Biomedical Engineering Reference
In-Depth Information
bronectin dimers bind to transmembrane integrin receptors. Integrin
α
β
1
is the primary receptor for mediating assembly linking fibronectin to the
actin cytoskeleton. On the outside of the cell, interactions between fibronectin
and integrins promote fibronectin-fibronectin association and fibril forma-
tion perhaps by inducing conformational changes in bound fibronectin. The
thickness of fibronectin fibrils varies substantially from 10 to 1000 nm in
diameter, extended fibronectin molecules are about 3 nm in diameter indi-
cating that fibrils probably range from a few to several hundred fibronectin
molecules across. The organization of fibronectin within the fibrils is not
well understood but is probably determined by intermolecular interactions
involving some subset of the following domains [79]. Detailed hypotheses
on the mechanism of the fibronectin fibrillogenesis were derived in con-
ceptually and experimentally elegant studies by Vogel et al. [39]. Part of
this work was also the proof that shear force—applied upon expansion of
a fibronectin monolayer at the air-water interface using a Langmuir trough-
could be sufficient to create fibronectin-fibronectin interconnections, even
without the protein linked to cellular receptors. Shear stress was also ap-
plied to fibronectin solutions in work done by Brown et al. [80] to produce
fibronectin mats, a macroscopic fibronectin-based structure, which had been
successfully used as culture carriers. The materials were prepared from hu-
manplasmafibronectinbyapplyingfluidshearforcesdirectlytoaviscous
solution of fibronectin. Structural analysis revealed that mechanical shear re-
sulted in the formation of an orientated fibrous protein material that was
less soluble than its non-sheared counterpart. Addition of fibrinogen and fur-
ther modifications achieved by the processing [81] were investigated with
respect to the structure and cell guidance characteristics of the fibronectin
cables.
Recent reports suggest that fibronectin-rich matrices may play an im-
portant role in the formation of specialized matrices—the accumulation of
fibronectin fibrils may be seen to contribute a transient auxiliary functional-
ity as, for instance, discussed for the process of angiogenesis [82].
Fibrin, the polymerized form of the coagulation plasma protein fibrino-
gen, is, together with fibronectin deposited into wounds from the circulation
shortly after injury and influences the reorganization of damaged tissues as
a provisional matrix. Fibronectin deposition is continued by wound fibro-
blasts and macrophages, or migrating keratinocytes [83]. In response to in-
jury, resident fibroblasts in the surrounding tissue proliferate and then mi-
grateintothewoundedsite[84].Oncewithinthewound,fibroblastsproduce
type I procollagen as well as other matrix molecules and deposit these extra-
cellular matrix molecules in the local milieu. Fibroblasts can use a fibrin and
fibronectin matrix to move through the wound. When exposed to a chemo-
tactic gradient, they will migrate along, rather than across, the fibronectin
fibrils. Fibrin plays an important role in healing and regeneration in the de-
veloped organism, but not in embryonic and fetal development. As a result
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