Is extraembryonic angiogenesis in the chick embryo controlled by the endoderm? A morphology study

Visualizing the spatiotemporal pattern of yolk sac membrane vascular network by enhanced local fractal analysis

OBJECTIVE

To establish methods for providing a comprehensive and detailed description of the spatial distribution of the vascular networks, and to reveal the spatiotemporal pattern of the yolk sac membrane vascular network during the angiogenic procedure.

METHODS

Addressing the limitations in the conventional local fractal analysis, an improved approach, named scanning average local fractal dimension, was proposed. This method was conducted on 6 high-resolution vascular images of the yolk sac membrane for 3 eggs at two stages (E3 and E4) to characterize the spatial distribution of the complexity of the vascular network.

RESULTS

With the proposed method, the spatial distribution of the complexity of the yolk sac membrane vascular network was visualized. From E3 to E4, the local fractal dimension increased in 3 eggs, 1.80 ± 0.02 vs. 1.85 ± 0.02, 1.72 ± 0.03 vs. 1.83 ± 0.02, and 1.77 ± 0.03 vs. 1.82 ± 0.02, respectively. The mean local fractal dimension in the most distal area from the embryo proper was the lowest at E3 while the highest at E4. At E3, the most peaks of the local fractal dimension were located in the vein territories and shifted to artery territories at E4.

CONCLUSIONS

The spatial distribution of the complexity of the yolk sac membrane vascular network exhibited diverse patterns at different stages. In addition from E3 to E4, the increment of complexity at the intersection areas between arteries and sinus terminalis was with the most advance. This is consistent with the physiologic evidence. The present work provides a potential approach for investigating the spatiotemporal pattern of the angiogenic process.

Comparison of anti-angiogenic properties of pristine carbon nanoparticles

Angiogenesis is vital for tumour formation, development and metastasis. Recent reports show that carbon nanomaterials inhibit various angiogenic signalling pathways and, therefore, can be potentially used in anti-angiogenic therapy. In the present study, we compared the effect of different carbon nanomaterials on blood vessel development. Diamond nanoparticles, graphite nanoparticles, graphene nanosheets, multi-wall nanotubes and C60 fullerenes were evaluated for their angiogenic activities using the in ovo chick embryo chorioallantoic membrane model. Diamond nanoparticles and multi-wall nanotubes showed the greatest anti-angiogenic properties. Interestingly, fullerene exhibited the opposite effect, increasing blood vessel development, while graphite nanoparticles and graphene had no effect. Subsequently, protein levels of pro-angiogenic growth factor receptors were analysed, showing that diamond nanoparticles decreased the expression of vascular endothelial growth factor receptor. These results provide new insights into the biological activity of carbon nanomaterials and emphasise the potential use of multi-wall nanotubes and diamond nanoparticles in anti-angiogenic tumour therapy.

Extraembryonic origin of circulating endothelial cells

Circulating endothelial cells (CEC) are contained in the bone marrow and peripheral blood of adult humans and participate to the revascularization of ischemic tissues. These cells represent attractive targets for cell or gene therapy aimed at improving ischemic revascularization or inhibition of tumor angiogenesis. The embryonic origin of CEC has not been addressed previously. Here we use quail-chick chimeras to study CEC origin and participation to the developing vasculature. CEC are traced with different markers, in particular the QH1 antibody recognizing only quail endothelial cells. Using yolk-sac chimeras, where quail embryos are grafted onto chick yolk sacs and vice-versa, we show that CEC are generated in the yolk sac. These cells are mobilized during wound healing, demonstrating their participation to angiogenic repair processes. Furthermore, we found that the allantois is also able to give rise to CEC in situ. In contrast to the yolk sac and allantois, the embryo proper does not produce CEC. Our results show that CEC exclusively originate from extra-embryonic territories made with splanchnopleural mesoderm and endoderm, while definitive hematopoietic stem cells and endothelial cells are of intra-embryonic origin.

