Vessel Wall - Tissue Functioning and Remodeling in the Circulatory and Ventilatory Systems - page 350

7.3.1 Large Vessels

The wall of large blood vessels has a circumferentially layered structure. The
internal thin intima is composed of the inner endothelium and a subendothelial
tunica of connective tissue. The endothelium is a monolayer of endothelial cells
supported at its abluminal surface by a basal lamina. The endothelium modulates
wall structure and functions.
The internal elastic lamina (IEL) made of numerous azimuthal bundles of elastic
fibers delimits the intima from the media. The fenestrated internal elastic lamina
is characterized by a quasi-uniform distribution of pores. Water and solutes are
transported through these fenestrae. Cells communicate and migrate via these pores.
The number of fenestrations is greater at branching points.
The middle muscular tunica — media — is formed by layers of circumferential
smooth myocytes and connective tissue with azimuthally arranged elastic fibers.
Elastin sheets, or lamellae, are fenestrated to facilitate material diffusion. Action
potentials trigger SMC contraction after a delay ranging from 80 to 100 ms that
lasts from 10 to 15 s [ 653 ]. According to the vessel type, media contains varying
amounts of collagen and elastic fibers, elastic lamellae, and proteoglycans secreted
by smooth myocytes. Media of arteries is larger than that of veins of similar size.
The external elastic lamina (EEL), a thick elastic circumferential band, is located
between the media and adventitia.
The external adventitia consists mainly of connective tissue with predominant
fibroblasts, some smooth myocytes, many macrophages, elastic fibers and predomi-
nant longitudinally arranged collagen fibers. In large vessels, the adventitia contains
nerves, vessels — vasa vasorum , and possibly lymphatic vessels. Media of arteries
is much thinner than that of veins of similar size.

7.3.1.1 Elastic Arteries

The media is the main site of histological specializations of artery walls (Table 7.1 ).
Vessels proximal to the heart are elastic arteries.
Wall distension during pressure wave propagation for efficient windkessel effect
is facilitated by thin concentric fenestrated lamellae 5 of elastin in a thick media,
between which are smooth myocytes and collagen fibers (Table 7.2 ). The lamella
number depends on vessel size, between-lamella space being nearly constant. Inside
the elastic lamella, elastic fibers have a helical orientation. 6

5 Lamella: diminutive of lamina, or lamna: thin plate, membrane, layer.
6 The orientation of elastin fibers in a given rabbit artery slice is parallel to the local artery axis or
nearly perpendicular to it, whether the elastin fibers belong to the fenestrated internal elastic lamina
or medial elastic bundles (P. Farand, personnal communication). Therefore, elastin fibers of the
internal elastic lamina and media sustain longitudinal and circumferential loadings, respectively.
Bridges exist between axially and circumferentially oriented elastin fibers.