31.4 Repair Flashcards
Give a definition for pathology in reference to wound repair.
These are the mechanisms by which cells, tissues and organs defend themselves against injury and insult
What are the four main defence systems?
- Haemostasis (blood clotting)
- Inflammation
- Immunity and immune responses
- Wound repair
What are the goals and outcomes of wound repair?
- Removal of dead and damaged cells
- Removal of the fibrin clot (fibrinolysis)
- Removal of other molecules in the exudate
- Restoration of damaged structures
What are two important things to note about wound repair?
- Repair starts at the same time as inflammation
- Repair is a dynamic process
What are the good outcomes of wound repair?
Resolution and repair
- Complete restoration of normal function
- E.g. haematopoietic system (blood lost through haemorrhage or trauma will be replaced relatively quickly by haemopoietic stem cells)
- E.g. damage to the gut epithelium (very high capacity for repair)
- E.g. bone fracture in young people (if set properly, complete restoration of structure and function)
What are some examples of bad outcomes of wound repair?
- Excessive repair
- Hypertrophic scars (scar does not extend beyond the wound, raised scar tissue)
- Keloid (scar does extend beyond the boundaries of the wound, raised scar tissue)
- Replacement of normal tissue with fibrous scar
- E.g. in myocardial infarct, myocardium is replaced with scar tissue, which affects function through being less able to contract and upsetting the electrolyte balance around the tissue
- Continued ulceration
- Chronic, non-healing wounds
Following inflammation, what is formed at the site of injury?
Granulation tissue
What is granulation tissue?
- NB NOT the same as granuloma (small area of inflammation)
- Opposite of necrosis
- This is new connective tissue full of newly forming blood vessels that are needed for wound healing
- This tissue is created and modified by fibroblasts
- Pink in colour (due to new blood vessels)
- Granular due to punctate haemorrhage (small capillary haemorrhages into the skin that form petechiae, small brown/red spots)
- Fibroblasts lay down extracellular matrix (ECM), especially type III collagen
- This is later replaced by type I collagen, the main constituent of scar tissue
What is shown here?
Granulation tissue.
* Arrows indicate macrophages in granulation tissue
* Also some indication of angiogenesis
Which cell type is highly abundant in granulation tissue?
Fibroblast rich tissue - secrete collagen I and III and GAGs
Which cell types are involved in wound repair?
Fibroblasts
Macrophages and neutrophils
Endothelial cells
Describe what happens during angiogenesis
PART OF GRANULATION TISSUE
Endothelial cell migration
Endothelial cell proliferation (VEGF and TGF alpha) = vascular budding along fibronectin molecules
Proteolysis of ECM (collagenases)
Tube formation
What is the role of angiogenesis in wound repair?
In wound repair, angiogenesis forms new vessels from pre-existing vessels through invasion of the wound clot and organisation of a microvascular network through the granulation tissue.
How are lymphatic vessels involved in wound repair?
- Their regeneration and presence is important for wound healing, and this is seen through:
- VEGFR3 (vascular endothelial growth factor receptor 3)-expressing lymphatic vessels found in early granulation tissues, with regression in later stages
- The ligand for VEGFR3 (VEGFC) increases in response to tissue injury
- Wounds with impaired lymphatic systems are more susceptible to infection and a failure to heal properly
- This is because a function of the lymphatic system is to remove debris from a site of infection - preventing this could increase bacterial colonisation and trap other growth factors/matrix proteins
- Excessive oedema can also compress other vessels and limit blood flow, causing the area to become ischaemic
- The lymphatic system is also closely linked to the immune response, and so is necessary to fight infection
- VEGFR3 (vascular endothelial growth factor receptor 3)-expressing lymphatic vessels found in early granulation tissues, with regression in later stages
How does angiogenesis occur and what is its role in wound repair?
