the inner cell mass stopped, no visceral endoderm formed, and finally the egg cylinder disintegrated. It follows that FGFR2 is required for early postimplantation development between implantation and the formation of the egg cylinder. We suggest that FGFR2 contributes to the outgrowth, differentiation, and maintenance of the inner cell mass and raise the possibility that this activity is mediated by FGF4 signals transmitted by FGFR2. The role of early FGF signaling in pregastrulation development as a possible adaptation to mammalian (amniote) embryogenesis is discussed.

Isolation and characterization of endothelial progenitor cells from mouse embryos

The cardiovascular system develops early in embryogenesis from cells of mesodermal origin. To study the molecular and cellular processes underlying this transition, we have isolated mesodermal cells from murine embryos at E7.5 with characteristic properties of endothelial progenitors by using a combination of stromal cell layers and growth conditions. The isolated embryonic cells displayed unlimited stem-cell-like growth potential and a stable phenotype in culture. RNA analysis revealed that the embryonic cells express the endothelial-specific genes tie-2 and thrombomodulin (TM) as well as the early mesodermal marker fgf-3. The GSL I-B4 isolectin, a marker of early endothelial cells, specifically binds to the isolated cells. The in vitro differentiation with retinoic acid and cAMP led to a 5- to 10-fold induction of flk-1, von Willebrand Factor (vWF), TM, GATA-4 and GATA-6. Electron microscopy revealed that in vitro differentiation is associated with increased amounts of rER and Golgi, and a dramatic increase in secretory vesicles packed with vWF. When cultured in Matrigel, the embryonic cells assume the characteristic endothelial cobblestone morphology and form tubes. Injection into chicken embryos showed incorporation of the embryonic cells in the endocardium and the brain vasculature. The expression of TM, tie-2, GATA-4 and GATA-6 suggests that the isolated embryonic endothelial cell progenitors are derived from the proximal lateral mesoderm where the pre-endocardial tubes form. The properties of the endothelial cell progenitors described here provide a novel approach to analyze mediators, signaling pathways and transcriptional control in early vascular development.

Avian VEGF-C: cloning, embryonic expression pattern and stimulation of the differentiation of VEGFR2-expressing endothelial cell precursors

VEGF-C is a recently discovered secreted polypeptide related to the angiogenic mitogen VEGF. We have isolated the quail VEGF-C cDNA and shown that its protein product is secreted from transfected cells and interacts with the avian VEGFR3 and VEGFR2. In situ hybridization shows that quail VEGF-C mRNA is strongly expressed in regions destined to be rich in lymphatic vessels, particularly the mesenteries, mesocardium and myotome, in the region surrounding the jugular veins, and in the kidney. These expression sites are similar to those observed in the mouse embryo (E. Kukk, A. Lymboussaki, S. Taira, A. Kaipainen, M. Jeltsch, V. Joukov and K. Alitalo, 1996, Development 122, 3829–3837). We have observed VEGFR3-positive endothelial cells in proximity to most of the VEGF-C-expressing sites, suggesting functional relationships between this receptor-ligand couple. The comparison of the VEGF and VEGFR2 knockout phenotypes had suggested the existence of another ligand for VEGFR2. We therefore investigated the effect of VEGF-C on VEGFR2-positive cells isolated from the posterior mesoderm of gastrulating embryos. We have recently shown that VEGF binding triggers endothelial differentiation of these cells, whereas hemopoietic differentiation appears to be mediated by binding of a so far unidentified VEGFR2 ligand. We show here that VEGF-C also triggers endothelial differentiation of these cells, presumably via VEGFR2. These results indicate that VEGF and VEGF-C can act in a redundant manner via VEGFR2. In conclusion, VEGF-C appears to act during two different developmental phases, one early in posterior mesodermal VEGFR2-positive endothelial cell precursors which are negative for VEGFR3 and one later in regions rich in lymphatic vessels at a time when endothelial cells express both VEGFR2 and VEGFR3.