Emergence of haematopoietic stem cells during development

Self-renewable haematopoietic stem cells (HSCs) become segregated during development into a finite pool, from which they are mobilized upon physiological requirement. A central feature characterizing developmental haematopoiesis is that definitive organs become colonized by HSCs originating from a central source. The emission of HSCs occurs more or less continuously during a protracted period in parallel or successive sites. The most recently discovered of these sites is the placenta. The allantois, which is one of the components of the placenta, probed before it becomes vascularised, turns out to be a location where clonogenic precursors become committed. The placenta is thus a site of intrinsic haematopoiesis. Until this finding, the aorta and periaortic tissues were held to be the sites of definitive HSC commitment. The haematopoietic process in the aorta is prominent, particularly in avian embryos, and displays striking anatomical relationships between endothelial and haematopoietic cells. This made it possible to investigate the cytological and molecular relationship between the two types of cells. Somite exchanges between quail and chicken disclosed two distinct lineages, a dorsal one, purely endothelial, and a ventral one, hemangioblastic. The latter, also termed hemogenic endothelium, builds at first the whole inside lining of the aorta, and is then progressively replaced by cells of somitic origin, beginning with the aortic roof; it emits haematopoietic cells when located in the floor of the aorta and disappears. These events involve a changing molecular pattern, with expressions of transcription factor Runx1 and receptor VEGF-R2 as faithful markers of the lineage switch. Taking advantage of the stereotyped anatomical arrangement at the aortic level, which is favourable to dissect the mechanisms of HSC commitment, the analysis of developmental haematopoiesis should progress still further.

TaqMan®-based quantification of invasive cells in the chick embryo metastasis assay

The chick embryonic metastasis (CEM) assay is a fast in vivo method to investigate the invasive properties of tumor cells. Until now, most quantification methods were semiquantitative and time-consuming. Here we describe a rapid quantification method using TaqMan® technology to quantify the invaded tumor cells in the chorioallantoic membrane of fertilized eggs. This method is based on specific detection of human ALU sequences. Moreover, it provides high sensitivity over a wide linearity range.

Vasculogenesis and angiogenesis

Two distinct mechanisms, vasculogenesis and angiogenesis implement the formation of the vascular network in the embryo. Vasculogenesis gives rise to the heart and the first primitive vascular plexus inside the embryo and in its surrounding membranes, as the yolk sac circulation. Angiogenesis is responsible for the remodeling and expansion of this network. While vasculogenesis refers to in situ differentiation and growth of blood vessels from mesodermal derived hemangioblasts, angiogenesis comprises two different mechanisms: endothelial sprouting and intussusceptive microvascular growth (IMG). The sprouting process is based on endothelial cell migration, proliferation and tube formation. IMG divides existing vessel lumens by formation and insertion of tissue folds and columns of interstitial tissue into the vessel lumen. The latter are termed interstitial or intervascular tissue structures (ITSs) and tissue pillars or posts. Intussusception also includes the establishment of new vessels by in situ loop formation in the wall of large veins. The molecular regulation of these distinct mechanisms is discussed in respect to the most important positive regulators, VEGF and its receptors flk-1 (KDR) and flt-1, the Angiopoietin/tie system and the ephrin-B/EpH-B system. The cellular mechanisms and the molecular regulation of angiogenesis in the pathological state are summarized and the differences of physiological and pathological angiogenesis elaborated.

Induction of the avian coelom with associated vitelline blood circulation by Rauber's sickle derived junctional endoblast and its fundamental role in heart formation

