Wound Healing

Question 1

Phases of wound healing

Answer 1

1.Hemostasis
2. Inflammation
3. Proliferation
4. Remodelling

Question 2

Hemostasis phase of wound healing

Answer 2

Tissue injury, damage to capillaries, initiates coagulation cascade via activation of fibrinogen.

1. Platelet aggregation and fibrin scaffold allows for migration of cells.
2. Platelets - chief effector cells during hemostasis.
3. Hemostasis plug formation, Alpha granules release several growth factors and cytokines- PDGF, TGF-B - chemoattractants.
4. Release of proangiogenic and antiangiogenic growth factors - VEGF, platelet derived stromal cell-derived factor 1 - critical for revascularization.

Question 3

Duration of inflammation phase of wound healing

Answer 3

Typically lasts 48 hours

Question 4

Inflammation phase of wound healing

Answer 4

Coagulation cascade, activation of complement, bacterial degradation, facilitates and triggers inflammatory phase.

1. Provisional matrix provides scaffolding for cell recruitment and attachment required during the subsequent phases of wound healing.
2. Inflammatory cells are attracted especially neutrophils that fills wound cavity. These remove dead tissues by phagocytosis, prevent infection by oxygen-dependent and independent killing mechanisms.
3. Degrade ECMs by proteases and prepare wound for healing. Prolonged persistence of neutrophils - proposed to be primary factor in conversion of acute wounds into non-healing chronic wounds.
4. Monocytes/macrophages enters wound 48-72 hours post-injury. Attracted by monocyte chemoattractant protein 1. Critically regulates early and late stages of wound repair. Predominant cell - 3 days post-wounding. Phagocytose debris and bacteria, produces growth factors ( FGF-2, PDGF, EGF, TGF-B. TNF-A, GM-CSF, IFN-A, IL-1, IL-8) for fibrosis and neovascularization.
5. Lymphocytes last cell to enter wound, enters between days 5 and 7 post-wounding. CD4 and CD8 cells play a role in subsequent proliferative phase. Mast cells also appears later, function is not known.

Question 5

Proliferative phase of wound healing

Answer 5

Occurs from days 4 to 21.
Certain facets of proliferative phase such as repitheliazation probably begin almost immediately following injury.

1. Keratinocytes adjacent to wound alters their phenotype hours after injury. Keratinocytes become free to move and migrates laterally due to regression of desmosomal connections between keratinocytes and basement cell membrane. Formation of actin filaments in cytoplasm of keratinocytes provides them with locomotion. They move via interactions with ECM proteins via specific Integrin mediators.
2. Granulation tissue gradually replaces provisional fibrin matrix, it is a platform for migration. It is largely composed of 3 cells - fibroblasts, macrophages, endothelial cells. They form the ECM and new blood vessels. Granulation tissue begins to appear by day 4 post-injury. Fibroblasts are the workhorses, produces ECM and provides scaffold for keratinocyte migration.
3. Macrophages continue PDGF, TGF-B production, induces fibroblast proliferation, migration and ECM deposition as well as endothelial cell stimulation to produce new blood vessels. Provisional fibrin matrix replaced by thinner Type III collagen, which in turn will be replaced by type I collagen during remodelling phase.
4. Endothelial cells - form new blood vessels through angiogenesis and vasculogenesis. The latter involves, recruitment and assembly of bone-marrow derived progenitor cells. Macrophages release proangiogenic factors such as VEGF, FGF-2, angiopoietin 1 and thrombospondin.
5. At a certain point, all the processes need to be turned off and formation of granulation tissue/ECM halted. As it a regulated event, once collagen matrix has filled wound cavity, fibroblasts rapidly disappear and newly formed blood vessels regress. This occurs through gradual programmed cell-death or apoptosis.

Question 6

Longest phase of wound healing

Answer 6

Remodelling phase

Question 7

Duration of Remodelling phase of wound healing

Answer 7

21days upto 1 year

Question 8

Remodelling phase

Answer 8

1. Once the wound has filled with granulation tissue and after keratinocyte migration has re-epithelialized it, process of wound remodelling occurs. Likely remodelling phase begins with the programmed regression of blood vessels and granulation tissue.
2. Characterised by processes of wound contraction and collagen remodelling.
   The process of wound contraction is produced by myofibroblasts - fibroblasts with intracellular actin microfilaments, capable of force generation and matrix contraction. Myofibroblasts contact the wound through specific integrin-mediated cell-matrix interactions with dermal environment.
3. Collagen remodelling occurs by replacement of type III collagen laid down by fibroblasts during proliferative phase to type I collagen over next few weeks to months. Largely mediated with matrix metalloproteinases, secreted by macrophages, fibroblasts, endothelial cells. Breaking strength of healing wound improves slowly, reflecting the turnover in collagen subtypes and increased collagen cross-linking.
4. At 3 weeks, beginning of remodelling phase, wounds have only 20% of strength of unwounded skin, ultimately only possess 70%-80% of breaking strength of unwounded skin at 1 year.