MoD S5 - Healing and Repair Flashcards
What is the definition of regeneration?
The replacement of dead or damaged cells by functional, differentiated cells derived from stem cells
What are stem cells?
What happens to a stem cell when it divides?
Undifferentiated cells with limitless proliferation potential
Daughter cells either:
- Remain as stem cells
- Differentiate into specialised cells
What are stem cells used for?
Internal repair system that replaces lost or damaged cells
Can possibly be used to treat degenerative disease
What are the three major types of stem cell?
Unipotent
Multipotent
Totipotent
Where can stem cells be found in the gut?
What is their function there?
At the base of crypts in the mucosa
Replace cells lost from the mucosa
Blood cells are derived from what type of stem cell?
Multipotent haemopoietic stem cells
What is the function of stem cells in the embryo?
Differentiate into the different tissues of the body
How are cells classified by propensity to regeneration?
Give a breif description and example of each type
Labile cells:
- Normally in active cell division
- Rapid proliferation
- E.g. Epithelial cells
Stable cells:
- Resting state - G0, but can divide in the right conditions
- Speed of regeneration variable
- E.g. Hepatocytes
Permanent cells:
- Unable to divide - G0
- Unable to regenerate
- E.g. Neurones, Cardiomyocytes
Give two examples of stable cell proliferation
Renal tubular epithelium, when damaged can proliferate and replace lost and damaged cells
Hepatocytes can regenerate when they are damaged during cirrhosis of the liver (nodules)
What factors control regeneration?
Growth factors
Contact between cell and basement membrane and adjacent cells
What do growth factors do?
Promote proliferation in the stem cell population via extracellular signals that are transduced into the cell hence promoting expression of genes controlling the cell cycle
Give some examples of growth factors
Proteins:
- Epidermal GF
- Platelet derived GF
- Fibroblast GF
- Some Cytokines
Hormones:
- Oestrogen & Testosterone
- Growth hormone
What types of cell signalling do growth factors use?
Autocrine
Paracrine
Endocrine
Give an example mechanism by which a growth factor might induce cell proliferation
Binds to extracellular receptor
Receptor has intrinsic enzyme activity and will autophosphorylate (tyrosine kinase)
This sets of a phosphorylation cascade which results in increased transcription of genes which regulate the cell cycle
How is contact between cell and basement membrane and adjacent cells involved in regulating regeneration?
Signalling is done through adhesion molecules
Signals inhibit proliferation when the cell being signalled is in contact with other cells (Contact inhibition)
Loss of contact will promote proliferation
In what group of pathologies do the mechanisms controlling cellular proliferation become deranged?
Cancer
Give an example of a mechanism by which contact inhibition is achieved
E-cadherin is a protein spanning the cell membrane that will dimerise with other E-cadherin molecules on adjacent cells
The end result of this dimerisation is contact inhibition
What is ‘fibrous repair’?
The replacement of functional tissue by scar tissue
What are the 2 main outcomes of inflammation or injury and how do they come about?
Hint: relate this back to proliferation potential of the cells that were damaged
When injury leads to necrosis of permanent cells:
- Fibrous repair and scarring occurs
When injury leads to necrosis of stable or labile cells:
- If collagen framework preserved, insult is resolved
- If collagen framework destroyed, Fibrous repair and scarring
What are the 3 main components of fibrous repair and all together, what do they form?
Cells that migrate in
Blood vessels - Angiogenesis
Extracellular matrix protein production and remodelling
Granulation tissue
What are the important cell types in fibrous repair?
How is each type involved?
Inflammatory cells:
- Phagocytosis of debris - neutrophils and macrophages
- Chemical mediators - lymphocytes and macrophages
Endotheial cells:
- Angiogenesis
Fibroblasts/Myofibroblasts
- Produce extracellular matrix proteins E.g. Collagen
- Myofibroblasts can contract the wound
Why is angiogenesis vital to wound healing?
