The Questions of Developmental Biology

The central cells of the blood islands differentiate into the embryonic blood cells. As the
blood islands grow, they eventually merge to form the capillary network draining into the two
vitelline veins, which bring food and blood cells to the newly formed heart.
Three growth factors may be responsible for initiating vasculogenesis (see Figure 15.15).
One of these, basic fibroblast growth factor (FGF2) is required for the generation of
hemangioblasts from the splanchnic mesoderm. When cells from quail blastodiscs are dissociated
in culture, they do not form blood islands or endothelial cells. However, when these cells are
cultured in FGF2, blood islands emerge and form endothelial cells (Flamme and Risau 1992).
FGF2 is synthesized in the chick embryonic chorioallantoic membrane and is responsible for the
vascularization of this tissue (Ribatti et al. 1995). The second protein involved in vasculogenesis
is vascular endothelial growth factor (VEGF). VEGF appears to enable the differentiation of
the angioblasts and their multiplication to form endothelial tubes. VEGF is secreted by the
mesenchymal cells near the blood islands, and the hemangioblasts and angioblasts have receptors
for VEGF (Millauer et al. 1993). If mouse embryos lack the genes encoding either VEGF or the
major receptor for VEGF (the Flk1 receptor tyrosine kinase), yolk sac blood islands fail to
appear, and vasculogenesis fails to take place (Figure 15.16E; Ferrara et al. 1996). Mice lacking
genes for the second receptor for VEGF (the Flt1 receptor tyrosine kinase) have differentiated
endothelial cells and blood islands, but these cells are not organized into blood vessels (Fong et
al. 1995; Shalaby et al. 1995). A third protein, angiopoietin-1 (Ang1), mediates the interaction
between the endothelial cells and the pericytes smooth musclelike cells they recruit to cover
them. Mutations of either angiopoietin-1 or its receptor lead to malformed blood vessels, deficient
in the smooth muscles that usually surround them (Davis et al. 1996; Suri et al. 1996; Vikkula et
al. 1996).
Angiogenesis: sprouting of blood vessels and remodeling of vascular beds
After an initial phase of vasculogenesis, the primary capillary networks are remodeled. At
this time, veins and arteries are made. This process is called angiogenesis (see Figure 15.15).
First, VEGF acting alone on the newly formed capillaries causes a loosening of cell contacts and
a degradation of the extracellular matrix at certain points. The exposed endothelial cells
proliferate and sprout from these regions, eventually forming a new vessel. New vessels can also
be formed in the primary capillary bed by splitting an existing vessel in two. The loosening of the
cell-cell contacts may also allow the fusion of capillaries to form wider vessels the arteries and
veins. Eventually, the mature capillary network forms and is stabilized by TGF-β (which
strengthens the extracellular matrix) and platelet-derived growth factor (PDGF, which is
necessary for the recruitment of the pericyte cells that contribute to the mechanical flexibility of
the capillary wall) (Lindahl et al. 1997).
A key to our understanding of the mechanism by which veins and arteries form was the
discovery that the primary capillary plexus in mice actually contains two types of endothelial
cells. The precursors of the arteries contain EphrinB2 in their cell membranes. The precursors of
the veins contain one of the receptors for this molecule, EphB4 tyrosine kinase, in their cell
membranes (Figure 15.17; Wang et al. 1998). If EphrinB2 is knocked out in mice, vasculogenesis
occurs, but angiogenesis does not. It is thought that EphB4 interacts with its ligand, EphrinB2,
during angiogenesis in two ways. First, at the borders of the venous and arterial capillaries, it
ensures that arterial capillaries connect only to venous ones. Second, in non-border areas, it might
ensure that the fusion of capillaries to make larger vessels occurs only between the same type of
vessel.