Blood vessels form in two ways: vasculogenesis, whereby vessels arise from blood islands (Fig. 5.14), and angiogenesis, which entails sprouting from
Figure 5.14 Blood vessels form in two ways: vasculogenesis (top), in which vessels arise from blood islands, and angiogenesis (bottom), in which new vessels sprout from existing ones. During vasculogenesis, fibroblast growth factor 2 (FGF-2) binds to its receptor on subpopulations of mesoderm cells and induces them to form hemangioblasts. Then, under the influence of vascular endothelial growth factor (VEGF) acting through two different receptors, these cells become endothelial and coalesce to form vessels. Angiogenesis is also regulated by VEGF, which stimulates proliferation of endothelial cells at points where new vessels will sprout from existing ones. Final modeling and stabilization of the vasculature are accomplished by platelet-derived growth factor (PDGF) and transforming growth factor p (TGF-p).
existing vessels. The first blood islands appear in mesoderm surrounding the wall of the yolk sac at 3 weeks of development and slightly later in lateral plate mesoderm and other regions (Fig. 5.15). These islands arise from mesoderm cells that are induced by fibroblast growth factor 2 (FGF-2) to form heman-gioblasts, a common precursor for vessel and blood cell formation. Heman-gioblasts in the center of blood islands form hematopoietic stem cells, the precursors of all blood cells, whereas peripheral hemangioblasts differentiate into angioblasts, the precursors to blood vessels. These angioblasts proliferate and are eventually induced to form endothelial cells by vascular endothelial growth factor (VEGF) secreted by surrounding mesoderm cells (Fig. 5.14). This same factor then regulates coalescence of these endothelial cells into the first primitive blood vessels.
Once the process of vasculogenesis establishes a primary vascular bed, additional vasculature is added by angiogenesis, the sprouting of new vessels (Fig. 5.14). This process is also mediated by VEGF, which stimulates proliferation of endothelial cells at points where new vessels are to be formed. Maturation and modeling of the vasculature are regulated by other growth factors, including platelet-derived growth factor (PDGF) and transforming growth factor ft (TGF-ft), until the adult pattern is established.
As mentioned, the first blood cells arise in the blood islands of the yolk sac, but this population is transitory. The definitive hematopoietic stem cells arise from mesoderm surrounding the aorta in a site called the aorta-gonad-mesonephros region (AGM). These cells will colonize the liver, which becomes the major hematopoietic organ of the fetus. Later, stem cells from the liver will colonize the bone marrow, the definitive blood-forming tissue.
The gastrointestinal tract is the main organ system derived from the endodermal germ layer. This germ layer covers the ventral surface of the embryo and forms the roof of the yolk sac (Fig. 5.16A). With development and growth of the brain vesicles, however, the embryonic disc begins to bulge into the am-niotic cavity and to fold cephalocaudally. This folding is most pronounced in the regions of the head and tail, where the head fold and tail fold are formed (Fig. 5.16).
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