Characterised by:
- Endothelial cell migration
- This is via fibronectin (glycoprotein involved in cell-adhesive interactions, induced intracellular signalling changes)
- Endothelial cell proliferation
- Mediated by vascular endothelial growth factors (VEGFs, secreted by macrophages and platelets) and transforming growth factor alpha (TGF-ɑ, likely to be secreted by cells of endothelial origin)
- Proteolysis of ECM
- This is achieved via collagenases, tPA and uPA
- Endothelial cell tube formation
In wound repair, angiogenesis forms new vessels from preexisting vessels through invasion of the wound clot and organisation of a microvascular network through the granulation tissue
*
Why is blood clotting important for wound healing?
- Blood clots form at sites of injury to prevent bleeding
- This occurs within minutes or even seconds
- They prevent further blood loss, which is obviously beneficial
- Blood clots also seal off a potential entrance into the body for pathogens
Which factor tightens and stabilises the platelet plug?
Factor XIIIa
How are platelets activated?
Endothelial cell damage = express von-Willebrand factor and collagen exposed from damaged vessels which activates platelets
What does vWF bind to during activation?
Platelet receptor glycoprotein Ib/Ia
What happens after platelet activation?
Platelet adhesion and aggregation
-release of thrombin
-exocytosis of alpha granules:
vWF, Clotting factor V, PDGF (platelet activating factor which recruits fibroblasts) and fibrinogen deposition
What are the three stages of blood clotting?
- Vascular spasm/ vasoconstriction
- Formation of the platelet plug
- Coagulation
What is coagulation?
- Series of reactions that are divided into three pathways:
* Contact/intrinsic pathway, where a negatively charged particle initiates a cascade resulting in the formation of factor X
* Tissue factor/extrinsic pathway, where tissue damage caulses the release of tissue factor, creating a smaller cascade to facilitate the formation of factor X
* Common pathway, which merges both pathways in the production of thrombin from prothrombin due to the action of factor X
* Vitamin K, calcium and phospholipids are necessary cofactors for this process- Production of thrombin then allows the cleavage of fibrinogen into fibrin, which forms the mesh that will adhese to and strengthen the platelet plug, therefore completing haemostasis
What is the common pathway in the coagulation cascade?
Xa activates II which converts fibrinogen to fibrin mesh (fibrin deposition)
How are blood clots removed to restore blood flow?
Fibrinolysis by plasmin (proteolytic enzyme) which is activated by urokinase secreted by macrophages
What is the role of macrophages in repair?
*Debris removing activities
*Secrete chemical mediators (IL-1) which attract fibroblasts
*Secrete VEGF and TGF alpha which are involved in angiogenesis
*Tissue remodelling and development.
*Defence against infection.
What do fibroblasts initially lay down in granulation tissue, and then what is this later replaced by?
- Extracellular matrix in the form of type III collagen
- This is later replaced by type I collagen
What are the roles of fibroblasts in wound repair?
- Contribute to both initiation and resolution phases
- Primary source of ECM production, which provides a scaffold for cells and plays a key role in determining cell phenotype and function
- Fibroblasts secrete:
- Collagen III and then collagen I
- Glycosaminoglycans
How does infection inhibit wound repair?
- Infection of the wound triggers the body’s immune response
- This causes tissue damage and inflammation, alongside slowing down the healing process
- Prolonged inflammation in response to incomplete microbial clearance in an infected wound results in the prolonged elevation of inflammatory cytokines
- This elongated inflammatory phase prevents the transition to a healing phase
- Metalloproteases are also released as a result of inflammation, which degrade ECM
- Bacteria are also likely to form biofilms, which will slow healing
How do glucocorticoids affect wound repair?
- They slow wound repair
- They are used as anti-inflammatories, which inhibit wound repair through global anti-inflammatory effects and suppression of cellular wound responses
- This includes fibroblast proliferation and collagen synthesis
- Systemic steroids cause wounds to heal with incomplete granulation tissue and reduced wound contraction
How does malnutrition affect wound repair?
