Biomedical Engineering Reference
In-Depth Information
Fig. 6.1.
(a) Fluorescence microscopy images of cross sectional preparations of the human left
vertebral artery (cerebral), fixed at 30 % stretch. Immunohistochemical staining of the arterial wall
reveals elastin (green) localized in the internal elastic lamina, cell nuclei (blue, DAPI stain), col-
lagen fibres (red). (b) 3D reconstruction of confocal microscopy image slices taken of a human
anterior cerebral artery (ACA) revealing the autofluorescent internal elastic lamina, under (A) zero
stretch and (B) 30 % strain. All three scale bars = 50 microns
Collagen in the arterial wall
Collagen molecules in the arterial wall are assembled into fibrils that are packed to-
gether to form collagen fibres. In Type 1 collagen, thick fibrils (
74 nm) are densely
packed to form collagen fibres (2-10 mm in diamter) that are packed together in bun-
dles [80]. In contrast, Type III collagen is formed as individual fibres (0.5-1.5 mm
in diameter), composed of loosely packed thin fibrils (
∼
45 nm). The mechanical
behaviour of the vascular wall is affected by the orientation and unloaded geometry
of these fibres as well as their distribution across the wall layers. These features vary
with location in the vasculature as welll as in health and disease.
∼
Elastic fibres in the arterial wall
Mammalian elastic fibres (0.2-1.5
m) are composed of a core of twisted, rope-like
structures of highly cross-linked elastin protein surrounded by fibrillin microfibrils
(10-12 nm in diameter) [80, 96]. The microfibrils are believed to contribute to elas-
togenesis and elastic fibre function [104, 111].
In cerebral vessels the elastic fibres are nearly all localized in a fenestrated (win-
dowed) sheet of elastic fibres, the IEL (Fig. 6.1b), [63]. SEM has revealed the pres-
ence of folds in the IEL, projecting into the lumen and running parallel to the longi-
tudinal axis of the artery [68]. This folded state has been found under zero applied
loading and the folds can be “pulled out” by applying loads perpendicular to the ax-
ial direction of the vessel, Fig. 6.1b. The main function of the fenestrae in the IEL
appears to be the enhancement of passage of water, nutrients and electrolytes across
the wall. However, the size and number of these fenestrae change in time and have
been shown to affect the mechanical properties of the vessel [40].
μ