Provides access to the wound for inflammatory cells and fibroblasts as well as oxygen and nutrients
How does angiogenesis begin?
Endothelial proliferation induced by proangiogenic growth factors such as VEGF
Pre-existing blood vessels will sprout off new blood vessels (they don’t arise de novo)
Describe the process of angiogenesis
Growth factors such as VEGF produced at sites of chronic inflam, tissue injury or hypoxia
Endothelial proteolysis of basement membrane
Migration of endothelial cells into surrounding interstitium via chemotaxis
Endothelial proliferation into primitive tubular structures
Maturation and tubular remodelling
Linkage with venous system
Recruitment of periendocardial cells
How is angiogenesis involved in malignancy?
Tumours require rich blood supply
Tumours require angiogenesis to supply metabolic demand once tumour has grown to a larger size
Potential therapeutic target
What are the functions of the extracellular matrix?
Supports and anchors cells
Separates tissue components (E.g. basement membrane)
Sequesters growth factors
Facilitates cell communication and migration
Describe collagen
Structure?
Types?
Provides extracellular framework Composed of triple helices of various polypeptide alpha chains Fibrillar collagens (I - III) Amorphous collagen (IV - VI) Remodelled by specific collagenases
Describe the synthesis of collagen fibres
Hint: FIBRES
Polypeptide alpha chains synthesised in ER
Enzymatic modification steps including Vit C dependent hydroxylation
Alpha chains align and cross link to form procollagen triple helix
Soluble procollagen secreted from cell
After secretion cleaved to form tropocollagen
Tropocollagen will polymerise to form fibrils
Bundles of fibrils form fibres
List 4 major defects of collagen synthesis
Vit C Deficiency - Scurvy!
Ehlers-Danos syndrome
Oseogenesis imperfecta
Alport syndrome
Describe some of the features of scurvy
Inadequate Vit C dependent hydroxylation of alpha chains leads to defective helix formation
Lacks strength, vunerable to enzymatic degradation
Causes heamorrhage due to affecting collagen supporting blood vessels
What is the major defect in Ehlers-Danos syndrome?
Defective conversion of procollagen to tropocollagen
Apart from collagen what are some of the other constituents of the ECM in fibrous repair?
Matrix glycoproteins:
- Organise and orientate cells
- Support cell migration
- E.g. Fbronectin, laminin
Proteoglycans:
- Matrix organisation, cell support, regulate availability of growth factors
Elastin:
- Provides elasticity
What are the 3 main stages of fibrous repair?
Inflammatory cells infiltrate
Clot replaced by granulation tissue
Maturation
Describe the process of Fibrous repair
Inflammation:
- Blood clot forms
- Acute inflammation occurs, neutrophils digest clot
- Chronic inflammation begins and macrophages and lymphocytes are recruited
Granulation tissue replaces clot:
- Capillaries begin to sprout and infiltrate in response to pro-angiogenic growth factors
- Myo/fibroblasts migrate and differentiate and begin producing ECM (Collagen and friends)
Maturation:
- Cell population falls
- Collagen increases, matures and remodels
- Myofibroblasts contract, reducing volume of defect
- Vessels differentiate and are reduced
- Left with fibrous scar
How is fibrous repair controlled?
Complex and poorly understood:
- Inflammatory cells recruited via chemotaxis
- Pro-angiogenic factors release by platelets and ECM in response to hypoxia E.g. VEGF
- Macrophages produce pro-fibrotic cytokines (E.g. IL-1, TNF-alpha) leading to fibroblast proliferation and ECM production
What is healing by primary intention?
Occurs following an incised wound in the skin with apposed edges
Minimal clot and granulation tissue
Epidermis regenerates
Dermis undergoes fibrous repair (granulation tissue transitions to scar tissue)
Sutures out at 10 days (when tissue has approx 10% strength)
Maturation of scar continues for up to 2yrs resulting in minimal scarring with good strength
Risk of trapping infection
What is healing by secondary intention?
