Distribution and migration of angiogenic cells from grafted avascular intraembryonic mesoderm

Neural crest and the patterning of vertebrate craniofacial muscles

Patterning of craniofacial muscles overtly begins with the activation of lineage-specific markers at precise, evolutionarily conserved locations within prechordal, lateral, and both unsegmented and somitic paraxial mesoderm populations. Although these initial programming events occur without influence of neural crest cells, the subsequent movements and differentiation stages of most head muscles are neural crest-dependent. Incorporating both descriptive and experimental studies, this review examines each stage of myogenesis up through the formation of attachments to their skeletal partners. We present the similarities among developing muscle groups, including comparisons with trunk myogenesis, but emphasize the morphogenetic processes that are unique to each group and sometimes subsets of muscles within a group. These groups include branchial (pharyngeal) arches, which encompass both those with clear homologues in all vertebrate classes and those unique to one, for example, mammalian facial muscles, and also extraocular, laryngeal, tongue, and neck muscles. The presence of several distinct processes underlying neural crest:myoblast/myocyte interactions and behaviors is not surprising, given the wide range of both quantitative and qualitative variations in craniofacial muscle organization achieved during vertebrate evolution.

Morphogenesis of embryonic CNS vessels

This chapter focuses on the morphology of blood vessel formation in and around the early central nervous system (CNS, i.e., brain and spinal cord) of avian embryos. We discuss cell lineages, proliferation and interactions of endothelial cells, pericytes and smooth muscle cells, and macrophages. Due to space limitations, we can not review the molecular control of CNS angiogenesis, but refer the reader to other chapters in this book and to recent publications on the assembly of the vasculature (1,2).

Stem cell-derived endothelial cells/progenitors migrate and pattern in the embryo using the VEGF signaling pathway

Endothelial precursor cells respond to molecular cues to migrate and assemble into embryonic blood vessels, but the signaling pathways involved in vascular patterning are not well understood. We recently showed that avian vascular patterning cues are recognized by mammalian angioblasts derived from somitic mesoderm through analysis of mouse-avian chimeras. To determine whether stem cell-derived endothelial cells/progenitors also recognize global patterning signals, murine ES cell-derived embryoid bodies (EBs) were grafted into avian hosts. ES cell-derived murine endothelial cells/progenitors migrated extensively and colonized the appropriate host vascular beds. They also formed mosaic vessels with avian endothelial cells. Unlike somite derived-endothelial cells, ES cell-derived endothelial cells/progenitors migrated across the host embryonic midline to the contralateral side. To determine the role of VEGF signaling in embryonic vascular patterning, EBs mutant for a VEGF receptor (flk-1(-/-)) or a signal (VEGF-A(-/-)) were grafted into quail hosts. Flk-1(-/-) EB grafts produced only rare endothelial cells that did not migrate or assemble into vessels. In contrast, VEGF-A(-/-) EB grafts produced endothelial cells that resembled wild-type and colonized host vascular beds, suggesting that host-derived signals can partially rescue mutant graft vascular patterning. VEGF-A(-/-) graft endothelial cells/progenitors crossed the host midline with much lower frequency than wild-type EB grafts, indicating that graft-derived VEGF compromised the midline barrier when present. Thus, ES cell-derived endothelial cells/progenitors respond appropriately to global vascular patterning cues, and they require the VEGF signaling pathway to pattern properly. Moreover, EB-avian chimeras provide an efficient way to screen mutations for vascular patterning defects.

Microvascular assembly and cell invasion in chick mesonephros grafted onto chorioallantoic membrane

Embryonic tissues, in common with other tissues, including tumours, tend to develop a substantial vasculature when transplanted onto the chorioallantoic membrane (CAM). Studies conducted to date have not examined in any detail the identity of vessels that supply these grafts, although it is known that the survival of transplanted tissues depends on their ability to connect with CAM vessels supplying oxygen and nutrients. We grafted the mesonephros, a challenging model for studies in vascular development, when it was fully developed (HH35). We used reciprocal chick-quail transplantations in order to study the arterial and venous connections and to analyse the cell invasion from the CAM to the organ, whose degeneration in normal conditions is rapid. The revascularization of the grafted mesonephros was produced by the formation of peripheral anastomoses between the graft and previous host vasculatures. The assembly of graft and CAM blood vessels occurred between relatively large arteries or veins, resulting in chimeric vessels of varying morphology depending on their arterial or venous status. Grafts showed an increased angiogenesis from their original vasculature, suggesting that the normal vascular degeneration of the mesonephros was partially inhibited. Three types of isolated host haemangioblast were identified in the mesonephros: migrating angioblast-like cells, indicating vasculogenesis, undifferentiated haematopoietic cells and macrophages, which might have been involved in the angiogenesis. Tomato lectin was found to bind activated macrophages in avian embryos.

