1. Repair and wound healing
Dr Vishal Saxena
MBBS, MD(Path), FICMR

2. Introduction Inflammation sets off repair.
Repair involves two processes:
Regeneration: replacement of dead cells
by proliferation of cells of the same type.
2. Replacement by connective tissue
(fibrous tissue) or fibrosis.

3. Regeneration Replacement of injured cells by cells of the same type.
Depends on the ability of cells to replicate.
Labile cells (e.g. stem cells in epidermis ) and stable cells (e.g. hepatocytes) are able to replicate.
Permanent cells are unable to replicate:
Cardiac and striated muscles are replaced by scar tissue (fibrosis)
Neurons cannot be replaced.

4. Labile cells Divide actively during life to replace lost cells.
Are capable of regeneration after injury.
Include cells of the epidermis and gastrointestinal mucosa, cells lining the surface of the genitourinary tract and hematopoietic cells of bone marrow.

5. Stable cells
Characteristically undergo few divisions but are capable of division when activated.
Are capable of regeneration following injury.
Include hepatocytes, renal tubular cells, parenchymal cells of many organs and numerous mesenchymal cells (e.g. smooth muscle, cartilage, connective tissue & endothelium).

6. Permanent cells Are incapable of division and regeneration.
Include neurons and myocardial cells.
Are replaced by scar tissue (typically fibrosis ; gliosis in CNS) after irreversible cell injury and cell loss.

7. Regeneration requirements:
Preservation of the basement membrane (BM) and a relatively intact connective tissue infrastructure.
e.g. first degree burn
Factors that stimulate parenchymal cell regeneration (e.g. growth factors & hormones).

8. Repair by connective tissue
Replacement of damaged area by a connective tissue (fibrous tissue) scar.
Occurs if :
Tissue architecture is destroyed
BM is not intact (e.g. third degree burn)
Requires neutrophil transmigration
To liquefy injured tissue and then
Macrophage transmigration to remove the debris.
Depends on the formation of granulation tissue.

9. Granulation tissue : microscopy
Circle: Fibroblasts
Arrows: new blood vessels
Granulation tissue : microscopy

10. Growth factors in wound healing
Monocyte chemotaxis : PDGF, FGF, TGF - 
Fibroblast migration : PDGF, EGF,FGF, TGF- , IL-5
Fibroblast proliferation : PDGF, EGF,FGF, TNF
Angiogenesis : VEGF, BFGF. Endostatin inhibits.
Collagen synthesis : TGF - , PDGF
Collagen secretion : PDGF, FGF, EGF,TNF, TGF-  inhibits

11. Repair by Fibrosis
Healing : is a fibroproliferative response which patches up the damaged tissue.
It involves :
Inflammatory response, with removal of dead tissue.
Proliferation & migration of parenchymal & connective tissue cells.
Angiogenesis & formation of granulation tissue.
ECM synthesis & collagen deposition.
Tissue remodeling & wound contraction.
Acquisition of wound strength.

12. Angiogenesis from preexisting blood vessels

13. Steps in the angiogenesis from preexisting blood vessels
Vasodilatation: NO; VEGF ( > vascular permeability)
Proteolytic degradation of BM: metalloproteinases
Disruption of cell- cell contact of endothelial cells: plasminogen activator
Migration of endothelial cell towards angiogenic stimulus

14. Proliferation of the endothelial cells
Maturation of endothelial cells; remodeling into capillary tubes
Recruitment of periendothelial cells (pericytes & smooth muscle cells) for support & formation of mature vessel

15. Granulation tissue
A specialized type of newly formed connective tissue.
Pink, soft and has granular appearance.
Is Highly vascular.
Composed of newly formed blood vessels (angiogenesis) and fibroblasts.
Accumulates in the ECM and eventually produces dense fibrotic scar.

16. Angiogenesis
Critical for wound healing, tumor growth and development of collateral circulation.
Vasculogenesis
primitive blood vessel formation during embryonic development from angioblasts.

17. Angiogenesis or neovascularization
Formation of new blood vessels from preexisting vessels.
Enhanced by
Fibronectin and
Basic fibroblast growth factor
(BFGF)
Vascular endothelial growth factor
(VEGF)

18. Collagen: Repair by connective tissue (fibrosis) also requires:
Initial production of type III collagen.
Collagen:
Is a triple helix of cross linked alpha chains.
Cross linking increases the tensile strength of collagen.
Type I collagen in skin, bone and tendons has greater tensile strength than type III collagen in the early phase of tissue repair.

19. Vitamin C deficiency is associated with poor wound healing.
Enzyme: Lysyl oxidase necessary for cross linking of collagen fibrils. Copper is a cofactor of lysyl oxidase. Deficiency of this metal results in poor wound healing.
Hydroxylation of proline and lysine (necessary for collagen synthesis) requires presence of vitamin C.
Vitamin C deficiency is associated with poor wound healing.
Remember: type IV collagen is found in basement membrane

20. Dense scar tissue produced from granulation tissue must be remodeled.
Remodeling increases the tensile strength of scar tissue.
Metalloproteinases (collagenases) replace type III collagen with type I collagen, increasing tensile strength to approx. 80% of the original.