In histological sections through chicken blastoderms of different ages we describe the temporospatial relationship between junctional endoblast, the formation of blood islands (appearing first from a peripherally migrating mesoblastic blastema), and the formation of coelomic vesicles developing later in/and from a more superficially extending mesoblastic blastema (coelomic mesoblast). After unilateral removal of the Rauber's sickle-derived junctional endoblast in early streak blastoderms (stage 2-4; Vakaet [1970] Arch Biol 81:387-426) and culture to stage 11 (Hamburger and Hamilton [1951] J Morphol 88:49-92), we observed that the early formation of the coelomic cavity was locally or totally disturbed in the operated area. Besides the simultaneous absence of blood islands, the coelomic vesicles did not form normally. Instead of regularly aligned coelomic vesicles, progressively forming the coelomic cavity by fusion, some voluminous irregular cavities appeared. Thus, the extent of the coelomic cavity was greatly reduced and the operated side was considerably smaller than the unoperated side. Furthermore, in the youngest operated blastoderms the cranial portion of the involved coelomic cavity (hemipericardial cavity) exhibited rudimentary development and usually did not reach the region of the foregut endoderm. This resulted in the absence of the myoepicardium and associated endocardium at this side. In another experiment, after removal of the junctional endoblast at one side of the chicken blastoderm, a fragment of quail junctional endoblast was placed isotopically. This resulted, after further in vitro culture, in the restoration of the formation of coelomic vesicles and accompanying subjacent blood islands in the immediate neighborhood of the apposed quail junctional endoblast. Also, the pericardium and primary heart tube developed normally. Similarly, by using the quail-chicken chimera technique, we demonstrated that the splanchnic mesoderm cells of the pericardium develop in intimate association with the most cranial part of the junctional endoblast (derived from the Rauber's sickle horns). Our experiments indicate that the coelom and, in particular, the pericardium and primary heart tube form progressively (in time and space) under the inductory influence of Rauber's sickle and junctional endoblast.

In the absence of Rauber's sickle material, no blood islands are formed in the avian blastoderm

Using the quail-chick chimera technique, we followed the fate of Rauber's sickle cells in older whole blastoderms (cultured for approximately 2 days): after removal of the autochthonous Rauber's sickle from an unincubated chicken blastoderm, a quail Rauber's sickle was grafted isotopically and isochronically in its place. In transverse sections through these chimeras, the grafted quail Rauber's sickle cells were seen to have transformed into a broad row or ridge of quail junctional endoblast cells extending at the inner border of the area containing blood islands. After unilateral removal of the junctional endoblast from an intermediate streak chicken blastoderm (Stage 3; Hamburger and Hamilton [1951] J Morphol 88:49-92), we observed during further in vitro culture that at the operated side, in the area previously occupied by this junctional endoblast, blood islands no longer developed. If after such a unilateral removal of the chicken junctional endoblast quail junctional endoblast was apposed in its place, then blood islands reappeared in the operated area. The intimate contact between the apposed quail junctional endoblast and the recently formed blood islands, derived from peripherally migrating mesoderm, was very obvious on sections through such chimeras. We further demonstrate that Rauber's sickle vs. junctional endoblast is indispensable for the anlage of blood islands in avian blastoderms. Indeed, in the absence of Rauber's sickle material no blood islands develop (even when mesoderm is present after ingression of the upper layer via a primitive streak) in the isolated central region of the area centralis of unincubated chicken blastoderms after culture in vitro. Also, no junctional endoblast and no sickle canal appear in these explants. By contrast, if a Rauber's sickle fragment is placed on such an isolated central blastoderm region, then blood islands develop. These blood islands start to develop from peripherally migrating mesoderm in the neighborhood of the Rauber's sickle-derived junctional endoblast.

Ontogeny of the endothelial system in the avian model

The avian model provides an experimental approach for dissecting the origin, migrations and differentiation of cell lineages in early embryos. In this model, the endothelial network was shown to take place through two processes depending on the origin of endothelial precursors: vasculogenesis when angioblasts emerge in situ, angiogenesis when angioblasts are extrinsic. Two different mesodermal territories produce angioblasts, the somite which only gives rise to endothelial cells and the splanchnopleural mesoderm which also produces hemopoietic stem cells. Potentialities of the mesoderm are determined by a positive influence from the endoderm and a negative control from the ectoderm. The presence of circulating endothelial precursors in the embryonic blood stream is also detected.