Endoderm is required for vascular endothelial tube formation, but not for angioblast specification

Angioblasts, the precursor cells that comprise the endothelial layer of blood vessels, arise from a purely mesodermal population. Individual angioblasts coalesce to form the primary vascular plexus through a process called vasculogenesis. A number of reports in the literature suggest that signals from the adjacent endoderm are necessary to induce angioblast specification within the mesoderm. We present evidence, using both embryological and molecular techniques, indicating that endoderm is not necessary for the induction of angioblasts. Xenopus embryos that had endoderm physically removed at the onset of gastrulation still express vascular markers. Furthermore, animal caps stimulated with bFGF form angioblasts in the absence of any detectable endodermal markers. These results show that endoderm is not required for the initial formation of angioblasts. While Xenopus embryos lacking endoderm contain aggregates of angioblasts, these angioblasts fail to assemble into endothelial tubes. Endothelial tube formation can be rescued, however, by implantation of endodermal tissue from sibling embryos. Based on these studies in Xenopus, and corroborating experiments using the quail embryo, we conclude that endoderm is not required for angioblast specification, but does play an essential role in the formation of vascular tubes.

Assembly of trunk and limb blood vessels involves extensive migration and vasculogenesis of somite-derived angioblasts

Vascular development requires the assembly of precursor cells into blood vessels, but how embryonic vessels are assembled is not well understood. To determine how vascular cells migrate and assemble into vessels of the trunk and limb, marked somite-derived angioblasts were followed in developing embryos. Injection of avian somites with the cell-tracker DiI showed that somite-derived angioblasts in unperturbed embryos migrated extensively and contributed to trunk and limb vessels. Mouse-avian chimeras with mouse presomitic mesoderm grafts had graft-derived endothelial cells in blood vessels at significant distances from the graft, indicating that mouse angioblasts migrated extensively in avian hosts. Mouse graft-derived endothelial cells were consistently found in trunk vessels, such as the perineural vascular plexus, the cardinal vein, and presumptive intersomitic vessels, as well as in vessels of the limb and kidney rudiment. This reproducible pattern of graft colonization suggests that avian vascular patterning cues for trunk and limb vessels are recognized by mammalian somitic angioblasts. Mouse-quail chimeras stained with both the quail vascular marker QH1 and the mouse vascular marker PECAM-1 had finely chimeric vessels, with graft-derived mouse cells interdigitated with quail vascular cells in most vascular beds colonized by graft cells. Thus, diverse trunk and limb blood vessels have endothelial cells that developed from migratory somitic angioblasts, and assembly of these vessels is likely to have a large vasculogenic component.

Physiology of angiogenesis

Angiogenesis is a key prerequisite for growth in all vertebrate embryos and in many tumors. Rapid growth requires efficient transport of oxygen and metabolites. Hence, for a better understanding of tissue growth, biophysical properties of vascular systems, in addition to their molecular mechanisms, need to be investigated. The purpose of this article is twofold: (1) to discuss the biophysics of growing and perfused vascular systems in general, emphasizing non-sprouting angiogenesis and remodeling of vascular plexuses; and (2) to report on cellular details of sprouting angiogenesis in the initially non-perfused embryonic brain and spinal cord. It is concluded that (1) evolutionary optimization of the circulatory system corresponds to highly conserved vascular patterns and angiogenetic mechanisms; (2) deterministic and random processes contribute to both extraembryonic and central nervous system vascularization; (3) endothelial cells interact with a variety of periendothelial cells during angiogenesis and remodeling; and that (4) mathematical models integrating molecular, morphological and biophysical expertise improve our understanding of normal and pathological angiogenesis and account for allometric relations.