Fibroblast growth factor 2 control of vascular tone

Vascular tone control is essential in blood pressure regulation, shock, ischemia-reperfusion, inflammation, vessel injury/repair, wound healing, temperature regulation, digestion, exercise physiology, and metabolism. Here we show that a well-known growth factor, FGF2, long thought to be involved in many developmental and homeostatic processes, including growth of the tissue layers of vessel walls, functions in vascular tone control. Fgf2 knockout mice are morphologically normal and display decreased vascular smooth muscle contractility, low blood pressure and thrombocytosis. Following intra-arterial mechanical injury, FGF2-deficient vessels undergo a normal hyperplastic response. These results force us to reconsider the function of FGF2 in vascular development and homeostasis in terms of vascular tone control.

Regulated human erythropoietin receptor expression in mouse brain

Erythropoietin (Epo) is known for its role in erythropoiesis and acts by binding to its receptor (EpoR) on the surface of erythroid progenitors. EpoR activity follows the site of hematopoiesis from the embryonic yolk sac to the fetal liver and then the adult spleen and bone marrow. Expression of EpoR has also been observed in selected cells of non-hematopoietic origin, such as the embryonic mouse brain during mid-gestation, at levels comparable to adult bone marrow. EpoR transcripts in brain decrease during development falling by birth to less than 1-3% of the level in hematopoietic tissue. We have now recapitulated this pattern of expression using a human EpoR transgene consisting of an 80-kb human EpoR genomic fragment. The highest level of expression was observed in the embryonic yolk sac and fetal liver, analogous to the endogenous gene, in addition to expression in adult spleen and bone marrow. Although activity of this transgene in brain is initially lower than the endogenous gene, it does exhibit the down-regulation observed for the endogenous gene in adult brain. The expression pattern of hybrid transgenes of an hEpoR promoter fused to beta-galactosidase in 9. 5-day embryos suggested that the hEpoR promoter region between -1778 and -150 bp 5' of the transcription start site is necessary to direct EpoR expression in the neural tube. EpoR expression in the neural tube may be the origin of the EpoR transcripts detected in brain during development. These data demonstrate that both the mouse and human EpoR genes contain regulatory elements to direct significant levels of expression in a developmentally controlled manner in brain and suggest that in addition to its function during erythropoiesis, EpoR may play a role in the development of selected non-hematopoietic tissue.

Developmental biology of hematopoiesis

Hematopoietic stem cells are at the top of a hierarchy that regulates the generation of a vast repertoire of blood cells during the lifetime of a vertebrate. Recent experiments, using a vast variety of embryonic systems, shed new light on the origin of stem cells and the genes that function to regulate and maintain hematopoietic differentiation programs. Two distinct populations of stem cells develop--derived initially from transient, extraembryonic source and later from a stable, intraembryonic source; it is possible that both are generated from a pro-HSC able to respond differentially to local inductions. The initial blood cells develop from ventral mesoderm. The blood-forming region develops as a result of signaling from specific, secreted, embryonic growth factors, including the bone morphogenetic proteins. Stem cells give rise to progenitors that are restricted progressively in their ability to contribute to specific lineages. This process is regulated by transcription factors, whose functions are confirmed through genetic analyses. The identification of highly conserved, embryonic signaling pathways and transcription regulatory genes illustrates the enormous utility of analyzing embryonic hematopoiesis in frog, chick, fish, and mouse systems to further our understanding of human stem cells.

Ureteric bud cells secrete multiple factors, including bFGF, which rescue renal progenitors from apoptosis

Kidney development requires reciprocal interactions between the ureteric bud and the metanephrogenic mesenchyme. Whereas survival of mesenchyme and development of nephrons from mesenchymal cells depends on signals from the invading ureteric bud, growth of the ureteric bud depends on signals from the mesenchyme. This codependency makes it difficult to identify molecules expressed by the ureteric bud that regulate mesenchymal growth. To determine how the ureteric bud signals the mesenchyme, we previously isolated ureteric bud cell lines (UB cells). These cells secrete soluble factors which rescue the mesenchyme from apoptosis. We now report that four heparin binding factors mediate this growth activity. One of these is basic fibroblast growth factor (bFGF), which is synthesized by the ureteric bud when penetrating the mesenchyme. bFGF rescues three types of progenitors found in the mesenchyme: precursors of tubular epithelia, precursors of capillaries, and cells that regulate growth of the ureteric bud. These data suggest that the ureteric bud regulates the number of epithelia and vascular precursors that generate nephrons by secreting bFGF and other soluble factors.

