Vasculature Growth - Tissue Functioning and Remodeling in the Circulatory and Ventilatory Systems - page 602

blood vessels from bone marrow-derived endothelial progenitor cells; (2) angiogen-
esis; and (3) arteriogenesis, i.e., growth and remodeling of arterioles into arteries
(collateralization).

10.1 Vasculogenesis

Mesodermal cells in the early embryo differentiate into endothelial progenitor
cells — angioblasts — and aggregate. A functional vasculature develops from
angioblasts.
During vasculogenesis, angioblasts determine “blood pockets” that lengthen to
form irregular capillaries. Vascular plexi formed by aggregating angioblasts remodel
and mature into organized vascular networks of large and small, ramified or merged,
vessels. Parts of vascular plexi contain blood islands. Outer cells of the blood islands
give rise to endothelia, inner cells to hematopoietic progenitors [ 1167 ]. Fusion of
angioblast pockets forms primary capillary plexi . Ducts connect each other in a
non-hierarchical, inhomogeneous network of primitive vessels.
Immediately after vasculogenesis, angioblasts and endothelial cells undergo
specification to either arterial or venous fate. Angioblasts that aggregate to form
blood islands, which fuse and remodel in response to hemodynamic stresses and
genetic factors, 1 create a primitive interlaced network of arterial and venous plexi.
Angioblasts also acquire arterial or venous fates and coalesce to generate the first
embryonic blood vessels: the dorsal aorta and cardinal vein. The dorsal aorta and
cardinal vein are indeed directly formed by the coordinated sorting and segregation
of arterial and venous angioblasts and their respective assembly.
After vasculogenesis and once associated with the heart pump, the primitive
network that conveys blood remodels with branching. Angiogenesis from the dorsal
aorta, cardinal vein, and vascular plexi builds a hierarchical network of arteries,
arterioles, capillaries, venules, and veins. Subsequent recruitment pericytes and vas-
cular smooth myocytes stabilizes nascent vessels and promotes vessel maturation.
In addition, sprouting of lymphatic endothelial cells from venous endothelia gives
rise to the closed-ended lymphatic network. Furthermore, a subset of hemogenic
arterial endothelial cells generates hematopoietic stem cells.

10.2 Angiogenesis

Angiogenesis is the development of new branching vessels from existing vascu-
lature. Angiogenesis thus involves migration and proliferation of endothelial cells

1 E.g., Hairy enhancer of Split [HES]-related transcriptional regulators HRT1 and HRT2 promotes
arterial differentiation; nuclear receptor NR2f2 favors venous differentiation [ 886 ].