L8: Regeneration and Repair

Question 1

When in the inflammatory response is repair necessary?

Answer 1

Acute phase–> fibrosis–> substantial damage/ tissue destruction
Chronic phase–> Prolonged inflammation

Question 2

What processes are involved in wound healing?

Answer 2

Injury–> haemostasis (blood clots)–> inflammation–> regeneration or repair

Question 3

What is regeneration?

Answer 3

Regrowth of cells–> no evidence/ minimal evidence of injury–> no scar formation

Physiological–> production of new cells–> haematopoesis (RBC, WBC) etc

Question 4

When can regeneration take place?

Answer 4

Minor injuries–> superficial skin damage

• Incision wound
• Abrasion

Question 5

Where do the new cells come from?

Answer 5

Stem cells –> differentiate into other cell types

• -> self renew
• -> replace damaged/ dead cells

Question 6

What are the different types of stem cells?

Answer 6

Totipotent–> potential to differentiate into any cell type
Multipotent–> several cell types
Unipotent–> one cell type

Question 7

Where are stem cells found?

Answer 7

Scattered throughout (not totipotent stem cells)
e.g. skin/epidermis (stratum basale), intestinal mucosa (bottom of crypts), liver (between hepatocytes) etc...

Question 8

Which tissue types are able to regenerate?

Answer 8

Labile tissue–> continuous replication of cells
e.g. epithelium, haematopoietic tissue
Stable tissue–> Normally low level of replication–> can undergo rapid proliferation–> response to stimuli
–> Go Quiescent cells
e.g. liver, kidneys, pancreas, bone, endothelium, smooth muscle
Permanent tissue–> cells do not replicate once left cell cycle
e.g. neurons, skeletal muscle, cardiac muscle

Question 9

What is a requirement for tissue regeneration?

Answer 9

Intact architecture

Question 10

What is the difference between regeneration and repair?

Answer 10

Regeneration–> replace damaged cells with new cells (regrowth of cells)
Repair–> Replacement of tissue with scar

Question 11

After tissue injury how do cells know whether to undergo regeneration or repair?

Answer 11

Necrosis of liable or stable tissues
–> Collagen framework intact–> regeneration
–> Not intact or on-going chronic inflammation–> Fibrosis repair
Necrosis of permanent tissue
–> Firbous repair (scar tissue)

Question 12

How does a scar form?

Answer 12

Bleeding and haemostasis
Inflammation
Proliferation
Remodelling

Question 13

Describe each stage of scar formation?

Answer 13

1- Bleeding and haemostasis

• -> Within seconds to minutes
• -> Prevent blood loss

2- Inflammation

• -> Acute then chronic
• -> Digestion of blood clot, removal of dead tissue
• -> Minutes- days

3- Proliferation

• -> Capillaries (angiogenesis)
• -> Fibroblasts–> produce and lay down collagen and elastin
• -> Myofibroblasts
• -> Extracellular matrix (ECM)
• -> Formation of granulation tissue–> new collagen laid down
• -> Days- weeks

4- Remodelling

• -> Maturation of scar
• -> Reduced cell population
• -> Increased collagen
• -> Myofibroblast contract–> seals gap
• -> Scar formation

Question 14

What is the function of proliferation?

Answer 14

Fill in the gaps
Capillaries supply O2 and nutrients
Contracts and closes the defect/ gap

Question 15

What are the cells involved in fibrous repair? What is their function?

Answer 15

Neutrophils and macrophages 
--> Phagocytosis
--> Release of chemical mediators
Lymphocytes
--> Eliminate pathogens 
--> Co-ordinate other cells 
Endothelial cells 
--> Proliferation 
--> Angiogenesis
Fibroblasts
--> secrete collagen and elastin
Myofibroblasts
--> Intracellular actin--> wound contraction

Question 16

What is the difference between fibroblasts and myofibroblast?

Answer 16

Fibroblasts–> spindle shaped nucleus, cytoplasmic extensions–> secrete collagen and elastin to form ECM

Myofibroblasts–> between and smooth muscle and a fibroblast–> similar appearance to fibroblasts
–> intracellular actin–> contracts–> wound contraction

Question 17

What is collagen?

