Vessel Wall - Tissue Functioning and Remodeling in the Circulatory and Ventilatory Systems - page 352

Tabl e 7. 3. Wall composition in elastin and collagen (dog, % dry defatted tissue) in different
segments of proximal arteries and elastic modulus range ( kPa ; Sources: [ 655 , 656 ]).

Collagen Elastin

Ascending aorta 15-20 40-50

Thoracic aorta 15-30 20-40

Abdominal aorta 25-35 15-20

Femoral artery 35-45 15-25

10 2 -10 6 10 2 -10 3 Elastic modulus

7.3.1.2 Muscular Arteries

Muscular arteries have thinner intima. The media is characterized by numerous
concentric layers of smooth myocytes with elastic fibers and few collagen fibers.
The external elastic lamina can be clearly observed. The wall content in elastin
and collagen varies according to the artery type, elastic, transitional, or muscular
(Table 7.3 ). The fibers have a helical orientation with a variable pitch angle from
the inner to the outer vessel wall. Cerebral arteries are thin walled without external
elastic lamina.
Adventitia that surrounds most blood vessels allows cell migration into and out
of the artery wall. Adventitial cells participate in vascular growth and remodeling.
Adventitia is a residence zone for progenitor cells that can adopt both vascular and
non-vascular fates. Endothelial progenitor cells form clusters between the media
and adventitia in arteries.
The morphogen Sonic hedgehog (Vol. 3 - Chap. 10. Morphogen Receptors)
stimulates production of angiogenic factors (e.g., VEGFa and angiopoietin-1) by
fibroblasts, and promotes angiogenesis. It also specifies arterial or venous identity
of endothelial cells and recruits mural cells. Sonic Hedgehog restricted to the
adventitia of artery wall favors maintenance of resident SCA1 vascular progenitor

cells [ 657 ]. 7 Some progenitors self-renew, whereas others differentiate.
These progenitors express transcription factors for SMC differentiation, includ-
ing serum response factor and myocardin family members 8 (i.e., myocardin and
myocardin-related transcription factors MRTFa and MRTFb), as well as transcrip-
tional corepressors of the SRF-dependent transcription (Kr uppel-like factor KLF4,
homeodomain-containing protein Hox7, 9

+

and forkhead box transcription factor

FoxO4) for progenitor maintenance.

7 Adventitial sonic Hedgehog has low levels in adult mouse arteries for postnatal growth and
remodeling of the artery wall.
8 Myocardin is a smooth and cardiac myocyte-specific transcriptional coactivator of serum
response factor. Members of the myocardin family of coactivators activate genes involved in cell
proliferation and migration and myogenesis.
9 A.k.a. Msh homeobox gene product Msx1.