Formation of the branching pattern of blood vessels in the wall of the avian yolk sac studied by a computer simulation

The present study was performed to provide data to support the notion previously believed but not proved experimentally or theoretically, that blood vessels are formed by the selection of capillaries in the network. In an attempt to understand the mechanism of formation of blood vessel branching structures, the transformation of a capillary network to a branching system in the wall of quail yolk sac was successively recorded by a series of photographs, and a computer simulation was carried out for the process of in vivo vascularization based on the photographs. The simulation demonstrated that a positive feedback system participated in the formation of a branching structure. That is, vessels which had been much used were enlarged, whereas less used vessels were reduced in their size and finally extinguished. The enlarged vessels became major components of the branching system. As the body of an embryo grew, it was observed that polygonal capillary networks enlarged, which led each polygon of the network to divide into a few finer polygons. Then, some of the capillary vessels were again selected and formed a branching system. This process repeated during the body growth, indicating that the vascular system developed adaptively to the body growth. A region where the growth was fast, received much blood flow and produced finer networks of capillaries. Thus, it was experimentally demonstrated for the first time that capillaries in the network are successively selected by a positive feedback mechanism and form blood vessels.

Neovascularization of the Xenopus embryo

The receptor tyrosine kinase, Flk-1 or VEGFR-2, and its ligand, vascular endothelial growth factor (VEGF) are required for the development of the embryonic vasculature. Targeted disruption of either gene in mice results in the failure of vascular system formation. The Xenopus homologues of flk-1 and VEGF have been cloned and their expression has been examined throughout early embryonic development. These studies indicate that flk-1 is expressed in groups of endothelial precursor cells which will form the major blood vessels of the embryo, including the posterior cardinal veins, the dorsal aorta, the vitelline veins, and the endocardium. VEGF expression is found in tissues adjacent to the mesenchyme containing the flk-1-expressing endothelial precursors. Expression of both flk-1 and VEGF is transient, appearing as the primary vascular plexus is forming and declining steadily after the onset of functional embryonic circulation. After establishment of the primary vascular structures, flk-1 expression is also observed in the intersegmental veins which form by an angiogenic mechanism. Overall, these results support a role for VEGF/flk-1 signaling in both vasculogenesis and angiogenesis in the Xenopus embryo. When VEGF is expressed ectopically in Xenopus embryos by microinjection of either plasmid DNA or synthetic mRNA, large, disorganized vascular structures are produced. This result indicates that ectopic VEGF is capable of altering the architecture of the developing vascular network.

Ligand-dependent development of the endothelial and hemopoietic lineages from embryonic mesodermal cells expressing vascular endothelial growth factor receptor 2

The existence of a common precursor for endothelial and hemopoietic cells, termed the hemangioblast, has been postulated since the beginning of the century. Recently, deletion of the endothelial-specific vascular endothelial growth factor receptor 2 (VEGFR2) by gene targeting has shown that both endothelial and hemopoietic cells are absent in homozygous null mice. This observation suggested that VEGFR2 could be expressed by the hemangioblast and essential for its further differentiation along both lineages. However, it was not possible to exclude the hypothesis that hemopoietic failure was a secondary effect resulting from the absence of an endothelial cell microenvironment. To distinguish between these two hypotheses, we have produced a mAb directed against the extracellular domain of avian VEGFR2 and isolated VEGFR2+ cells from the mesoderm of chicken embryos at the gastrulation stage. We have found that in clonal cultures, a VEGFR2+ cell gives rise to either a hemopoietic or an endothelial cell colony. The developmental decision appears to be regulated by the binding of two different VEGFR2 ligands. Thus, endothelial differentiation requires VEGF, whereas hemopoietic differentiation occurs in the absence of VEGF and is significantly reduced by soluble VEGFR2, showing that this process could be mediated by a second, yet unidentified, VEGFR2 ligand. These observations thus suggest strongly that in the absence of the VEGFR2 gene product, the precursors of both hemopoietic and vascular endothelial lineages cannot survive. These cells therefore might be the initial targets of the VEGFR2 null mutation.