Occurs after infarct, ulcer or any large wound
Unapposed wound edges
Large clot dries to form as scar or eschar
Epidermis regenerates from base up
Produces far more granulation tissue than primary intention healing and produces a larger scar (takes longer)
Myofibroblasts must produce much more contraction to reduce defect volume
What are the 4 stages of bones repair?
Haemotoma formation
Procallus formation
Bony callus formation
Remodelling
Describe healing of bone after a fracture
Haemotoma:
- Forms from ruptured periosteal and marrow cavity vessels
- Provides framework for ingress of macrophages, endothelial cells, fibroblasts and osteoblasts
- Necrotic tissue removed by macrophages
- Capillaries develop
Procallus:
- Normal granulation tissue forms + osteoblasts and osteoblast matrix
- Bones is laid down in irregular woven pattern sometimes with islands of cartilage
- Mineralisation begins
- External Hyaline cartilage callus provides support
Bony callus and remodelling:
- Woven bones gradually replaced by lamellar bones
- Lamellar bone remodelled to direction of mechanical stress
What local factors affect wound healing?
Type, size, location of wound
Apposition (important in skin, bone, nerves)
Lack of movement
Blood supply
Infection (gangrene, systemic)
Foreign material (dirt, glass, sutures, necrotic tissue)
Radiation damage
What are the general or systemic factors affecting healing?
Age
Drugs (steroids, hormones)
Dietary deficiency (general e.g. proteins, Vit.C, essential amino acids)
General health (Diabetes, rheumatoid arthritis)
Cardiovascular health
What are the common complication of wound healing?
Insufficient fibrosis:
- Wound dehiscence (rupture of sutures)
- Hernia, ulceration
- Risk factors: Obesity, malnutrition, steroids
Excessive fibrosis:
- Cosmetic scarring, keloids
- Cirrhosis, lung fibrosis
Excessive contraction:
- Obstruction of tubes or channels (strictures)
- Limitation of joint movement (contractures)
Describe regeneration of the liver following acute and chronic damage
Acute:
- Only organ capable of regeneration of lost tissue
- Can completely regerate lost hepatocytes and structural damage
Chronic:
- Liver cirrhosis
- Fibrous tissue replaces damaged liver tissue
- Normal hepatic structure lost, hepatocytes form ‘nodules’ of regenerating tissue between fibrous tissues
Describe the healing response to acute tubular necrosis
Death of kidney tubule cells
Due to toxins or hypoxia
Some epithelial cells remain due to patchy necrosis of tissue, structural framework also remains intact
Epithelial regeneration via mitosis occurs and epithelia is regrown on the basement membrane
This leads to complete regeneration
Describe healing of the heart post MI
Myocardium is completely incapable of regeneration of damaged tissue
Healing will always occur via fibrous repair
This leads to a loss of contractile force the heart can generate, may lead to heart failure or cardiogenic shock
Describe the healing of cartilage
Cannot normally regenerate
Chondrocytes cant migrate to damaged areas due to being trapped in lacunae
Avascular, so deposition of new matrix is slow
Hyaline cartilage damage may result in fibrocartilagenous scar tissue repair
What is Wallerian degeneration?
What follows?
Degeneration of a nerve fibre distal to a cut or crushing injury
Neurolemma doesn’t degenerate leaving a hollow tube
Axonal regrowth (sprouting) can occur in the CNS and PNS
If neurolemma is held in good apposition then the proximal nerve axon can sprout into the distal neurolemma and regenerate and reinnervate the target
If neurolemma is lost or not held in apposition, this is not possible
What is the response to CNS damage?
Gliosis
Macrophages and microglia invade site of damage and begin clearing cell debris (E.g. Degenerating axons)
Oligodendrocytes precursor cells then invade and begin the process of remyelination of demyelinated neurones
Surrounding astrocytes will proliferate and form a glial scar
This process will inhibit axonal regrowth, but can have other positive effects
