Collagen Organization Critical Role in Wound Contraction

Abstract

Background: Open wound closure by wound contraction produces a healed defect made up mostly of dermis. Generating thicker collagen fibers condenses granulation tissue, which pulls surrounding skin into the defect.

The problem: What is the mechanism for open wound contraction? Is it through the generation of contractile force using sustained myosin ATPase, thus causing cell contraction or by rapid myosin ATPase that condenses collagen fibrils into fibers?

Basic/clinical science addressed: The mechanism for wound contraction is not often debated after the discovery of the myofibroblast. Myofibroblasts are the major cell phenotype in maturing granulation tissue. It is concluded, not quite accurately, that myofibroblasts are responsible for wound contraction. As wound contraction progresses, polarized light microscopy reveals birefringence patterns associated with ever-increasing thickening of collagen fibers. Collagen fibers thicken by eliminating water between fibrils. Wound contraction requires collagen synthesis and granulation tissue compaction. Both myofibroblasts and fibroblasts synthesize collagen, but fibroblasts, not myofibroblasts, compact collagen. Free-floating fibroblast-populated collagen lattices (FPCL) contract by rapid myosin ATPase, thus resulting in thicker collagen fibers by elongated fibroblasts. The release of an attached FPCL, using sustained myosin ATPase, produces rapid lattice contraction, now populated with contracted myofibroblasts in the absence of thick collagen fibers.

Discussion: In vivo and in vitro studies show that rapid myosin ATPase is the motor for wound contraction. Myofibroblasts maintain steady mechano-tension through sustained myosin ATPase, which generates cell contraction forces that fail to produce thicker collagen fibers. The hypothesis is that cytoplasmic microfilaments pull collagen fibrils over the fibroblast's plasma membrane surface, bringing collagen fibrils in closer contact with one another. The self-assembly nature of collagen fixes collagen fibrils in regular arrays generating thicker collagen fibers.

Conclusion: Wound contraction progresses through fibroblasts generating thicker collagen fibers, using tractional forces; rather than by myofibroblasts utilizing cell contraction forces.

H. Paul Ehrlich

Figure 1.

Contraction of a pair of tattooed full excision open wound in the dorsum of an adult rat. On the left is a pair of wounds at 1 day, and on the right is the same pair of wounds at day 18. Note that the tattoo marks at the corner of the wounds have moved very little, whereas the tattoo marks on the edges have moved to the center of the wound.

Figure 2.

The birefringence pattern from Sirius red-stained polarized light microscopy of contracting open rat wounds is presented. (A) From young granulation tissue of a 5-day-old wound. (B) More mature granulation tissue from a 10-day-old contracting wound. Note a change in the diameters of collagen fibers. Magnification 40×.

Figure 3.

A diagram of the scheme for the rapid and sustained myosin ATPase is presented. The phosphorylation of MLC in rapid and sustained myosin ATPase activity occurs by different mechanisms. MLCK affixes a phosphate group from ATP onto MLC serine-19, which is necessary for optimal myosin ATPase activity. The inhibitor W-7 will block MLCK, thus inhibiting rapid myosin ATPase activity. MLCP removal of the phosphate group from MLC serine-19 is required for the relaxation step in cell locomotion through rapid myosin ATPase activity. Inhibiting MLCP activity, prolonging the phosphorylated state of MLC, defines the mechanism for sustained myosin ATPase activity, where MLCP is maintained in a chronic state. The inhibitor Y-27632 restores MLCP activity and terminates sustained myosin ATPase activity. MLC, myosin light chain; MLCK, myosin light chain kinase; MLCP, myosin light chain phosphatase.

Figure 4.

A diagram identifying the differences in FF-FPCLs and ADR-FPCLs is presented. The cell number, collagen concentration, and composition of culture medium are the same in both FPCLs. The difference is the time of releasing the lattice from the surface of the dish. FF, free floating; FPCL, fibroblast-populated collagen lattice; ADR, attached delayed released.

Figure 5.

The birefringence pattern from polarized light microscopy from a 1-day-old contracting FF-FPCL that shows the birefringence pattern from parallel aligned collagen fibers localized at the ends of an elongated fibroblast. With polarized light microscopy, minimal cellular morphology is achieved. Fibroblasts translocate collagen fibrils by passing them over their plasma membrane surface, aligning them in parallel arrays. Magnification 80×.