Answer 17

Abundant mammalian protein
Provides extracellular framework
29 different types
Type 1–> bones, tendon, ligament, skin, sclera
Type 4–> basement membrane, lens, glomerular filtration

Question 18

How is collagen synthesised?

Answer 18

1. Peptide of gly-x-y–> alpha chain preprocollagen –> in ER of (myo)fibroblasts
2. Hydroxylation of Pro and Lys aa–> vitamin C dependent–> procollagen
3. Release from ribosome
4. Glycosylation
5. Triple helix–> alpha chains crosslinked–> in cytoplasm
6. Secreted from cell
7. Cleave N and C terminal–> tropocollagen
8. Tropocollagen crosslinked–> microfibrils–> fibrils and collagen fibres

Question 19

What are some of the diseases associated with defective collagen?

Answer 19

Acquired–> developed later in life
Scurvy

Inherited–> born with it
Ehlers- Danlos syndrome
Osteogensis Imperfecta
Alport Syndrome

Question 20

What is Scurvy?

Answer 20

Vit C deficiency
Inadequate hydroxylation of pre-pro collagen
Defective triple-helix= defective collagen
Unable to heal wounds properly, tendancy to bleed, tooth loss

Question 21

What is Ehlers-Danlos syndrome?

Answer 21

Heterogenous group of 6 inherited disorders
Collagen fibres lack adequate tensile stress and strength
Hyperextensible, fragile, susceptible to injury
Hypermobile joints

Question 22

What is osteogenesis imperfecta?

Answer 22

Brittle bone disease
Little bone tissue and extreme skeletal fragility
Blue Sclerae

Question 23

What is Alport Syndrome?

Answer 23

X-linked disease
Type IV collagen dysfunctional
Usually male
Haematuria in childhood–> chronic renal failure

Question 24

How are regeneration and repair controlled?

Answer 24

Cells communicate with each other to produce a proliferative response

Question 25

What are the mechanisms of communication between cells?

Answer 25

Direct cell-cell/ Cell-stromal contract Local Mediators (Growth factors) Hormones

Question 26

What are growth factors? What do they do? What are some examples?