Vasculogenesis and angiogenesis as mechanisms of vascular network formation, growth and remodeling

Two distinct mechanisms, vasculogenesis and angiogenesis implement the formation of the vascular network in the embryo. Vasculogenesis gives rise to the heart and the first primitive vascular plexus inside the embryo and in its surrounding membranes, as the yolk sac circulation. Angiogenesis is responsible for the remodeling and expansion of this network. While vasculogenesis refers to in situ differentiation and growth of blood vessels from mesodermal derived hemangioblasts, angiogenesis comprises two different mechanisms: endothelial sprouting and intussusceptive microvascular growth (IMG). The sprouting process is based on endothelial cell migration, proliferation and tube formation. IMG divides existing vessel lumens by formation and insertion of tissue folds and columns of interstitial tissue into the vessel lumen. The latter are termed interstitial or inter-vascular tissue structures (ITSs) and tissue pillars or posts. Intussusception also includes the establishment of new vessels by in situ loop formation in the wall of large veins. The molecular regulation of these distinct mechanisms is discussed in respect to the most important positive regulators, vascular endothelial growth factor (VEGF) and its receptors flk-1 (KDR) and flt-1, the Angiopoietin/tie system and the ephrin-B/EpH-B system. The cellular mechanisms and the molecular regulation of angiogenesis in the pathological state are summarized and the differences of physiological and pathological angiogenesis elaborated.

Vasculogenesis in the day 6.5 to 9.5 mouse embryo

The process of vasculogenesis was characterized in the 6.5- to 9.5-day mouse embryo and in allantoic culture by analysis of spatial and temporal expression patterns of the endothelial or hematopoietic lineage-associated proteins, TAL1, Flk1, platelet/endothelial cell adhision molecule (PECAM), CD34, VE-cadherin, and Tie2. The study establishes that: (1) TAL1 and Flk1 are coexpressed in isolated mesodermal cells that give rise to endothelial cells and thus can be defined as angioblasts; (2) hematopoietic cells of blood islands express TAL1, but not Flk1; (3) vasculogenesis in the embryo proper is initiated by mesoderm fated to give rise to the endocardium; (4) the maturation/morphogenesis of blood vessels can be defined in terms of a sequential pattern of expression in which TAL1 and Flk1 are expressed first followed by PECAM, CD34, VE-cadherin, and later Tie2; and (5) TAL1 expression is down-regulated in endothelial cells of mature vessels.

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.

Immunolocalization of the vascular endothelial growth factor receptor-2 in the subepicardial mesenchyme of hamster embryos: identification of the coronary vessel precursors

The earliest evidence of the development of the cardiac vessels in mammals is the emergence of subepicardial blood islands, which are thought to originate from mesenchymal progenitors. In order to identify these progenitor cells, we have studied the immunohistochemical localization in the heart of Syrian hamster embryos of the type 2 vascular endothelial growth factor receptor, the earliest molecule known to be expressed in the vasculogenic cell lineage. Only a few immunoreactive subepicardial mesenchymal cells were present by 10 days post coitum. By 11 days post coitum, the subepicardial mesenchymal cells became abundant at the dorsal part of the ventricle, the atrioventricular and the conoventricular grooves. About 20% of cells were labelled with the antibody. Immunoreactive cells were isolated or formed pairs, short cords, rounded clusters or ring-like structures at the subepicardium or, occasionally, within the ventricular myocardium. Other labelled cells were simultaneously cytokeratin immunoreactive. By 12 days post coitum, most immunoreactive mesenchymal cells have been replaced by a capillary network. We propose that an active process of vascular differentiation occurs between 10 and 12 days post coitum in the subepicardium of this species, and it might be a suitable model for the study of vasculogenetic mechanisms.

TAL1/SCL is expressed in endothelial progenitor cells/angioblasts and defines a dorsal-to-ventral gradient of vasculogenesis

In this study we establish that TAL1/SCL, a member of the helix-loop-helix family of transcription factors, and an important regulator of the hematopoietic lineage in mice, is expressed in the endothelial lineage of avians. The earliest events of vascular development were examined using antibodies to TAL1/SCL, and the QH1 antibody, an established marker of quail endothelial cells. Analyses using double immunofluorescence confocal microscopy show that: (i) TAL1/SCL is expressed by both quail and chicken endothelial cells; (ii) TAL1/SCL expression precedes that of the QH1 epitope; and (iii) TAL1/SCL, but not QH1, expression defines a subpopulation of primordial cells within the splanchnic mesoderm. Collectively these data suggest that TAL1/SCL-positive/QH1-negative cells are angioblasts. Further, using TAL1/SCL expression as a marker of the endothelial lineage, we demonstrate that in addition to the previously described cranial-to-caudal gradient, there is a dorsal-to-ventral progression of vasculogenesis.