Embryonic endothelial cells transdifferentiate into mesenchymal cells expressing smooth muscle actins in vivo and in vitro

All blood vessels are lined by endothelium and, except for the capillaries, surrounded by one or more layers of smooth muscle cells. The origin of the embryonic vascular smooth muscle cell has until now been described from neural crest and locally differentiating mesenchyme. In this study, we have substantial evidence that quail embryonic endothelial cells are competent in the dorsal aorta of the embryo to transdifferentiate into subendothelial mesenchymal cells expressing smooth muscle actins in vivo. At the onset of smooth muscle cell differentiation, QH1-positive endothelial cells were experimentally labeled with a wheat germ agglutinin-colloidal gold marker (WGA-Au). No labeled subendothelial cells were observed at this time. However, 19 hours after the endothelial cells had endocytosed, the WGA-Au-labeled subendothelial mesenchymal cells were observed in the aortic wall. Similarly, during the same time period, subendothelial cells that coexpressed the QH1 endothelial marker and a mesenchymal marker, alpha-smooth muscle actin, were present. In such cells, QH1 expression was reduced to a cell membrane localization. A similar antigen switch was also observed during endocardial-mesenchymal transformation in vitro. Our results are the first direct in vivo evidence that embryonic endothelial cells may transdifferentiate into candidate vascular smooth muscle cells. These data arouse new interpretations of the origin and differentiation of the cells of the vascular wall in normal and diseased vessels.

In vitro and in vivo differentiation into B cells, T cells, and myeloid cells of primitive yolk sac hematopoietic precursor cells expanded > 100-fold by coculture with a clonal yolk sac endothelial cell line

The yolk sac, first site of hematopoiesis during mammalian development, contains not only hematopoietic stem cells but also the earliest precursors of endothelial cells. We have previously shown that a nonadherent yolk sac cell population (WGA+, density < 1.077, AA4.1+) can give rise to B cells, T cells and myeloid cells both in vitro and in vivo. We now report on the ability of a yolk sac-derived cloned endothelial cell line (C166) to provide a suitable microenvironment for expansion of these early precursor cells. Single day 10 embryonic mouse yolk sac hematopoietic stem cells wer expanded > 100 fold within 8 days by coculture with irradiated C166 cells. Colony-forming ability was retained for at least three passages in vitro, with retention of the ability to differentiate into T-cell, B-cell, and myeloid lineages. Stem cell properties were maintained by a significant fraction of nonadherent cells in the third passage, although these stem cells expressed a somewhat more mature cell surface phenotype than the initial yolk sac stem cells. When reintroduced into adult allogeneic immunocompromised (scid) hosts, they were able to give rise to all of the leukocyte lineages, including T cells, B cells, and myeloid cells. We conclude that yolk sac endothelial cells can support the stable proliferation of multipotential hematopoietic stem cells, thus generating adequate numbers of cells for study of the mechanisms involved in their subsequent development and differentiation, for in vivo hematopoietic restitution, and for potential use as a vehicle for gene transfer.

Embryonic angiogenesis: a review

Supply with nutrients is essential from early embryonic stages onwards. Therefore, circulatory organs form the first functioning organ system. With the exception of the heart, this system is at first formed by only one cell type, the endothelial cell. Emergence, behavior, and differentiation of endothelial cells are discussed in this review. At first, endothelial cells develop from angioblasts (primary angiogenesis/angioblastic development), later they develop from preexisting endothelial cells (secondary angiogenesis/angiotrophic growth). The composition of the extracellular matrix may promote or inhibit angiogenesis. Various growth factors which can be bound to the extracellular matrix may have been found, but only two of them (VEGF, P1GF) seem to influence endothelial cell behavior directly. Heterogeneity and organ-typical differentiation of endothelial cells seem to be dependent on cell-cell signaling within each organ.

