Lecture 10: Healing - generation & healing

Question 1

What is Generation

Answer 1

• Replacement of injured cells by cells of the same kind.
• Some cells types don’t readily regenerate.
• Restoration of tissue function
• Usually possible for minor injuries.

Question 2

What is repair

Answer 2

• Replacement of injured cells but not fully - can cause loss or reduction of normal tissue function/structure.
• Occurs during large scare damage or in tissues w/o regenerative capacity.

Question 3

Regeneration can occur differently for different cells types

Answer 3

• Labile cells - constant regeneration (mitotically active - skin and gut epithelia)
• Can regenerate if damaged when signalled (quiet and quiescent -usually inactive). i.e. liver hepatocytes
• Limited regeneration (permanent-terminally differentiated). i.e. neurons and cardiomyocytes.

Question 4

STEM cells - types, differentiation

Answer 4

• Unipotent, multipotent, pluripotent
• Divide infrequently (quiescently) but are immortal.
• Differentiate asymmetrically to produce another STEM cell and TAC cell, which differentiates into a specific cell afterwards. Quick and short proliferation.

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Question 5

TAC

Answer 5

Transient amplifying cell.

STEM –> TAC + STEM –> STEM + specific epith

Question 6

Too many stem cell consequence

Answer 6

Tumor/cancer - too much differentiation, too many cells.

Question 7

Too few stem cell consequence

Answer 7

Tissue atrophy, reduced organ size - not enough regeneration/healing.

Question 8

Describe activation of inflammation and regeneration of epithelial cells in labile cells (i.e. Small intestine).

Answer 8

1) Release of PAMPs (i.e. LPS) by pathogen
2) LPS/PAMPs damages membrane of epithelia
3) PAMPs bind to PRR (i.e. TLR4) of underlying tissue-resident macrophages.
3) Macrophage is activated, differentiates and changes function, releasing COX2.
3) Arachadonic acid –COX2–> Prostaglandins
4) Prostaglandins activate inflammation and contribute to proliferation of STEM cells.
5) STEM cells –> STEM + TAC –> epithelium.

Question 9

Describe regeneration of stable cells (i.e. Hepatocytes and bile duct)

Answer 9

1) DAMPs/PAMPs bind to TLR (PRR) of liver resident macrophages - Kupffer cells.
2) Activated Kupffer cells release cytokines and growth factors that induce hepatocyte mitosis.
3) Mesenchymal and Hepatocyte STEM cells also contribute to regeneration by division and differentiation.

Up to 70% of liver can be lost and regenerated.

Question 10

Repair occurs when:

Answer 10

• Parenchymal (functional) and stromal (supportive) tissues are damaged heavily.
• Repair occurs via formation of temporary C.T. (granulation tissue) that resolves a scar.
• Can work alongside regeneration in certain tissues.

Question 11

Phases of repair:

Answer 11

1) Haemostasis (clotting - takes few mins)
2) Inflammation (debridement - takes 0-3 days)
3) Proliferation (takes 3-12 days)
4) Remodelling (scar tissue formation - takes 3 days to 6 months).

Question 12

M0 macrophages

Answer 12

Inactive macrophages, waiting for signal to differentiate.

Question 13

M1 macrophages:

Answer 13

Differentiate from M0 macrophages.
Release pro-inflammatory cytokines: IL-6, IL-1, ROS.

Question 14

M2 macrophages:

Answer 14

Contribute to wound healing, secreting cytokines and growth factors.
- PDGF (platelet-derived growth factor)
- TGFb (Transforming growth factor beta)
- TNFa/EGF. (tumor necrosis factor alpha/endothelial growth factor).

Question 15

Pro-resolution/M3 macrophages

Answer 15

Support re-modelling, release cytokines and GF.
- TGFb and IL-10.

Question 16

Wound healing/repair process:
1) Hemostasis

Answer 16

• Activation of coagulation cascade - platelets aggregate, degranulate, thus producing fibrin.
• Transglutaminase (enzyme) crosslinks Fibrin to fibronectin and other ECM proteins.

Question 17

Clotting in Hemostasis provides:

Answer 17

• Temporary mechanical stability
• Barrier to microbes, prevents dessication
• Matrix in which cells involved in repair can migrate to
• Matrix rich in cytokines and growth factors such as PDGF, TGFb, VEGF secreted from platelets.

Question 18

Wound healing/repair process:
2) Debridement

Answer 18

Removal of necrotic tissue instigated by neutrophils, then macrophages.

• phagocytose tissue debris and bacteria, antigen-presenting cells, show to T cells to activate adaptive immune system. Phagocytosis triggers neutrophils to conduct apoptosis - apoptotic bodies are efferocytosed by M1 macrophages, triggering M2 healing response.
• Secrete proteases (i.e. collagenases) that liquify tissue
• Generate ROS - chemotactic, mitogenic, and antimicrobial
• M1 Macrophages secrete mitogenic growth factors for healing. - triggering M2 macrophages

Question 19

Specific role of growth factors in 2) Debridement

Answer 19

Macrophages secrete mitogenic growth factors for:

PDGF - fibroblast migration and proliferation
TGFb - synthesis of stromal proteins
VEGF - angiogenesis
EGF and TGFa- epith cell migration and proliferation

Question 20

Wound healing/repair:
3) Granulation: Matrix formation

hint: fibroblast > myofibroblast (functions)

Answer 20

PDGF, TGFb, and fibronectin and mechanical tension induce fibroblast migration into clot and differentiate > myofibroblasts.

Myofibroblasts:
- lay down collagen to “plug” the wound
- express SM actin and contractile stress fibres
- focal adhesions link stress fibres to EXM and extracellular fibronectin.
- contraction pulls together edges of wound and accelerates healing
- die by apoptosis at the end of granulation phase.

Question 21

Wound healing/repair:
4) Granulation: blood supply

Answer 21

• Endothelial cells produce new capillaries (angiogenesis) - VEGF.
• Pericytes produced in response to PDGF, surround endothelial cells and stabilise them.
• Vasculogenesis: Endothelial STEM cells from bone marrow generate new blood cells

Question 22

Wound healing/repair:
5) Re-epithelialisation

Answer 22

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Question 23

Wound healing/repair:
6) Remodelling

Answer 23

• Granulation tissue replaced with acellular scar tissue - collagen is deposited.
• MMP breakdown collagen, fibroblasts put down collagen until equilibrium is reached.
• When equilibrium is reached Collagen III is replaced with Collagen I, increases strength by cross-linking.

Question 24

Incorrect repair:

Answer 24

• Can lead to chronic wounds or excessive scarring
• Leads to excessive collagen production or defective remodelling

Question 25

Failed wound healing:

Answer 25

Leads to ischemia, compromised vasculogenic progenitor cell activity, broken ends of bones not being brought back together.Excess

Question 26

Excessive scarring is caused by:

Answer 26

Excessive inflammatory response, excessive production of fibrogenic cytokines, prolonged presence of myofibroblasts.