Answer 26

``` Polypeptide that act on cell surface Cause other cells to enter the cell cycle and proliferate --> reach M phase and divide Helpful for healing Examples: Epidermal Growth Factors Vascular Endothelial Growth Factors Platelet Derived Growth Factors Tumour Necrosis Factors ``` defective in cancer

Question 27

What are the different types of cell communication for local mediators and hormones?

Answer 27

Autocrine--> respond to signalling molecules produced by self Paracrine--> signals from cells in local area Endocrine--> signals released into bloodstream act on target cells

Question 28

How does epidermal growth factor work?

Answer 28

Binds to EGF receptor (EGFR) Produced by keratinocytes, inflammatory cells and macrophages Mitogenic for epithelial cells, hepatocytes and fibroblasts

Question 29

How does VEG-F work?

Answer 29

Inducer of BV development (vasculogenesis) Growth of new BV (angiogenesis) In chronic inflammation, wound healing and tumours

Question 30

What does platelet derived GF do?

Answer 30

Stored in platelet alpha granules Released upon activation Produced by macrophages, endothelial cells, smooth muscle cells and tumour cells Migration and proliferation of fibroblasts, SMC, and monocytes

Question 31

What does TNF do?

Answer 31

Induces fibroblast migration and proliferation and collagenase secretion

Question 32

Why is direct cell-cell contact important?

Answer 32

Important to stop cell proliferation--> contact inhibition Cadherins (cell surface molecules) interact--> inhibit proliferation defective in cancer

Question 33

What are the two classifications of wound healing?

Answer 33

Primary Intention Secondary Intention Determined by the size of the wound

Question 34

What is closing by primary intention? What are the different stages?

Answer 34

Incision, closed, non-infected and sutured wounds (apposed edges) Disruption of epithelial BM but death of only an limited number of cells Stages 1- haemostasis (seconds to minutes) --> narrow space--> clotted blood--> dehydration of surface--> scab 2- inflammation (minutes to hours) --> neutrophils appear at margin--> inhibit bacteria 3- Migration of cells (up to 48 hours) --> macrophages --> scavenge dead neutrophils--> activated, secrete cytokines--> attract other cells (fibroblasts) 4- Regeneration (3 days) --> replace neutrophils--> granulation tissue invades (fibroblasts and new capillaries)--> produce collagen 5- Early scarring (7-10 days)--> filled with granulation tissue--> fibroblast proliferate, deposit collagen--> fibrous mass 6- Scar maturation (1 month- 2 years)--> mass of fibrous tissue many collagen fibres--> few cells and vessels--> appears white

Question 35

What is healing by secondary intention?

Answer 35

Significant tissue loss Unapposed edges (infection, ulcer, abcess) Abundant clot, inflammation and granulation tissue Considerable wound contraction required (myofibroblasts) Dermis requires significant repair Epidermis regenerates from edges

Question 36

How does bone fracture healing take place?

Answer 36

1. Haemotoma formation - -> BV in bone and periosteum break - -> Mass of clotted blood forms - -> Bone cells at edge of trabecular die - -> Swelling and inflammation occur--> cytokines released - -> Phagocytic cells and osteoclast begin to remove dead and damaged tissue - -> Macrophages will eventually remove the clot 2. Soft callus - -> New BV infiltrate the periosteum - -> Procallus (soft callus) of granulation tissue forms - -> Fibroblast produce collagen fibres that span trabeculae - -> Other fibroblast produce cartilage - -> Osteoblast in periosteum and endosteum start laying down bone 3. Hard callus - -> New bone trabeculae being to appear - -> Trabeculae develop become hard (bony) callus - -> Endochondrial Ossification replaces cartilage with bone - -> Intranmembraneous ossification produce bone in gaps 4. Remodelling - -> Cancellous bone remodelled to compact bone - -> Material bulging out and in removed by osteoclasts - -> Final step--> remodelling--> original bone shape

Question 37

What are the local factors affect wound healing?

Answer 37

Local and systemic factors Local: --> Size, location and type of wound--> primary or secondary intention, regeneration or scarring --> Mechanical stress--> weight-bearing/ loading will take longer --> Blood supply--> O2 and nutritens, myofribroblast and fibroblast supplied--> takes longer if no blood supply --> Infection--> produces peristant tissue injury and inflammation --> Foreign bodies--> inflammation and injury --> Haematoma--> large can slow healing --> Necrotic tissue--> needs clearing --> Protection--> help to keep the wound clean and free --> Surgical techniques --> bad techniques slows healing

Question 38

What are the systemic factors that affect healing?

Answer 38

Age--> elderly heal more slowly Anaemia, hypoxia and hypovolaemia--> poor O2 delivery to healing tissue Obesity--> increased tension on wounds Diabetes--> impairs blood supply Malignancy--> Wasting of body (caused by it) Genetic disorders Drugs--> steroids, cytotoxics, antibiotics Vitamin deficiency--> Vit C deficiency inhibits collagen synthesis Malnutrition or proteins loss--> lack of essential substances

Question 39

What are some of the complications associated with fibrous repair?

Answer 39

1. Insufficient fibrosis--> wound dehiscence (edges of wound don't meet)--> obesity, elderly, malnutrition and steroid used (skin thinner less for suture to hold to) 2. Excessive fibrosis--> Keloid scar--> over production of collagen, exceeds borders of scar 3. Adhesion--> fibrous bands, cause obstruction of tubes (if around organ or tube) 4. Loss of function--> replacement of specialised tissue by fibrous tissue --> heart- myocardium can't contract 5. Disruption of architecture--> liver cirrhosis 6. Excessive scar contraction--> constriction of tubes, fixed flexion deformaties (contractures)

Question 40

How does healing affect certain tissues?

Answer 40

- Cardiac muscle--> regenerative capacity limited--> scar formation--> compromised function - Liver--> can regenerate, architecture damaged--> cirrhosis - Peripheral nerves--> axons degenerate--> proximal stumps sprout and elongate--> Schwann cells guide back to tissue they innervate--> 1-3mm/day - Cartilage--> no blood supply--> doesn't heal well - CNS--> permanent tissue--> replaced by CNS support elements