Angiogenesis and the skin: a primer

Angiogenesis is the development of a blood supply to a given area of tissue. This area of tissue may be part of normal embryonic development, revascularization of a wound bed, or the stimulation of vessel growth by inflammatory or malignant cells. Angiogenesis is of crucial importance to the dermatologist, as it is of key importance in pathologic processes such as psoriasis, warts, and cutaneous malignancy, and it is required for optimal wound healing. Other dermatologic processes wherein angiogenesis is defective or uncontrolled are decubitus ulcers, stasis ulcers, pyogenic granulomas, hemangiomas, Kaposi's sarcoma, and possibly Spitz nevus, hypertrophic scars, and keloids. Recent advances in the understanding of growth factors will likely lead to advances in the treatment of skin cancer and psoriasis, and more rapid healing of wounds. In this review, I hope to summarize the most important growth factors, inhibitors of angiogenesis, and future directions in research and therapeutics involving angiogenesis.

Blood island formation in attached cultures of murine embryonic stem cells

Differentiation of murine embryonic stem cells in suspension culture results in the formation of cystic embryoid bodies that develop blood islands. In this study pre-cystic embryoid bodies were attached to a substratum, and the program of differentiation was monitored. The attached ES cell cultures formed blood islands on a cell layer that migrated out from the center of attachment and beneath a mesothelial-like cell layer. Morphological and in situ marker analysis showed benzidine-positive hematopoietic cells surrounded by vascular endothelial cells that expressed PECAM and took up DiI-Ac-LDL. Waves of morphological differentiation were evident, suggesting a graded response to differentiation signals. Electron microscopy of the blood islands showed that they were similar to blood islands of cystic embryoid bodies and mouse yolk sacs, and cell-cell junctions were evident among the blood island cells. RNA expression analysis was consistent with the presence of hematopoietic precursor cells of several lineages and a primitive vascular endothelium in the cultures. Thus a program of vascular and hematopoietic development can be elaborated in attached ES cell cultures, and these blood islands are accessible to experimental manipulation.

Characterization of fibroblast growth factor-2 binding to ribosomes

We have examined the binding of FGF-2 to ribosomes and have found that in NIH 3T3 cells that synthesize high amounts of all FGF-2 forms, both 18 kDa and HMW forms of FGF-2 bind to ribosomes. Ribosomes purified from these cells were treated with RNase or puromycin to identify the binding site of FGF-2 on the ribosome. Neither RNase nor puromycin treatment affected the in vivo binding of FGF-2 to ribosomes suggesting that FGF-2 binds ribosomal protein or rRNA, but not mRNA. The stoichiometry of binding in these cells was approximately 1 FGF-2 molecule bound per 1 ribosome. Binding was unaffected by high salt treatment indicating that FGF-2 tightly associates with polysomes. An in vitro binding experiment performed with purified ribosomes and recombinant FGF-2 suggested that the binding site is saturable. HBNF, a protein with similar charge and size to FGF-2, bound 15-fold less than FGF-2 to purified ribosomes. These results indicate that the binding of FGF-2 to ribosomes is specific.

Vascular endothelial cell lineage-specific promoter in transgenic mice

Vascular endothelial cells play essential roles in the function and development of the cardiovascular system. However, due to the lack of lineage-specific markers suitable for molecular and biochemical analyses, very little is known about the molecular mechanisms that regulate endothelial cell differentiation. We report the first vascular endothelial cell lineage-specific (including angioblastic precursor cells) 1.2 kb promoter in transgenic mice. Moreover, deletion analysis of this promoter region in transgenic embryos revealed multiple elements that are required for the maximum endothelial cell lineage-specific expression. This is a powerful molecular tool that will enable us to identify factors and cellular signals essential for the establishment of vascular endothelial cell lineage. It will also allow us to deliver genes specifically into this cell type in vivo to test specifically molecules that have been implicated in cardiovascular development. Furthermore, we have established embryonic stem (ES) cells from the blastocysts of the transgenic mouse that carry the 1.2 kb promoter-LacZ reporter transgene. These ES cells were able to differentiate in vitro to form cystic embryoid bodies (CEB) that contain endothelial cells determined by PECAM immunohistochemistry. However, these in vitro differentiated endothelial cells did not express the LacZ reporter gene. This indicates the lack of factors and/or cellular interactions which are required to induce the expression of the reporter gene mediated by this 1.2 kb promoter in this in vitro differentiation system. Thus this system will allow us to screen for the putative inducers that exist in vivo but not in vitro. These putative inducers are presumably important for in vivo differentiation of vascular endothelial cells.

