Session 5 Healing and Repair
Dr Israa Al-Humairi
References
• Robbins Basic Pathology, 10th edition. 2018
• Muir’s Textbook of Pathology. 15th edition. 2014
• Understanding the terms: a-Resolution, b-fibrous repair, c-Regeneration, d-Labile, stable and permanent cells. • Description and discussion of: a- Healing of a clean incised skin wound. b- Healing of large skin defect c- Control mechanism in the above process. d- Structure and function of type 1 collagen e- Factors influencing the efficacy of healing and repair.
Objectives
Describing special aspect of healing and repair in various tissues including:
• Cardiac muscle
• Bone
• Liver
• Peripheral nerve
Objectives
Ist objective
• Understanding the terms:
a-Resolution,
b-fibrous repair,
c-Regeneration,
d-Labile, stable and permanent cells.
a
ACUTE INFLAMMATION,CHRONIC INFLAMMATION,
OR FIBROUS SCARRING?
Acuteinsult
Acuteinflammation
Damageslight?
Yes
Resolutionpossible
No
Chronicinsult
Chronicinflammation
Repairand
SCARRING
Repair of damaged tissue occur by 2 types of reactions:
1- Regeneration: Proliferation of the residual cells and
maturation of tissue stem cells. Normal structure is
restored
2- Connective tissue deposition: Response to injury
involving both regeneration and scar formation (fibrosis).
Normal structure is permanently altered.
Chronic Injury Repair (Regeneration with Fibrosis)
NORMAL
After years of chronic injury: fibrosis & loss of tissue with inadequate regeneration
Fibrosis: interstitial and subpleural
What determines regeneration vs.
repair with fibrosis?
1. What is capacity of injured cells for regeneration?
2. Is extracellular matrix framework damaged or largely intact?
Left: hepatocyte damage with intact matrix: complete regeneration of normal (e.g., after acute hepatitis A)
Right: hepatocyte AND matrix damage: some regeneration with reparative scarring (e.g., cirrhosis)
Cells proliferate during repair
1-Remannts cell of injured organ: to restore normal structure and function . 2- Vascular endothelial cells: create new blood vessels. 3- Fibroblast: source of fibrosis in scar formation
Cell types: capacity for regeneration
Cell type Examples Regenerative capacity
Labile Epithelial surfaces (skin, G.I tract) and hematopoietic cells
Unlimited; characterized by continuous regeneration
Quiescent
(stable)
Most internal organs (liver, kidney, endocrine); mesenchymal cells (fibroblast, smooth muscle, vascular)
Limited, normally not proliferating but in response to stimuli
Permanent CNS neurons; skeletal and cardiac muscle cells
Unable to divide; repaired by replacement with scar
Stem Cells
• Stem cells: normal undifferentiated cells with two features: – Self-renewal – Can generate differentiated
(mature) cells • Critical for regeneration of
cells in self-renewing tissues • Regenerative medicine:
therapeutic applications of stem cells to repair damaged tissues that do not typically regenerate after injury: e.g. heart, brain, skeletal muscle
Adult stem cell: fibroblast in tissue culture
Stem Cells: Origins and Types Adult (somatic) SCs: restricted capacity to generate certain cell types; thus “lineage-committed”
Embryonic or Pluripotent SC: capable of generating all tissue types
Multipotent SC: more restricted than embryonic SC; eventually become “lineage committed”
Niches: microenvironments in which somatic stem cells reside
Liver adult SCs (“oval cells”): reside in canals of Hering (thick arrow); canals carry hepatocyte’s bile secretion to portal bile ductule (thin arrow)
Niche cells dialogue with SC to regulate tissue’s demand for differentiated cells
Stem cells: Therapy
Production of induced pluripotent stem cells (iPS cells)
Factors controlling regeneration
Complex and poorly understood
• Growth factors (EGF,PDGF,FGF)
• Contact with the basement membrane and
adjacent cells (signalling through integrin).
Fibrous repair
Replacement of the injured cells with connective
tissue leading to formation of a scar.
