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

Blood Vessel Patterning at the Embryonic Midline

The reproducible pattern of blood vessels formed in vertebrate embryos has been described extensively, but only recently have we obtained the genetic and molecular tools to address the mechanisms underlying these processes. This review describes our current knowledge regarding vascular patterning around the vertebrate midline and presents data derived from frogs, zebrafish, avians, and mice. The embryonic structures implicated in midline vascular patterning, the hypochord, endoderm, notochord, and neural tube, are discussed. Moreover, several molecular signaling pathways implicated in vascular patterning, VEGF, Tie/tek, Notch, Eph/ephrin, and Semaphorin, are described. Data showing that VEGF is critical to patterning the dorsal aorta in frogs and zebrafish, and to patterning the vascular plexus that forms around the neural tube in amniotes, is presented. A more complete knowledge of vascular patterning is likely to come from the next generation of experiments using ever more sophisticated tools, and these results promise to directly impact on clinically important issues such as forming new vessels in the human body and/or in bioreactors.

Gastrulation and angiogenesis, not endothelial specification, is sensitive to partial deficiency in vascular endothelial growth factor-a in mice

Mouse embryogenesis is dose sensitive to vascular endothelial growth factor-A (VEGF-A), and mouse embryos partially deficient in VEGF-A die in utero because of severe vascular defects. In this study, we investigate the possible causes that underlie this phenomenon. Although the development of vascular defects in VEGF-A-deficient embryos seems to suggest that endothelial differentiation depends on the presence of a sufficient level of VEGF-A, we were surprised to find that endothelial differentiation per se is insensitive to a significant loss of VEGF-A activity. Instead, the development of the multipotent mesenchymal cells, from which endothelial progenitors arise in the yolk sac, is most highly dependent on VEGF-A. As a result of VEGF-A deficiency, dramatically fewer multipotent mesenchymal cells are generated in the prospective yolk sac. However, among the small number of mesenchymal cells that do enter the prospective yolk sac, endothelial differentiation occurs at a normal frequency. In the embryo proper, vasculogenesis is initiated actively in spite of a significant VEGF-A deficiency, but the subsequent steps of vascular development are defective. We conclude that a full-level VEGF-A activity is not critical for endothelial specification but is important for two distinct processes before and after endothelial specification: the development of the yolk sac mesenchyme and angiogenic sprouting of blood vessels.

Expression of vascular endothelial growth factor isoforms in the Japanese quail embryo

Vascular endothelial growth factor (VEGF) i s required for vascular development. In the quail, four VEGF isoforms formed by alternative splicing, are translated into proteins with 122, 146, 166, and 190 amino acids. VEGF isoforms differ biochemically, with variable affinities for heparan sulfate proteoglycans, the extracellular matrix and VEGF receptors. There are few data on the functional significance of VEGF isoforms. RT-PCR was used to examine isoform expression during quail vascular development. Our results suggest that all quail isoforms are expressed during establishment of the vascular pattern in whole embryos and extraembryonic tissues at apparently equal levels. No isoform-specific expression patterns were detected in isolated endoderm or between embryo halves.

Embryonic and adult vasculogenesis

Two mechanisms account for the formation of blood vessels, vasculogenesis and angiogenesis. Unfortunately, the terms vasculogenesis and angiogenesis literally have the same meaning, i.e., the genesis of blood vessels, and thus do little to distinguish between the two processes. Despite the nomenclature, the two processes are clearly distinct. Vasculogenesis, the de novo formation of blood vessels from mesoderm, is driven by the recruitment of undifferentiated mesodermal cells to the endothelial lineage and the de novo assembly of such cells into blood vessels. Angiogenesis is the generation of new blood vessels from endothelial cells of existing blood vessels, a process driven by endothelial cell proliferation. Recent years have seen dramatic changes in our understanding of the process of vasculogenesis, expanding the scope of its occurrence beyond the earliest stages of development to include involvement in neovascular processes throughout development as well as in the adult. In this review, emphasis is placed on discussion of emerging perspectives on the process of vasculogenesis in both the embryo and the adult.

Hematopoietic stem cells and their precursors: developmental diversity and lineage relationships

Within the context of the developing embryo, restrictions in cell lineage potential occur through cell-cell interactions and signaling molecules, leading to changes in genetic programs and to the emergence of disparate tissues containing functionally distinct cell types including somatic stem cells. Tissue maintenance in the adult is thought to occur through specific stem cells, and in the case of the hematopoietic system, through hematopoietic stem cells (HSCs). These cells arise in midgestation within the region of the embryo containing the dorsal aorta, gonads, and mesonephros (AGM) and are thought to maintain a distinct hematopoietic lineage-restricted fate. However, recent transplantation experiments suggest that within the adult, HSCs previously thought to be restricted can, under certain circumstances, display unexpected lineage potentials. With these surprising and controversial results, it is becoming apparent that a better understanding of the developmental processes, molecular programs and lineage relationships leading to the emergence of adult stem cells will provide insight into the incremental steps involved in lineage determination, and perhaps possibilities for the manipulated differentiation of stem cells. The most widely studied, accessible stem cell and cellular differentiation hierarchy is that of the hematopoietic system. With the issue of stem cell potential in the forefront, the focus of this review is on the development of the hematopoietic system: how HSCs arise in the embryo, the lineage relationships of hematopoietic cells as they are generated, and the identification of precursor cells fated to the hematopoietic lineage throughout ontogeny.