Blood-borne seeding by hematopoietic and endothelial precursors from the allantois

Until now the allantois has not been considered as a hematopoietic organ. Here we report experimental evidence demonstrating the in situ emergence of both hematopoietic and endothelial precursors in the avian allantoic bud. When the prevascularized allantoic bud from a quail embryo was grafted in the coelom of a chicken host, hematopoietic and endothelial cells later were found in the bone marrow of the host. Because the graft was located at a distance from the limb bud, these cells could reach the bone marrow only by the circulatory pathway. This blood-borne seeding may be accomplished by distinct hematopoietic and endothelial precursors, or by hemangioblasts, the postulated common precursors of these two lineages; we consider the latter interpretation more likely. We also show by reverse transcription-PCR that the allantois region expresses very early the GATA genes involved in hematopoiesis and some beta-globin chain genes.

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.

Development of pericyte-like cells during angiogenesis in quail chick chimeras as detected by combined Feulgen reaction and immunohistochemistry

Perivascular fibroblasts have been proposed as possible precursor cells for microvascular pericytes. To investigate the development of pericytes during angiogenesis we examined interspecific grafts between chick and quail embryos. Limb buds of three-day old quail embryos were transferred to the chorioallantoic membrane (CAM) of ten to fourteen day-old chick embryos. Six days after grafting, the limb buds were explanted and histologically examined by combined Feulgen reaction and immunohistochemistry using an antibody to quail endothelial and hemopoietic cells (QH-1). Limb buds were found to be vascularized by a network of capillaries which were partially derived from sprouts of the chick CAM microvasculature. Numerous hybrid capillaries were detected, consisting of host endothelial cells (chick) and graft pericytes (quail). These results provide further support for the idea that microvascular pericytes can evolve from perivascular fibroblasts.

The angiogenic potentials of the cephalic mesoderm and the origin of brain and head blood vessels

We have used two molecular markers to label blood vessel endothelial cells and their precursors in the early avian embryo. One marker, called Quek1, is the avian homologue of the mammalian VEGF receptor flk-1 and the other is the MB1/QH1 monoclonal antibody. Quek1 is expressed in a subset of mesodermal cells from the gastrulation stage. Quek1 positive cells later form blood vessel endothelial cells and express the MB1/QH1 antigen which is specific for endothelial and hemopoietic cells of the quail species. These two markers allowed us first to show that the cephalic paraxial mesoderm has angiogenic potentials which are much more extended than its trunk counterpart (the somites). Secondly, the origin of the endothelial cells lining the craniofacial and head blood vessels was mapped on the 3-somite stage cephalic mesoderm via the quail-chick chimera technique, in which well defined mesodermal territories are exchanged between stage-matched embryos of both species in a strictly isotopic manner. We found that the anterior region of the cephalic paraxial mesoderm is largely recruited to provide the forebrain and the upper face with their vasculature. This means that large volumes of tissues are vascularized by a discrete region of the cephalic mesoderm, the fate of which is otherwise to give rise to muscles. The widespread expansion of the angiogenic cells arising from the anterior paraxial mesoderm must be related to the high growth rate of the anterior region of the neural primordium, yielding the telencephalon and of the neural crest-derived facial structures which are themselves devoid of angiogenic potencies.(ABSTRACT TRUNCATED AT 250 WORDS)

Proliferation pattern of capillary endothelial cells in chorioallantoic membrane development indicates local growth control, which is counteracted by vascular endothelial growth factor application

The density and distribution of whole mount BrdU-anti-BrdU labeled endothelial cells (days 6-15) in the chick chorioallantoic membrane (CAM) was analyzed with computer-assisted microscopy. A significant loss of proliferative activity was noted after day 10: the density of labeled nuclei (in 10(-2) mm-2) decreased from a median 7.78 (days 6, 8, 10) to 2.42 (days 12, 14, 15). CAMs initially showed random patterns of labeled endothelial cells, but changed to clearly focal patterns after day 12. A regular arrangement of labeled nuclei was never seen. After application of vascular endothelial growth factor (VEGF) to the day 13 CAM, a significant increase in proliferative activity (11.50) and a random distribution of labeled endothelial cells was observed on day 15. Development of CAM precapillary vessels was assessed in terms of length density (in mm-1, mean +/- standard deviation), which was augmented three-fold from day 6 (1.22 +/- 0.05) to day 14 (3.54 +/- 0.23) and then remained nearly constant. VEGF application from day 13 to 15 raised arterial length per unit area to 4.53 +/- 0.77. It is concluded that normally a local regulation of endothelial proliferation and differentiation develops in the CAM, which doubles capillary endothelial cell density but simultaneously adapts to the decreasing need for endothelial cells, and thus maintains the quasi two-dimensional vessel pattern. However, proliferative foci persist in the capillary layer after day 10, and precapillary vessel density continues to increase until day 14. VEGF enhances DNA synthesis in all capillary endothelial cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Differentiation of the rat stria vascularis