WHY FIBROSIS? Severe or chronic tissue damage
may alter both cells and ECM of organ, such that
repair cannot be accomplished by regeneration
of parenchymal cells only.
FIBROUS REPAIR: The development of a fibrous scar.
–Blood clot forms.
–Acute inflammation around the edges.
–Macrophages infiltrate the clot.
–Capillaries and lymphatics sprout and infiltrate.
–Myofibroblasts infiltrate and differentiate.
–Glycoproteins and COLLAGEN are produced
–Cell population falls, vessels differentiate and are reduced
in number.
–Collagen matures AND CONTRACTS.
Direct observation of fibrous repair.
1) Exudate clots.
2) Neutrophils infiltrate
and digest clot
3) Macrophages and
lymphocytes are recruited
Direct observation of fibrous repair.
4) Vessels sprout, myofibroblasts
make glycoproteins
5) Vascular network; collagen
synthesised; macrophages
reduced
6) Maturity. Cells much reduced;
collagen matures, contracts,
remodels
Angiogenesis: 2 Mechanisms Angiogenesis = formation of new blood vessels after infancy (neovascularization)
Mechanism B: angioblast-like endothelial-precursor cells (EPCs) recruited from bone marrow, homing to site of angiogenesis
Angioblast: embryonic precursor of endothelial cells, pericytes, vascular smooth muscle cells
Mechanism A: sprouting new vessels from pre-existing vessels (1) vasodilation (NO, VEGF) (2) proteolysis of basement membrane by metalloproteinases (3) migration of ECs (4) proliferation of ECs behind the migrating front of cells (5) maturation of ECs (6) recruitment pericytes & smooth muscle cells to support new vessel
Angiogenesis
Fibrosis: 3 sequential phases
Phase 1: Migration & Proliferation of Fibroblasts
– Fibroblasts migrate into injured tissue and replicate
– Proliferation is stimulated by growth factors secreted by macrophages (main source), activated endothelial cells, and platelets: TGF-beta, PDGF, EGF, FGF; cytokines IL-1 and TNF
– TGF-beta is the most effective growth factor promoting fibrosis
• Produced by most cell types in granulation tissue
• Promotes both migration and proliferation of fibroblasts
• Increases synthesis of collagen and fibronectin
• Decreases degradation of ECM by metalloproteinases (stabilizes ECM as it develops into mature fibrosis)
Tissue Repair: 3 sequential phases
Phase 2: Deposition of Extracellular Matrix
– Fibroblasts become less mitotic and more synthetic
– Collagen synthesis begins 3-7 days post-injury and continues for weeks
– Growth factors for collagen synthesis similar to fibroblast proliferation
– Net collagen deposition depends both on increased synthesis and decreased degradation
Phase 3: Maturation and Remodeling
– Balance between ECM synthesis and degradation remodeling of tissue
– Decreasing vascularity and fibroblast proliferation
– Increasing collagen synthesis and cross-linking: fibrosis gradually acquires tensile strength
Histology of Early & Late Repair: balance of angiogenesis & fibrosis
Trichrome histochemical stain (mature collagen stains blue)
Early response (3-7 days): granulation tissue proliferating capillaries & fibroblasts with minimal mature collagen
Late response (> 4 weeks): fibrosis Mature collagen dominates the picture, with decreased vessel density
3 Phases of Wound Healing in Skin
Control of repair Poorly understood
• Angiogenesis: various angiogenic cytokines e.g VEGF, bFGF
• Fibrosis: various profibrotic cytokines, e.g. PDGF, TGF
• Limitation of fibrosis and remodelling: Hardly anything known.
• Description and discussion of:
a- Healing of a clean incised skin wound.
b- Healing of large skin defect
c- Control mechanism in the above process.
d- Structure and function of type 1 collagen
e- Factors influencing the efficacy of healing
and repair.
2nd Objective
Healing of skin wound
1- Healing by first intention or primary union
2- Healing by second intention or secondary union
Wound Healing by Primary Union (first intention)
First intention:
Wound damages few keratinocytes and dermal cells, disrupts short segment of basement membrane.