Differentiation of endothelial cells: analysis of the constitutive and activated endothelial cell phenotypes

Endothelial cells line the inside of all blood vessels, forming a structurally and functionally heterogeneous population of cells. Their complexity and diversity has long been recognized, yet very little is known about the molecules and regulatory mechanisms that mediate the heterogeneity of different endothelial cell populations. The constitutive organ- and microenvironment-specific phenotype of endothelial cells controls internal body compartmentation, regulating the trafficking of circulating cells to distinct vascular beds. In contrast, surface molecules associated with the activated cytokine-inducible endothelial phenotype play a critical role in pathological conditions including inflammation, tumor angiogenesis, and wound healing. Differentiation of the endothelial cell phenotypes appears to follow similar mechanisms to the differentiation of hematopoietic cells, with the exception that endothelial cells maintain transdifferentiating competence. The present review offers a scheme of endothelial cell differentiation and discusses the possible applications of differentially expressed endothelial cell molecules as targets for directed therapeutic intervention.

Hypoblastic tissue and fibroblast growth factor induce blood tissue (haemoglobin) in the early chick embryo

We have investigated the temporal and the causal basis of blood tissue specification in the chick embryo. Earlier workers have shown that the prospective blood-forming area is specified in a horseshoe-shaped area at the posterior side of the embryo. We found that cultured explants from the posterior marginal zone at stages XI to XIII (consisting of the posterior marginal zone and part of Koller's sickle) have a high propensity to form haemoglobin (Hb), which could be inhibited at stage XI by adding antibody against basic fibroblast growth factor (bFGF) to the neutral culture medium; this treatment had no effect from stage XII onwards. The same result was found when whole embryos were cultured with an antiserum raised against bFGF, or with heparin. In another series of experiments, we found that cultured pieces from the inner-core of stage XIII epiblasts (with or without hypoblast tissue) were able to form Hb, whereas inner-core pieces from the pre-hypoblast stages, namely stages X and XI, did not form Hb. The capacity to form Hb, however, could be conferred upon the inner-core pieces from stage X epiblasts if bFGF at a concentration of 75–150 ng/ml was added to the culture medium. Furthermore, and most pertinently, the capacity to form Hb could be conferred on stage X inner-core pieces when they were co-cultured with hypoblast from a stage XIII embryo in a sandwich explant. Thus the inductive role of the hypoblast appears to be mediated via bFGF.(ABSTRACT TRUNCATED AT 250 WORDS)

In vitro studies on the existence of endothelial precursor cells in the subectodermal avascular region of quail wing buds

In vitro studies were performed to investigate the angiogenic capacity of different parts of the avian limb bud. Small pieces of wing mesenchyme of the vascularized core or of the avascular subectodermal region were obtained from quail embryos at stages 18 to 25, and were cultured. The identification of the avascular wing mesenchyme was made possible after injection of India ink via the vitelline vein or by bleeding control during in vivo dissection. Tissue cultures were treated with the QH-1 antibody or/and the endothelial cell marker DiI-Ac-LDL. Endothelial cells were found in cultures of the mesenchymal core and in those of the avascular subectodermal wing mesenchyme. Moreover, their appearance was independent of the stage of the donor embryo. Although there were no vessels, the subectodermal wing mesenchyme was able to produce endothelial cells that proliferated and differentiated under in vitro conditions. Thus, endothelial precursor cells probably existed within the avascular wing mesenchyme. These cells might be identical with the QH-1-positive isolated cells that have been described in immunohistochemical studies of this region; they may contribute to the growing capillary plexus of the limb bud.