Lingual papillae of the growing rat as a model of vasculogenesis

BACKGROUND

Capillary sprouting is an important mechanism that initiates neovascularization. Because observation of capillary sprouting and its morphological staging can be problematic, we sought to establish a simple model of capillary growth.

METHODS

Rats were obtained at gestational days 15, 16, and 20, at birth, and at postnatal day 10. Scanning electron microscopy (SEM) of vascular casts, freeze-fractured and epithelium-exfoliated specimens, as well as transmission electron microscopy (TEM) of tissue sections were used.

RESULTS

In day 15 fetuses, the filiform papillae and their connective tissue cores had not been formed, but a simple capillary network without regional differences was present. In day 16 fetuses, mesenchymal cells started to form papillary connective tissue cores, and, inside the epithelium, ridges were found. Capillary sprouts arose from the preexisting sinusoidal capillaries by elongation and widening, invaded into connective tissue cores in day 20 fetuses, and gradually bifurcated to form capillary loops in the prospective giant conical papillae of the newborn rat. In postnatal day 10 rats, the capillary network beneath the papillae became bilayered.

CONCLUSION

Vascular formation in the lingual papillae in growing rats offers an easy model for the observation of capillary sprouting. In this model, the sprouts arise from preexisting sinusoidal capillaries and not from veins, as usually observed in other models. The mechanism of capillary growth is the elongation of (preexisting) sinusoidal capillaries into the developing connective tissue cores and toward the forming epithelial ridges.

Two distinct endothelial lineages in ontogeny, one of them related to hemopoiesis

We have shown previously by means of quail/chick transplantations that external and visceral organs, i.e., somatopleural and splanchnopleural derivatives, acquire their endothelial network through different mechanisms, namely immigration (termed angiogenesis) versus in situ emergence of precursors (or vasculogenesis). We have traced the distribution of QH1-positive cells in chick hosts after replacement of the last somites by quail somites (orthotopic grafts) or lateral plate mesoderm (heterotopic grafts). The results lead to the conclusion that the embryo becomes vascularized by endothelial precursors from two distinct regions, splanchnopleural mesoderm and paraxial mesoderm. The territories respectively vascularized are complementary, precursors from the paraxial mesoderm occupy the body wall and kidney, i.e., they settle along with the other paraxial mesoderm derivatives and colonize the somatopleure. The precursors from the two origins have distinct recognition and potentialities properties: endothelial precursors of paraxial origin are barred from vascularizing visceral organs and from integrating into the floor of the aorta, and are never associated with hemopoiesis; splanchnopleural mesoderm grafted in the place of somites, gives off endothelial cells to body wall and kidney but also visceral organs. It gives rise to hemopoietic precursors in addition to endothelial cells.

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.

Coordinate expression of vascular endothelial growth factor receptor-1 (flt-1) and its ligand suggests a paracrine regulation of murine vascular development

Vascular endothelial growth factor (VEGF) is a candidate regulator of blood vessel growth during embryonic development and in tumors. To evaluate the role of VEGF receptor-1/flt-1 (VEGFR1/flt-1) in the development of the vascular system, we have characterized the murine homolog of the human flt-1 gene and have analyzed its expression pattern during mouse embryogenesis. Receptor binding studies using transfected COS cells revealed that the murine flt-1 gene encodes a high affinity receptor for VEGF. The apparent Kd for VEGF binding, as determined by Scatchard analysis, was 114 pM, demonstrating that VEGFR1/flt-1 has a higher affinity to VEGF than VEGF receptor-2/flk-1 (VEGFR2/flk-1). By in situ hybridization, VEGFR1/flt-1 was detected in the yolk sac mesoderm already at the early stages of vascular development, while the receptor ligand was expressed in the entire endoderm of 7.5-day mouse embryos. A comparison with VEGFR2/flk-1 showed that the two receptors shared a common expression domain in the yolk sac mesoderm, but were expressed at different sites in the ectoplacental cone. The differential expression of the two VEGF receptors persisted in the developing placenta, where VEGFR1/flt-1 mRNA was detected in the spongiotrophoblast layer, whereas VEGFR2/flk-1 transcripts were present in the labyrinthine layer which is the site of VEGF expression. In the embryo proper, VEGFR1/flt-1 mRNA was specifically localized in blood vessels and capillaries of the developing organs, closely resembling the pattern of VEGFR2/flk-1 transcript distribution. In the developing brain, the expression of VEGF receptors in the perineural capillary plexus and in capillary sprouts which have invaded the neuro-ectoderm correlated with endothelial cell proliferation and brain angiogenesis. The data are consistent with the hypothesis that VEGF and its receptors have an important function both in the differentiation of the endothelial lineage and in the neovascularization of developing organs, and act in a paracrine fashion.