21. Examples of wound healing
Cutaneous Wound healing

22. first intention (primary union) or second intention (secondary union)
Skin wounds heal by
first intention (primary union) or
second intention (secondary union)

23. Healing by first intention/ primary union
Healing of a clean, uninfected, surgical wound approximated by sutures.
Incisional space filled with clotted blood & fibrin.
Dehydration of surface clot – scab.

24. Within 24 hours
Neutrophils at the margins move along the cut margins of the dermis
Deposition of BM
Fuse in the midline beneath the scab
Continuous thin epithelial layer that closes the wound

25. Day 3 :
Neutrophils replaced by macrophages
Granulation tissue fills incision space
Vertical orientation of collagen fibers
Thickening of epidermal layer

26. Day 5 :
Complete filling of the incision space by granulation tissue
Maximum neovascularisation
Abundant collagen; horizontal orientation
Epidermal maturation

27. 2nd week
Proliferation of fibroblasts
Continuous accumulation of collagen
Disappearance of inflammation & edema
By 4 weeks scar is made of cellular connective tissue covered by intact epidermis
Fibrous union

28. Healing by second intention
When the loss of cells and tissue is extensive
Regeneration of parenchymal cells is not complete
Differences from the primary healing
Large tissue defect; intense inflammatory reaction
Large amount of granulation tissue
Wound contraction: permanent contraction by myofibroblasts
Substantial scar formation & thinning of epidermis

31. Wound Strength Skin wounds 1 week old; 10% strength of unwounded skin
rapid increase in tensile strength as scar tissue accumulates over 2 months
Completely healed; 70-80% of unwounded skin strength
Scar tissue is never as strong as the original tissue !!
Wound contraction is caused by myofibroblasts.

32. Factors that influence healing
Systemic
Local

33. Systemic Factors that Delay/Retard Wound Healing
Nutrition
Protein deficiency, Vitamin C deficiency
inhibit collagen synthesis
Cu deficiency (cofactor in lysyl oxidase)
Metabolic status
diabetes mellitus:
Susceptibility to infection caused by impaired circulation and increased glucose.
Circulatory status
inadequate blood supply
atherosclerosis, vascular defects
Hormones
glucocorticoids inhibit collagen synthesis, decrease inflammation
Vitamin C:
responsible for hydroxylation of proline and lysine. Deficiency of vitamin C (scurvy) produces structurally weak collagen  poor wound healing.
Copper:
cofactor on lysyl oxidase which is responsible for cross linking of collagen fibrils. Deficiency of this element responsible for poor wound healing and predisposes to dissecting aortic aneurysm (since elastic tissue is also weak)
Glucocorticoids :
-occasionally used with antibiotics to prevent scar tissue formation (e.g. bacterial meningitis).
-inhibit wound healing by impairing collagen synthesis.

34. Local Factors that Delay/Retard Wound Healing
Infection
most important cause of delayed wounds healing
Persistent injury and inflammation
Staphylococcus MC pathogen
Mechanical factors
motion early in healing
Foreign material
Size, location & type of wound
wounds in ↑ vascularized areas (face) heal faster than in poorly vascularised areas (tendon, feet)
small wounds heal faster than larger
incisions heal faster than blunt trauma (contusions)
Most common infection that delays wound healing is by Staphylococcus aureus.

35. Complications of wound healing
Deficient scar formation
Excessive formation of repair components
Exaggerated contraction

36. Deficient scar formation
Can lead to two types of complications:
A. Wound Dehiscence (rupture of wound)
most common after abdominal surgery
coughing, vomiting,
B. Ulceration
inadequate vascularization

37. Abdomen, poor wound healing, dehisced surgical wound - Clinical presentation  Wound dehiscence is the disruption of apposed surfaces of a wound (most often abdominal), due to inadequate healing. Many factors, such as infection, inadequate blood supply, diabetes, and mechanical stress can impair healing.    

38. Excessive formation of repair components
1. Keloid
2. Exuberant granulation or proud flesh

39. Keloid
Raised scar (tumor like mass) due to excessive synthesis of type III collagen.
Genetic predisposition.
more common in African Americans
Tends to affect the earlobes, face, neck, sternum etc.
Micro: irregular thick collagen bundles extending beyond the original injury.

40. Head and neck, keloids - Clinical presentation  Keloids, nodular masses of hyperplastic scar tissue, occur when the wound healing process runs unchecked. They are more common in people of African descent. Surgical excision typically leads to repeated keloid formation.

41. Keloid gross and microscopy

42. Exuberant granulation (proud flesh)
Excessive granulation tissue
Protrudes above surrounding skin
Prevents re-epithelization

43. Contracture
Exaggerated contraction
Deformation of surrounding tissue or wound
Can compromise the movement of joints.
most common
palms, soles, anterior thorax following severe burns

45. Win 2009