Healing & Repair Flashcards
What determines the ability of a tissue to regenerate?
The ability of a tissue to regenerate depends on the type of cell that it is composed of:
- Labile cells are continuously dividing.
- Examples of labile cells include squamous epithelium, columnar epithelium, urothelium and haemopoietic cells.
- Stable cells are quiescent (i.e. in the G0 phase of the cell cycle but under stimulation can re-enter the cell cycle).
- Examples of stable cells include hepatocytes, pancreatic acinar cells, fibroblasts, smooth muscle cells and endothelium.
- Permanent cells are unable to divide.
- Examples of permanent cells include cardiac myocytes, neurones and skeletal muscle cells.
Give an overview of the mechanism of action of growth factors for inducing cell cycle progression.
- Growth factors bind to growth factor receptors, leading to phosphorylation of the receptor.
- The receptor triggers a cellular cascade that activates RAS through phosphorylation of RAS-bound GDP, producing GTP.
- Active RAS triggers PI3K-Akt-mTOR and RAF-MAPK pathways, which activate transcription of the MYC protein to induce cell cycle progression.
List 7 growth factors.
What are their sources and functions?
Examples of Growth factors:
1 - Epidermal growth factor (EGF).
- Sources of EGF include activated macrophages, keratinocytes and salivary glands.
- Functions include stimulation of cell division for many cell types, stimulation of epithelial cell migration and stimulation of granulation tissue formation.
2 - Transforming growth factor-alpha (TGF-a).
- Sources include activated macrophages and keratinocytes.
- Functions include stimulation of hepatocyte and epithelial cell proliferation.
3 - Hepatocyte growth factor (HGF).
- Sources of HGF include fibroblasts, liver stromal cells and endothelial cells.
- Functions include stimulation of hepatocyte and epithelial cell proliferation and increasing cell motility.
4 - Keratinocyte growth factor (KGF).
- KGF is produced by fibroblasts.
- Functions include stimulation of keratinocyte migration, proliferation and differentiation.
5 - Vascular endothelial growth factor (VEGF).
- VEGF is produced by mesenchymal cells.
- Functions include stimulation of endothelial cell proliferation and increasing vascular permeability.
6 - Fibroblast growth factors (FGFs).
- Sources include macrophages, mast cells and endothelial cells.
- Functions include inducing cell division and chemotaxis for fibroblasts and stimulating angiogenesis and ECM protein synthesis.
7 - TGF-beta.
- Sources include platelets, T-lymphocytes, macrophages, endothelial cells, epithelial cells, smooth muscle cells and fibroblasts.
- Functions include inducing chemotaxis for leukocytes an fibroblasts, stimulating ECM protein synthesis and suppressing acute inflammation.
Briefly describe the process of regeneration in the liver and bone.
Liver:
- Inflammatory cells (such as neutrophils) are recruited and inflammatory mediators are released at the site of injury (as with all healing processes).
- This is followed by formation of avascular islands of parenchymal cells.
- Quiescent hepatocytes in the avascular islands are then stimulated to proliferate by growth factors.
- This is accompanied by establishment of new sinusoidal channels, which give rise to the vasculature.
- Hepatocyte proliferation and revascularisation is also met with clearance of inflammatory cells and mediators (inflammation is resolved as with all normal healing processes).
Bone:
- Inflammatory cells (such as neutrophils) are recruited and inflammatory mediators are released at the site of injury (as with all healing processes).
- The bulk of bone is composed of the ECM substance, osteoid, which consists of type 1 collagen, glycosaminoglycans and proteoglycans. Therefore, regeneration is less reliant on osteocyte proliferation and more reliant on deposition and remodelling of osteoid.
- Osteoid deposition is met with clearance of inflammatory cells and mediators (inflammation is resolved as with all normal healing processes).
Describe (in more detail) the process of regeneration in bone.
Regeneration in bone:
1 - Rupture of vessels caused by a bone fracture leads to haematoma formation.
- Leakage of platelets leads to release of fibrin, forming an early fibrin meshwork, and platelet-derived growth factor (PDGF). Leakage of inflammatory cells leads to release of TGF-beta and FGF.
2 - These growth factors activate cells which engage in soft callus formation, which develops the meshwork for bone regeneration:
- Osteoclasts and osteoblasts, leading to remodelling and redeposition of osteoid.
- Progenitor cells, which differentiate into chondrocytes to produce a cartilage model, which is then replaced by bone in a process known as endochondral ossification.
3 - At 2-3 weeks, the soft callus becomes a bony callus, taking on a woven formation containing trabeculae of immature bone.
- Histologically, the osteoblast-lined perimeter of the weaves appears darker in colour on H&E stain, whereas the osteoid-filled centres of the weaves appear lighter.
4 - After 3 weeks, the immature bone matures to become lamellar bone, which has a normal structural integrity.
