Biomedical Engineering Reference
In-Depth Information
Chapter 7
7.1 Biological Introduction
Blood vessel formation and development is a multiscale process, driven by the
activation of endothelial cells (ECs, the main bricks of the capillary walls)
induced by the action of suitable biochemical stimuli which are released both
by environmental cells and by ECs themselves. Vascular progression involves
two different mechanisms: vasculogenesis and angiogenesis (for a review, see
[57, 358]). The former process mainly consists in the formation of a primitive
vascular network, that emerges from a directed and autonomous migration,
aggregation and organization of a dispersed population of endothelial cells.
The latter consists instead of the formation of new vessels from an existing
capillary or post-capillary venule. Angiogenic remodeling co-ordinates with
the establishment of blood flow and can occur through sprouting, i.e., by
the formation of new branches from the sides of existing capillaries, prun-
ing, resizing of the capillary volume of the thickness of the capillary wall, or
intussusception, i.e., by internal division of the vessel lumen.
Entering into more detail, in the embryo, the process of vasculogenesis
starts with the assembly of mesoderm-derived precursors of ECs in polygons
having well determined topological characteristics, dictated by the principal
and paradigmatic function of vasculature: the oxygen transport to the tissues.
After remodeling, these geometrical properties are more or less maintained
in the adult body, where the capillary network embedded in the tissues and
stemmed by the vascular tree has the same shape as the minimal unit par-
ticipating in the formation of the embryo vascular net. Though angiogenesis
already intervenes in the embryo to remodel the initial capillary network into
a mature and functional vascular bed comprised of arteries, capillaries, and
veins, its main role is played during the adult life, when it is involved in many
physiological processes, for instance, the vascularization of the ovary and the
uterus during the female cycle, of the mammary gland during lactation, and
of granulation tissues after wound healing.
However, vascular progression is a complex and highly regulated phe-
nomenon, controlled and balanced by coordinated molecular and cellular
events operating at different levels. When their equilibrium is disrupted vas-
cularization becomes pathological, as in the cases of chronic inflammatory
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