Lecture 10: Regeneration, Rpair And Fibrosis Flashcards
What is regeneration?
The replacement of lost cells by new cells of the SAME KIND
Healing can occur via which two ways?
Regeneration
Repair
Which kind of wounds can be fully healed by regeneration alone?
Wounds in which the epithelial lining ONLY is damaged.
Give an example of wounds in which the epithelial lining is the only thing that is damaged
- Abrasions of the skin
2. loss of intestinal villi which depend on epithelial cells for their shape.
The capacity of a tissue to regenerate arises largely from which type of cells?
Stem cells
How frequent are populations of stem cells in adult tissue?
Low frequency. < 10^-4 in bone marrow
What are stem cells?
Undifferentiated cells with the capacity to generate one, several or all differentiated cell types
What is the name given to a stem cell that has the capacity to generate only one differentiated cell type?
Unipotent stem cells
What is the name given to stem cells which have the capacity to generate several differentiated cell types?
Multipotent stem cells
What is the name given to stem cells which have the capacity to generate all differentiated cell types?
Pluripotent stem cells
What are stem cells like in their usual state?
They are usually quiescent (non-dividing)
Despite being normally quiescent, stem cells have indefinite what?
Proliferative potential
At what rate do stem cells divide?
At a rate that is dictated by tissue demand
What are stem cell progeny?
Transit amplifying cells
What kind of division do transit amplifying cells do?
Vigorous but transient division
Which two types of division can stem cells undergo?
- Asymmetrical cell division
2. Symmetrical cell division
What sort of cells are generated from asymmetrical division
One replacement stem cell
One differentiating stem cell
What sort of cells are generated in symmetrical division?
Identical daughter cells
Divisions with reduced probabilities of generating stem cells (I.e. too much asymmetrical divisions) lead to what?
Tissue degeneraton
Divisions with increased probabilities of generating stem cells (I.e. more symmetrical divisions) may lead to what?
Tumours
Which type of division is generated by a “big emergency”
Symmetrical division
What are the main cells that populate our tissues?
Differentiated cells
List the three types of states, tissues may be in
Mitotically active all the time,
Normally quiescent,
Terminally differentiated
What do mitotically active tissues do?
Regenerate readily after injury
What are some examples of mitotically active tissues?
Bone marrow haematopoietic cells
Skin
Gut
Respiratory and genitourinary epithelium
How is regeneration in colon crypts stimulated?
Via an lipopolisaccharide > macrophage > toll like receptor-4 > cyclooxyenase-2 > Prostaglandin E-2
pathway
What are normally quiescent tissues like?
Generally non- dividing but they can be induced into regenerative cell division by injury
What are some examples of normally quiescent tissues?
Endocrine glands
Liver
What happens when less than 70% of liver is lost?
Division of mature hepatocytes and stromal cells rapidly replace tissue
What other cells also contribute to tissue replacement when 70% of liver is lost?
Hepatic stem cells (via oval cells)
Mesenchymal stem cells
What do division of mature hepatocytes respond to?
Kupffer cell derived tumour necrosis factor, interleukin-6, hepatocyte growth factor, and transforming growth factor-α
What happens to terminally differentiated tissues?
They cannot re-enter the cell cycle and are not replaced
Give some examples of terminally differentiated tissues
Some neurons, cardiac myocytes
Which transcription factors may be used to reprogrammed differentiated cells?
Oct4, Sox2, Klf4, Myc
What are these differentiated cells reprogrammed into?
Induced pluripotent stem cells (iPS)
What does repair follow?
Loss of both functional (parenchymal) and connective (stromal) components of tissues
What does repair involve
The temporary formation of granulation tissue
The tissue cannot be what?
Reconstituted as ‘new’
What is the wound replaced by?
A fibrous scar
List the components of the repair cascade
Haemostasis
Inflammation
Proliferation
Remodelling
When does haemostasis occur?
Within a few hours
What happens in haemostasis
Platelets aggregate and degranulation at sites of blood vessel damage
Fibrinogen is converted into fibrin and cross linked with fibronectin and other ECM proteins by Transglutaminases
Why is clotting important?
It provides temporary mechanical stability
Forms a barrier to invading microorganisms
Prevents desiccation and further loss of plasma
Provides provisional matrix where cells involved in repair migrate
Generates matrix rich in cytokines and growth factors like platelet derived growth factor, transforming growth factor-β and vascular endothelial growth factor
What are the main cells involved in haemostasis
Platelets
What are the proteins involved in haemostasis
Fibrin, fibronectin, Transglutaminases
When does inflammation occur
Within a few days
Why does inflammation occur
The removal of necrotic tissue must occur before repair
What is debridement?
The removal of necrotic tissue
Which cells proceed first? Which second?
Neutrophils, followed by macrophages
What are the processes first carried out by neutrophils, and then by macrophages?
Phagocytose tissue debris, foreign material and bacteria
Secrete proteases like collagenases that liquefy necrotic tissue
Generate free radicals that inhibit microbial growth (and damage normal cells)
What other things to macrophages do?
Phagocytose dead neutrophils
Secrete growth factors for fibroblasts, capillary endothelial cells and epithelial cells
Process antigens to activate immunity
What type of cells also release growth factors that immediate repair ?
Platelets
How long does proliferation take?
A few weeks
Following injury, which cells proliferate?
Stromal and epithelial cells
What is Granulation tissue ?
Highly vascular connective tissue
Fibroblasts start proliferating and migrate where?
Into the provisional matrix
What do fibroblasts do?
They form a new matrix including collagen III initially, followed by collagen I, fibronectin and glycosaminoglycams
This new matrix formation is also termed what?