- Carbohydrate, energy, protein, fat, mineral and vitamin deficiency can all affect the healing process
- Energy sources (especially glucose as can be respired in anaerobic conditions) allow the synthesis of cellular ATP, which is needed for angiogenesis and deposition of new tissue
- Protein is needed for the synthesis of a number of components, with deficiency impairing angiogenesis, fibroblast proliferation, proteoglycan synthesis, collagen synthesis and wound remodelling
- Co-factors of iron and vitamin C are also needed for collagen synthesis
- Notable amino acids are arginine and glutamine
- Vitamins C, A and E show potent anti-oxidant and anti-inflammatory properties
- C and A deficiencies have also been shown to impair the wound healing process
- Magnesium, copper and zinc are all cofactors for various enzymes that are necessary for wound healing, iron is needed in some processes
- Mg in protein and collagen syntehsis
- Cu in cytochrome oxidase, superoxide dismutase (anti-oxidant) and optimal cross-linking of collagen
- Zn in RNA and DNA polymerase
- Fe is needed in hydroxylation of proline and lysine, which is necessary for collagen production
- Deficiency in vitamin K will also result in a failure of blood clotting
- So would hypocalcaemia, in theory, but toxic effects on the heart become significant before this has a chance to occur in vivo
How does radiation affect wound healing?
- Irradiated tissue will becoe hypoxic and fibroblasts will become dysfunction
- This leads to increased wound healing complications
- Radiation is more effective on dividing cells - as wound healing requires mass proliferation, radiation would therefore have an effect
What are the phases of cutaneous wound healing?
- Injury to epidermis and dermis
- Coagulation - platelet activation and fibrin deposition
- Early inflammation - occurs within the first 24hrs, PMN recruitment
- Late inflammation - occurs within 48hrs, macrophage recruitment and angiogenesis
- Granulation tissue formation (NOT to be mixed up with granuloma) - occurs at around 72hrs, defined by recruitment and proliferation of endothelial cells and fibroblasts
- Extracellular matrix deposition via fibroblast collagen synthesis
- Remodelling of collagen occurs over weeks to months
- Scar
Label the different layers of the skin on the diagram
What is healing by first intention?
- This is the healing that occurs when a clean laceration or surgical incision is made and then closed primarily, with sutures, Steri-Strips or another skin adhesive
- In general, these wounds heal fairly quickly (within 6-8 days)
What is healing by second intention?
- This is where a wound is healed from the base upwards through the laying down of new tissue
- This occurs due to the edges of the wound not being merged/the wound not being closed, and so granulation occurs instead
- In some cases, infection must also be resolved first, with includes an acute inflammatory response and the generation of pus before granulation tissue can be generated
Describe the process of first intention wound healing.
- Platelet thrombus forms within minutes
- Monocyte, neutrophil and macrophage recruitment occurs within hours
- Fibroblast infiltration into the wound begins within days
- After 7-10 days, no neutrophils are seen and have instead been replaced with macrophages and fibroblasts
- Laying down of collagen begins to occur (1 month to 2 years)
- After 1 month - 2 years, there is remodelling and the deposition of a collagen-containing scar
- Some fibroblasts may still be seen
What are the extremes of cutaneous wound healing?
How does continued ulceration occur?
- An ulcer is an open sore caused by a break in the mucosal lining or skin that then fails to heal
- This is a failure to restore normal function and develop due to a lack of ability to restore blood flow to the area/a lack of circulation
- Poor blood flow can occur as a result of diabetes, atherosclerosis and vein/valve issues
- When valves in the leg veins become damaged/are weak, there is a lack of blood flow back to the heart and accumulation of blood in the skin
- These are varicose ulcers
- Leg swelling and skin breakdown will result in formation of ulcers
- This can cause leakage of blood from the veins into the skin
- These are varicose ulcers
- Diabetic ulcers are related to failures in arterial flow, causing ischaemic areas to develop and ulcers to form
- Ulcers can also form as a result of pressure, which is often caused by prolonged immobility
How may a scar form?