The differentiation of the rat stria vascularis (SV) was investigated by light- and electron microscopy and by immunocytochemistry. Loss of the basal lamina at the epithelial-mesenchymal interface of SVs as indicated by immunoreactions of laminin and fibronectin induces the formation of vascular feet by basal infoldings of the marginal cells (MCs), and the development of the strial capillaries (SCs) by mesenchymal cells in a manner of vasculogenesis is progressing at the same time. The production of fibronectin in the rough endoplasmic reticulum of mesenchymal cells and the involvement of this glycoprotein in a mechanical linkage between the vasoformative mesenchymal cells and endothelial ones of the SCs are indicated by immunocytochemistry. The plasma membrane of the marginal cells (MCs) begins to show immunoreactions of Na+.K+ ATPase at postnatal day 5 and is conjugated to each other by tight junctions at postnatal day 14. The apical tubules of the differentiating MCs do not seem to be involved in the endocytotic activity but are involved in the plasma membrane supply for the rapid differentiation.

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.

Modelling of vascular growth processes: a stochastic biophysical approach to embryonic angiogenesis

In a computer simulation, growth of a capillary network is driven by a stochastic process on a planar hexagonal grid. Starting at a point source, the probabilities for the formation of new capillary elements depend on local biophysical knowledge. This knowledge is mainly derived from the flow theorem of Hagen-Poiseuille and the diameter exponent delta. The hexagonal grid is visualized as being supported by a cylinder or a sphere. An arterial tree results from the adaptive diameter augmentation, and is considered to have limited fractal properties. The dimension of its border, and the time course of growth and of blood pressure are compared with biological data from the chorioallantoic membrane (CAM) of incubated chicken eggs. The model is discussed in view of mechanosensitivity and cell-matrix interactions of endothelial cells, and CAM haemodynamics.

Role of mesenchymal cells in the neovascularization of the rabbit phallus

The neovascularization of the rabbit phallus at ages between prenatal days 15 and 21 was investigated by light- and electron microscopy, computer-aided light microscopic reconstruction, and immunocytochemistry. The phalli are embedded by an abundance of mesenchymal cells, which are in contact with the neighboring ones or with the endothelial lining of growing capillaries. They often form solid cell cords that eventually make contact with the growing capillaries. The computer-aided reconstruction of the serial light micrographs reveals that these cell cords are involved in connecting the adjacent capillaries. The incorporation of such mesenchymal cell projections into the endothelial lining, occasionally conjugated with simple attachment devices, is frequently observed by transmission and scanning electron microscopy. The contact areas between the mesenchymal and endothelial cells show immunoreactions of fibronectin. These results indicate the successive transformation of mesenchymal cells to endothelial cells of the growing capillaries. As endothelial cells of the growing capillaries show mitotic proliferation, such vasoformative mesenchymal cells seem to be involved in the acceleration of the neocapillarization of the rabbit phallus. Fibronectin actively produced in the mesenchymal cells may participate in their migration and the mechanical linkage with the endothelial cells.

Two molecules related to the VEGF receptor are expressed in early endothelial cells during avian embryonic development

We present the partial cloning and the expression patterns of two putative growth factor receptor molecules named Quek1 and Quek2 (for quail endothelial kinase) in chick and quail embryos from gastrulation to embryonic day 9 (E9). Quek1 and Quek2 show high homology to three interrelated murine and human genes, flk-1, KDR and flt. Flt was recently shown to be the receptor for the endothelial cell mitogen vascular endothelial growth factor (VEGF). In situ hybridization of Quek1 and Quek2 to sections of avian embryos showed that they are both expressed essentially by endothelial cells, that we identified with a monoclonal antibody (Mab) QH1 specific for endothelial and white blood cells of the quail. Quek1 is expressed in the mesoderm from the onset of gastrulation, whereas Quek2 message is first detected on QH1-expressing endothelial cells. The expression pattern of Quek1 suggests that it could identify the putative precursor of both endothelial and hematopoietic lineages, the hemangioblast. Quek1 and Quek2 are not expressed in all endothelial cells throughout life. At E9, after the initial phase of vasculogenesis, these genes are switched off in various compartments of the vascular network.