A new model of vasculogenesis and angiogenesis in vitro as compared with vascular growth in the avian area vasculosa

In cultures of dissociated quail epiblast the basic constituents of the vascular system, blood cells and endothelial cells can be induced by basic fibroblast growth factor (Flamme and Risau, Development, 116: 435-439, 1992). As we show here, in those cultures three types of vascular plexus differentiate spontaneously under different culture conditions: At the 3rd day a vascular plexus appears in situ closely resembling the vascular plexus of the quail area opaca vasculosa (vasculogenesis). Vascular sprouts are formed, extending long filopodia at their tips. Such filopodia are shown to build the first intervascular bridges in the growing vascular plexus of the area vasculosa at embryonic day 3. Connections of filopodia turn out to be precursors of new capillaries interconnecting pre-existing blood vessels (angiogenesis). Two further types of in vitro capillary plexus differentiate in long term endothelial cell cultures derived from induced angioblasts. Whereas one closely resembles so-called angiogenesis in vitro, the third type comprises mainly multinucleated giant endothelial cells lining loop like capillaries and represents a differentiation of aging endothelial cell culture. Thus, the present in vitro model is an approach to the sequence of angioblast induction, vasculogenesis, and angiogenesis.

Emergence of endothelial and hemopoietic cells in the avian embryo

During organogenesis, endothelial cells develop through two different mechanisms: differentiation of intrinsic precursors in organ rudiments constituted of mesoderm associated with endoderm, and colonization by extrinsic precursors in organs constituted of mesoderm associated with ectoderm (Pardanaud et al. 1989). On the other hand, both types of rudiment are colonized by extrinsic hemopoietic stem cells. In the present work we extend our former study by investigating the hemangioblastic (i.e. hemopoietic and angioblastic) potentialities of primordial germ layers in the area pellucida during the morphogenetic period. By means of interspecific grafts between quail and chick embryos, we show that splanchnopleural mesoderm gives rise to abundant endothelial cells, and to numerous hemopoietic cells in a permissive microenvironment, while somatopleural mesoderm produces very few cells belonging to these lineages, or none. Thus we confirm that the angioblastic capacities of the mesoderm differ radically, depending on its association with ectoderm or endoderm. Furthermore, at this embryonic period, both endothelial and hemopoietic potentialities are displayed by splanchnopleural mesoderm. However the site of emergence of intraembryonic hemopoietic stem cells appears spatially restricted by comparison to more widespread angioblastic capacities.

Induction of vasculogenesis and hematopoiesis in vitro

Despite a large number of investigations of embryonic vascular development, in particular in avian embryos, the conditions under which the endothelial and hematopoietic cell lineages emerge remain unknown. As we demonstrate here, both endothelial and hematopoietic cells can be induced by treatment of dissociated quail epiblast with fibroblast growth factors in vitro. These cells aggregate in characteristic blood islands. In long-term culture, the induced endothelial cells gave rise to vascular structures in vitro, i.e. vasculogenesis. No induction was observed in the absence of fibroblast growth factors, and other growth factors like TGF-beta, TGF-alpha and EGF were not capable of inducing blood island formation. Thus, the dissociated quail epiblast provides a remarkably simple test system to investigate cell lineage diversification in higher vertebrates.

