Biology Reference
In-Depth Information
smooth muscle called lamellar units. The adventitia is also rich in sheets of elastin
interspersed with collagen that run parallel to blood flow. The inner one-third
contains mostly elastin, while the outer layers are primarily composed of collagen
(White and Mazzacco
1996
). The hallmark disease involving elastin degradation in
the vascular system is aneurysm formation.
All aneurysms, regardless of size, have reduced adventitial and medial elastin,
suggesting that this is a key initial step in aneurismal formation. Exposure of the
external aortic layer of rabbits to elastase uniformly resulted in aneurysms that grew
progressively and displayed reduced adventitial and medial elastin (the rest of the
aorta was normal) (White and Mazzacco
1996
). Loss of elastin caused the stress to
be transmitted to the collagen fibers and thus dilation and eventual rupture were
caused by progressive collagen degradation after elastin loss.
In the skin, elastin forms a three-dimensional meshwork that extends from the
papillary down to the dermis. In the reticular dermis it has branching fibers whereas
it is more sheet-like in the deep dermis (Tsuji
1988
). As the fibers extend upward to
the dermal-epidermal junction they become smaller and lose elastin content until
they are almost exclusively fibers of glycoproteins. Although there are several skin
disorders involving elastin, the two most common disorders, photo-aging and
chrono-aging, both appear fairly similar with more frayed and tortuous elastin
fibers (Imayama and Braverman
1989
). These findings are more pronounced in
sun-exposed skin where there can be destruction of the dermal elastin meshwork
(O'Brien and Regan
1991
). In addition, sun-exposed skin appears to have more
inflammatory cells (macrophages and mast cells) and more elastin deposition that
aged skin alone (Bernstein et al.
1994
).
9.3.1
Inability to Repair Properly and Findings After Cleavage
The complexity of elastic fiber assembly is likely why adults are incapable of
adequately replacing them. Thus, diseases of elastin cleavage are irreversible
diseases requiring surgical intervention in the case of aneurysm, or supportive
care with possible lung volume reduction or transplant in the case of emphysema.
If elastin could be reformed properly, perhaps these diseases could be treated
medically or much less invasively.
Attempts to bioengineer elastin or elastin-based matrices are underway and
reviewed in Daamen et al. (
2007
). Approaches include the use of purified elastin
preparations, hydrolyzed elastin, protein engineering of either, tropoelastin frag-
ments, elastin-like peptides, and elastin hybrids. In many of these preparations, the
amounts of other matrix components such as collagen, glycosaminoglycans, and
growth factors can be varied to optimize function and cell adhesion. In addition,
amino acid sequences may be modified in some cases to further modify the
structure/function. Acellular tissue, as a source of matrix, has shown some promise
for reconstitution of bladder, esophagus, skin, and vasculature. Replacement and
regrowth of elastic fibers may be the only way to control these destructive diseases.