Repithelialisation
Fibroblasts may differentiate into what kind of cells?
Myofibrobalsts
What stimulates fibroblasts differentiation into myofibroblasts?
Platelet derived growth factor and transforming growth factor-β from platelets and macrophages
Contact with embryonic alternatively-spliced isoform of fibronectin , ED-A fibronectin which is synthesised in response to transforming growth factor - β
Mechanical tension
What do myofibroblasts do?
They lay down collagen fibres, like fibroblasts
Express α-smooth muscle actin, smooth muscle myosin and bundles of contractile myofilaments (like smooth muscle cells)
Focal adhesions link intracellular actin micro filaments with extracellular fibronectin
Attach to each other and to ED-A fibronectin and contract to ull edges of wound together to accelerate healing
Die by apoptosis at end of granulation phase
What features do myofibroblasts have which links intracellular actin micro filaments with extracellular fibronectin?
Focal adhesions
What are stress fibres?
Intracellular actin myofilaments
How does contraction improve healing
It reduces width of wound by 30-50% of original span
What is angiogenesis?
Endothelial cells forming new capillaries in the granulation tissue
Endothelial cells proliferate and migrate in what way?
Chemotactically
Endothelial cells proliferate and migrate chemotactically in response to what?
Macrophage derived Vascular endothelial growth factor
What do pericytes do?
Stabilise endothelial cells
How are pericytes produced?
In response to platelet derived growth factors
Where are plasma cells abundant?
In granulation tissue
What do plasma cells do?
They produce antibodies, and suppress bacterial growth
What is re-epithelialisation
Process by which epithelial cells recover an area from which they have been removed
Under normal conditions epithelial cells adhere to the basement membrane laminin using what kind of receptors?
Integrin receptors
Which two growth factors induce epithelial cells?
Epidermal growth factor and transforming growth factor -α
What do EGF and TGF-α induce epithelial cells to do?
Upregulate the EGF receptor,
Express new set of integrins and migrate along wound margin
Move through cross linked fibrin attached to underlying stroma by activating proteolytic enzymes
Regain their original phenotype when they have formed a continuous layer
What are the proteolytic enzymes that are activated by epithelial cells
Urokinase plasminogen activator
What does uPA do?
Converts plasminogen into plasmin
What does plasmin do?
Degrades fibrin and activates other proteases like matrix metalloproteinases
How long does remodelling take?
Montsh
What happens in the final phase of repair?
Granulation tissue is replaced with acellular scar tissue
What else happens in remodelling?
Disogranised collagen III fibres are replaced by parallel bundles of collagen I
Collagen accumulates to a maximum after 2-3 months when the equilibrium between collagen doosition and degradation by matrix metalloproteinases is restored
Strength of collagen continues to increase cross linking
What does excessive pathological scarring cause?
Disfigurement, pain, loss of mobility
What type of ECM component can be over expressed?
Collagen
What can over expression of ECM components arise from?
Excessive inflammatory response to injury resulting from infection or the presence of foreign bodies
Excessive production of fibrogenic cytokines like transforming growth factor β-1
Prolonged presence of myofibroblasts maybe as a result of reduced sensitivity to apoptosis
Fibrotic diseases account for how much of deaths in the industrialised world
45%
Why are these statistics under appreciated?
These deaths are reported as blocked arteries, pathological airway remodelling, chronic obstructive pulmonary disease, or heart/liver/kidney failure
Wound repair can evolve into what? (If injury is prolonged or healing response is days regulated
Progressive development of fibrosis
What are some agents that can cause sustained injury
Silica , other crystals like MSU, smoke in the lung
Inability to maintain telomeres, lung
Hypercholesterolemia, leading to liver damage
Alcohol abuse or viruses HBV, HCV causing liver damage
Obstruction of ureter leading to kidney fibrosis
Fibrosis results from what kind of myofibroblast responses?
Exaggerated myofibroblast responses
What kind of cells can contribute to fibrosis?
Macrophages, neutrophils and eosinophils
What do macrophages, neutrophils and eosinophils release?
Inflammatory cytokines that are also pro-fibrotic
What are some examples of these inflammatory cytokines released by macrophages neutrophils and eosinophils
Tumour necrosis factor
Interleukin-1
Transforming growth factorβ
Reactive oxygenated species
What do these inflammatory cytokines do
Perpetuate damage and promotes fibrosis
What are macrophages recruited by?
Persistent injury
What do macrophages release?
Tumour necrosis factor
Interleukin -1β
Transforming growth factor β
What is the effect of macrophages?
Pro fibrotic
What are neutrophils recruited by?
Macrophages,
Tumour necrosis factor
Interleukin -1β
What do neutrophils release?
Reactive oxidative species
Other toxin products
What is the effect of neutrophils?
Perpetuate tissue damage
What are eosinophils recruited by?
Allergic sensitisation
What do they release?
Interleukin-1β
Transforming growth factor β
What are the effects of eosinophils ?
Pro fibrotic
What does fibrosis suppress?
Blood flow
What is the effect of suppressed blood flow in the liver?
This can lead to portal hypertension aka elevated blood pressure
And endothelial cell damage
What is the effect of portal hypertension and EC damage
Create a cycle of tissue damage, macrophage/neutrophil recruitment and further fibrosis
Where can additional myofibroblasts differentiate from?
Fibroblasts
Bone marrow-derived monocytes via fibre yetis
Epithelial cells which may undergo epithelial-to-mesenchymal transition in the presence of mechanical tension, transforming growth factor β and hyaluronan
