With a reduction in inflammatory stimuli and therefore in the number
of inflammatory mediators, the wound milieu is altered to allow new
tissue formation. The wound enters the fibroplastic phase,
which is usually established within 5 days of wounding and lasts for
up to 4 weeks.
The macrophage, so critical to the
inflammatory stage of wound healing, is also essential to new tissue
development through macrophage-derived growth factors (MDGF),.
The MDGF list is long (though not as long as the list of Lincoln
bedroom visitors) and includes platelet-derived growth factor (PDGF),
transforming growth factor-alpha (TGF-a), transforming growth factor
beta (TGF-b), interleukin-1 (IL-1), and tumor necrosis factor
(TNF).
To produce
new tissue, fibroblasts proliferate in the wound and migrate
with the help of the growth factors and a very important glycoprotein
called fibronectin. Fibronectin acts as a conduit for fibroblasts. It
binds to both the wound matrix and the fibroblast, allowing the
fibroblast to march down it (the fibronectin), move along the
chemotactic gradient of growth factors, and take up residence in the
wound (image expandable to 27K JPEG).
Once in the wound
fibroblasts begin to synthesize collagen fibers and produce
proteoglycans, structural proteins, and adhesive proteins --
connective tissue ground substances. Vitamin C, iron, and copper are
essential to the synthesis of collagen, which constitutes 50% of the
protein found in scar tissue. Other fibrous protein in the wound
include elastin and reticulin (click on image to download 1.1 meg
Quicktime animation).
Collagen, however, cannot be
synthesized in the absence of adequate oxygen supply. But the wound
has derailed blood flow. Angiogenesis is required. Through
their enzymatic actions, growth factors break down the vascular
membrane of the venules. Endothelial cells that migrate through the
defect aggregate to form vascular buds (click on image to
download a 98K Quicktime movie).
Go back to Inflammation | NEW TISSUE FORMATION continues... |
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