A comparative study on the effects of tumor necrosis factor-alpha (TNF-alpha), human angiogenic factor (h-AF) and basic fibroblast growth factor (bFGF) on the chorioallantoic membrane of the chick embryo

The chorioallantoic membrane (CAM) assay is a widely used bioassay for testing angiogenic activities. In the present study we compared the gross and micromorphological effects of three angiogenic factors applied in Elvax carriers on the CAM: Tumor necrosis factor-alpha (TNF-alpha), human angiogenic factor (h-AF), and basic fibroblast growth factor (bFGF). Our question was whether the CAM responds to these factors which have very different actions with a stereotype or with a factor specific reaction. By microangiography and light microscopy, all positive reactions appeared as a spoke-wheel vascular pattern with a bundle of small capillary blood vessels in the center. These vessels were predominantly of a distended type in h-AF and TNF experiments, while narrower capillary vessels followed bFGF application. Chorioallantoic ectoderm and endoderm were thickened by cell accumulation and the mesenchymal stroma of the CAM was edematous and infiltrated with leucocytes in all three reactions. Additionally, bFGF experiments showed areas of densely arranged fibroblasts. Observations in vivo showed chorioallantoic tissue movements as a possible mechanism for the spokewheel vascular pattern. As compared with our results from studies of cytokinetics with bromodeoxyuridine, these current findings indicate that chemotaxis is responsible for the chorioallantoic angiogenic reaction rather than cellular proliferation.

Two-phase in vitro culture of explanted chick embryos

The present study describes a method of culturing chick embryos together with their surrounding area vasculosa on two different culture media in succession. Embryos in the 2nd day of incubation (stages 13, 14, 15 according to Hamburger and Hamilton, 1951) were explanted from the yolk with the aid of a ring of filter paper and transferred dorsal side up to a silicone culture dish containing the first culture medium (89.5% L-15, 10% fetal calf serum, 0.5% Antibiotics). The paper ring was clamped onto the wall of the culture dish by a steel ring so that the embryo was fixed for the culture period. After 4 +/- 1, 8 +/- 1, 12 +/- 1 hrs, the embryos were taken from the culture dishes and transferred to others containing a yolk-albumen mixture as culture medium; 81.2% of embryos survived the first phase of culture. On the second medium 50.3% of explanted embryos were still alive at stage 20 (HH), and 7.9% of them reached the 5th day of development (St 25 HH). The average length of survival in vitro was found to be influenced by both the length of the first culture phase and the stage at which embryos were explanted. This culture method may be useful for teratological tests, since in the first phase of culture, concentrations of test substances and the time of exposure can be exactly adjusted, and in the second phase, the embryo is allowed to develop quite normally, under conditions similar to those in ovo.

Embryonic angiogenesis factors

The vascular system develops during embryonic development by at least two distinct processes; vasculogenesis is the development of blood vessels from in situ differentiating angioblasts and angiogenesis is the sprouting of capillaries from pre-existing vessels. The molecular mechanisms involved in the regulation of these processes are poorly understood. Endoderm-mesoderm interactions seem to play an important role in angioblast differentiation and vasculogenesis. Soluble angiogenic factors may be involved in the vascularization of some embryonic organs, e.g. kidney and brain. Angiogenic growth factors have been isolated and purified from embryonic brain and identified as acidic and basic fibroblast growth factors. More specific endothelial cell growth factors such as platelet-derived endothelial cell growth factor and vascular endothelial growth factor may also play a role in embryonic angiogenesis.

Differentiation of endothelial cells in avian embryos does not depend on gastrulation

Unincubated quail eggs were treated with Cytochalasin B. By this means, gastrulation of the blastodiscs was inhibited. Fragments of these blastodiscs were grafted into wings buds of chick embryos, and the differentiation fate of graft-derived cells was studied. Results show that only endothelial cells differentiate from the grafts. They were even found outside the graft site in vessels made up of a chimeric endothelium. It can be concluded that determination, differentiation and migration of endothelial cells does not depend on gastrulation.