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.

Early formation of the vascular system in quail embryos

The relation between vascular development and translocation of the splanchnic mesodermal layers was studied in presomite to 20-somite quail embryos by scanning electron microscopy. In addition, serially sectioned embryos were stained immunohistochemically with monoclonal antibodies (alpha QH1 or alpha MB1) specific for endothelial and hemopoietic cells. By the formation of the foregut the anterior borders of the two splanchnic mesodermal layers of a presomite embryo are translocated to the lateral and ventral sides of the foregut and fuse in the ventral midline of a 4-somite embryo. Meanwhile the splanchnic mesoderm differentiates into a splanchnic mesothelial layer and a plexus of endothelial cells, facing the endoderm. From 4 somites onward the foregut is covered by a single endothelial plexus. At first the endothelial precursors bordering the anterior intestinal portal and those in the area of the ventral mesocardium lumenize, subsequently giving rise to the endocardium of the heart tube. Hereafter, the pharyngeal arch arteries and the dorsal aortae develop from the remaining precursors. During formation of the pharyngeal arches, the pharyngeal arch arteries maintain their connections with the splanchnic plexus through the developing ventral pharyngeal veins. After disappearance of the dorsal mesocardium, the midpharyngeal endothelial strand, which is a longitudinal strand of proendocardial cells, remains connected to the foregut. This strand will contribute to the formation of the pulmonary venous drainage into the left atrium. A bilateral accumulation of cardiac jelly developing between the promyocardium and proendocardial plexus only suggests that the heart develops from two tubes. The proendocardial layer, however, is not divided by the ventral mesocardium but initially forms just one endocardial heart tube.

Age-related changes in the position of centrosomes in endothelial cells of the rabbit aorta

The position of centrosomes in endothelial cells (EC) lining the aorta was examined in rabbits at different ages, using en face preparations and immunofluorescent staining with a serum that specifically labels centrioles. The results obtained show that in young rabbits (4 h-6 weeks) the great majority of the EC (61%) had centrosomes on the heart side of the nucleus, whereas in older rabbits (6-156 weeks) only 41% of the EC had centrosomes oriented toward the heart. The results suggest that the orientation of structures normally associated with centrosomes such as the microtubule organizing centers and the Golgi apparatus also change with age. The change in the orientation of centrosomes and associated structures along the longitudinal axis of the cell with age could affect the function and behaviour of EC and their ability to respond to injury.

Cytokinetic studies on the aortic endothelium and limb bud vascularization in avian embryos

Cytokinetic studies on the aortic endothelium using the BrdU/anti-BrdU-method were carried out on 2.5- to 6-day chick and quail embryos. The mitotic activity of the aortic endothelium is related temporally to the age of the avian embryo and spatially to the embryonic region where the aorta originates. The mitotic activity of the aortic endothelium decreases with increasing age of the embryos. In the limb buds, however, the mitotic rate of the aortic endothelial cells increases independently of the age of the embryo. This increase in the mitotic activity of the aortic endothelium at the appropriate levels coincides with the vascularization of the outgrowing limb buds. We concluded therefore that the aortic endothelium probably supplies endothelial cells for the formation of limb vessels at this stage. Thus our results suggest that angiogenesis (sprouting of capillaries from pre-existing vessels) takes place during limb vascularization in avian embryos. On the other hand, immunohistochemical studies with QH-1 or MB-1 antibody show, beside a capillary network in the central core of the wing bud, individual immunolabelled cells of mesenchymal character within the primarily avascular subectodermal region from the onset of vascularization onwards. We suggest that these cells have partly to be regarded as endothelial precursor cells, which have differentiated in situ from the local limb mesenchyme, and which will contribute to the developing vascular plexus. This means that not only angiogenesis, but also vasculogenesis (in situ from mesenchymal precursors differentiated endothelial cells) appears to be involved in limb vessel formation.

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.