When does scarring occur?
List the stages in scar tissue formation.
- Scarring occurs when repair cannot be achieved by regeneration alone.
Stages in scar tissue formation:
1 - Haemorrhage, leading to platelet and fibrin deposition.
2 - Acute inflammation (which, in contrast to normal healing, does not resolve throughout the scarring process and instead becomes chronic).
3 - Proliferation of endothelial cells and fibroblasts (which results in a bulging mass of cells beneath the fibrin deposit known as granulation tissue) and proliferation of epithelial cells.
4 - Connective tissue deposition.
What is granulation tissue composed of?
What is the purpose of granulation tissue?
Granulation tissue is composed of:
1 - Newly forming blood vessels.
2 - Inflammatory cells.
3 - Fibroblasts.
4 - Stroma.
- Granulation tissue attempts to give structural integrity back to damaged tissue (a provisional extracellular matrix) whilst other cells are proliferating.
How does fibrin appear on H&E stain?
Fibrin appears stranded and bright pink (eosinophilic) on H&E stain.
Summarise the process of angiogenesis.
Angiogenesis:
1 - A quiescent vessel is exposed to angiogenic factors such as VEGF.
2 - This leads to vasodilation and subsequent pericyte detachment by action of angiopoietin and basement membrane degradation by action of MMPs.
3 - A vascular stalk forms where the pericyte and basement membranes are detached.
4 - The vascular stalk elongates and migrates into the surrounding stroma to link with other stalks, forming new vessels.
- MMPs assist in the degradation of surrounding ECM and connective tissue to facilitate stalk migration.
Describe the process of connective tissue deposition during scar tissue formation.
What is the purpose of this process?
How does the scar change as it matures?
- As granulation tissue matures, fibroblasts begin to deposit connective tissue in a process known as fibroplasia. The connective tissue is deposited with little structural organisation.
- The connective tissue is primarily composed of collagen, but also contains proteoglycans and elastin.
- Fibroplasia is primarily regulated by TGF-beta, which is released by inflammatory cells such as M2 macrophages.
- The purpose of fibroplasia is to increase the structural integrity of the damaged tissue.
- As the scar matures:
1 - The connective tissue takes on a more organised horizontal structure. This dynamic remodelling is controlled by matrix metalloproteinases and their inhibitors.
2 - Fibroblasts decrease in size as they become less metabolically active.
3 - Some fibroblasts develop properties of smooth muscle cells as they differentiate into myofibroblasts to allow contraction of the wound, decreasing the size of the site of injury.
Describe the two types of skin healing.
1 - Skin healing by primary intention:
- Skin healing by primary intention occurs where there are small defects that can be closed by sutures (e.g. surgical wounds).
- Damage healed by primary intention is limited to a small amount of the epidermis and basement membrane.
- Healing by primary intention is relatively quick and leaves minimal scarring.
2 - Skin healing by secondary intention:
- Skin healing by primary intention occurs where there are large defects that cannot be closed (e.g. following severe trauma).
- Re-epithelialisation occurs first along the base of the defect and moves upwards.
- Healing by secondary intention is relatively slow, and involves more inflammation, granulation tissue, fibroplasia and, ultimately, more scarring.
- A skin graft can be used to aid healing in healing by secondary intention.
List 4 systemic factors and 4 local factors influencing wound healing.
Systemic factors influencing wound healing:
1 - Nutrition.
2 - Metabolic status.
3 - Circulatory status (e.g. diabetes impedes peripheral blood flow).
4 - Hormones.
Local factors influencing wound healing:
1 - Local blood supply.
2 - Infection.
3 - Presence of foreign bodies.
4 - Mechanical factors.
How does wound strength change over time?
- Wound strength after 1 week is 10% of the original tissue strength.
- This changes to 50-60% at 1 month.
- Tissue strength plateaus at 70-80% after 3 months, never reaching 100% of the original tissue strength.
List 3 examples of pathological healing in the skin.
1 - Keloid scars.
- These most often occur in people with darker skin following relatively minor trauma such as ear piercings or minor surgery.
- Histologically, keloid scars appear as thick deposits of collagen, losing the normal wave appearance.
- Freezing keloid scars early in development can stop their growth, but excision may just result in the formation of another keloid scar.
2 - Contractures.
- These occur in patients that have experienced large burn injuries, and the scar tissue deposited into the skin prevents normal movement.
3 - Chronic ulcers.
- These occur where (even relatively minor) injuries occur at regions with compromised blood supply.
- The lack of blood prevents the healing process altogether, leaving the wound vulnerable to infection.
What is cirrhosis?
- Cirrhosis is the typical response of the liver to prolonged severe injury.
- It is characterised by nodules of regenerating hepatocytes surrounded by fibrous tissue.
- Fibrosis distorts the liver architecture and damages the vasculature, impairing liver function.