Firboblast activate increases and the wound becomes fully plugged with collagen rather than keratinocyte regeneration
How may scars be removed in the skin?
stem cell populations within the stratum basale are regenerated from the surrounding epidermis and the overlying layers can be slowly regrown as keratinocytes move up and mature, overlying the scar tissue and causing it to fade as it remodels
What are some examples of the failure of wound repair?
- Organisation and scarring
- Failure of hepatic regeneration and fibrotic response to toxic insult
- E.g. cirrhosis of the liver, causing hepatocytes to die and be replaced by fibroblasts
- Results in islands of healthy, functional hepatocytes surrounded by scarring
- Failure of hepatic regeneration and fibrotic response to toxic insult
- Excessive fibrotic response to foreign bodies
- E.g. silicosis - working with silica in the air and aerosols can result in the build-up of shadows on the lung, which are the result of massive swathes of fibrosis
- Replacement of normal tissue with fibrous scar
- E.g. folling ischaemia, seen in MIs as injury in the myocardium causes replacement with fibroblasts
- Repeated injuries
- Repeated head collisions resulting in traumatic brain injury
- These build up over time to result in chronic trauma encephalopathy (CTE)
- Classic signs of CTE can be seen in autopsy, reduction in brain tissue due to repeated inflammation/CNS injury
- Repeated head collisions resulting in traumatic brain injury
- Continued ulceration
- Venous and arterial ulcers
What is the contraction phase of wound healing?
- This is where the wound itself closes/the two edges of the wound draw together
- Wound fibroblasts and myofibroblasts work together to draw the wound close
- They then contribute to the synthesis, bundling and alignment of collagen fibres, which are the primary constituent of scar tissue, that will eventually replace granulation tissue
What is the resolution phase of wound repair?
- This is essential for restoration of full functionality and a ‘normal’ appearance to the tissue
- Migrating and proliferating keratinocytyes meet at the wound edge as it seals
- They then stop and restratify (full mechanism yet to be elucidated)
- Epidermal appendages (e.g. hair follicles and sebaceous glands) must also be regenerated, as these do not readily form in scar tissue
- Thought to require re-enactment of the epidermal developmental program, of which Wnt signalling is essential
- Dense, unorganised ECM deposited in would healing is remodelled and reorganised
- Blood bessels also undergo refinement and maturation
- Most myofibroblasts undergo apoptosis at this stage
- Imperfect regulation of wound resolution can result in hyperproliferation and/or persistence of inflammatory reactions
- These can result in fibrosis and excessive scar formation
- Lasting scar formation is also particularly likely if the wound extends beyond the epidermis
What are glucocorticoids?
The endogenous anti-inflammatory agent present to suppress inappropriate inflammation within the body. Inhibition is reduced by:
- Reduced (pro-inflammatory) gene transcription
- Reduced activity of neutrophils and macrophages
- Decreased proliferation of T cells
- Decreased production of prostanoids, cytokines and histamine
What is a side effect of increased glucocorticoids?
Wound repair and healing is inhibited
Which factors encourage the production of fibrous tissue?
Growth factors which recruit fibroblasts
VEGF which increases vascular permeability and increase in plasma proteins fibrinogen and fibronectin
Which factors inhibit the formation of fibrous tissue?
Matrix malloproteinases that degrade the ECM
Which signals induce endothelial cell proliferation?
VEGF and TGFalpha
Which type of collagen is laid down first? What is this replaced by?
Type III collagen later matures to type I collagen
What is the role of proteases in wound repair?
Proteases are necessary for the removal of damaged proteins and extracellular molecules in the impacted region, making it easier for macrophages to remove the debris.
What is fibrinolysis and its role in wound repair?
hydrolysis of the fibrin meshwork and removal of the plug in the vasculature.
*tissue or urokinase (macrophage/neutrophil-derived) plasminogen activators to activated plasmin from its zymogen
What is plasmin?
Enzyme in blood that degrades many blood plasma proteins (including fibrin clots)