Example: surgical incision
Result: thin scar
Healing by Secondary Union (Second Intention)
Second intention: wounds that create a large defect Healing requires: --more inflammation --larger volume granulation tissue
--more collagen deposition --wound contraction
Example: deep traumatic abrasion
Result: wide scar, often with skin depression or elevation
Factors influencing wound healing
Systemic Factors Influencing Wound Healing
Factor Effect
Nutrition Profound effect; deficiencies of protein and vitamin C deficiency inhibit collagen synthesis
Metabolic status Diabetes mellitus delays healing (insulin necessary for nucleic acid & protein synthesis)
Circulatory status Inadequate blood supply slows healing; arterial atherosclerosis (limiting the inflow of arterial blood) or venous stasis (limiting outflow)
Steroid hormones Glucocorticoids inhibit wound healing But can beneficial in certain location ex eye
Local Factors Influencing Wound Healing
Factor Effect Infection Persistent inflammation; single most important cause of
delayed healing
Mechanical Early tension applied to wound may separate edges, delaying wound healing
Foreign bodies Fragments of metal, glass, wood, bone: prolong the inflammatory response and inhibit healing
Anatomic location Sites with rich vascularity (e.g., face) heal faster than sites with reduced vascularity (e.g., foot)
Type of wound Sharp incisions (e.g., surgical) heal faster than larger wounds (e.g., traumatic deep abrasion)
Pathologic Wound Repair: Spectrum
• Deficient scar formation due to a) wound dehiscence: separation of wound edges due to
mechanical forces, e.g., vomiting or coughing after abdominal surgical incision
b) wound ulceration: inadequate blood supply (e.g., atherosclerosis)
c) wound necrosis: infection & inadequate blood supply
• Excessive repair:
a) hypertrophic scar or keloid
b) Excessive granulation tissue
c) Contracture formation: deformity of tissues due to excessive or exaggerated wound contraction
Deficient Scar Formation: Chronic Ulceration
Chronic ulcer associated with venous stasis
Chronic ulcer associated with arterial atherosclerosis and compromised inflow
Hypertrophic scar (keloid)
Keloid: excess deposition of abnormally thick bundles of
collagen in dermis
Keloid after ear-piercing
Keloid formation has a genetic predisposition; more common in African-Americans
Wound Contracture, post-burn
After surgical skin graft repairs
Before treatment
Occur in large surface wounds that heal by secondary union
Mechanism: network of myofibroblasts at edges of wound, contracting tissues and producing excess ECM
Application: surgical closure of wounds Goal of suturing wounds: restore normal anatomic relationships to minimize size of the defect that will be filled by granulation tissue and subsequent fibrosis
smaller scar
LEFT: simple suture closing a superficial clean laceration
Right: subcuticular suture approximating edges of dermis in a deep clean laceration
Structure and function of type 1 collagen
Extracellular matrix (ECM)
• Definition
ECM is a network of proteins that constitute
a significant proportion of any tissue.
• Function
1- Mechanical support
2- Control of cell proliferation
3- Scaffolding for tissue renewal
4- Establishment of tissue microenvironment
Extracellular matrix (ECM)
• ECM occurs in 2 basic forms: 1-Interstitial matrix (filling spaces between cells) 2-Basement membrane (closely applied to cell surface)
• ECM components: 1) Fibrous structural proteins (collagen &elastin)
2) Adhesive glycoproteins: connect cells &
matrix (fibronectin, laminin)
3) Gel proteins: resilience and lubrication (proteoglycans, hyaluronan)
Histologic Structure of ECM
1: BM
2: IM
Structure of collagen
Collagens are composed of three separate
polypeptide chains braided into ropelike triple
helix. About 30 collagen types have been
identified.
Major Types of Collagen and Distribution
Collagen Type Tissue Distribution
Fibrillar Collagens
I Ubiquitous in hard and soft tissues
II Cartilage, intervertebral disc, vitreous
III Hollow organs, soft tissues
V Soft tissues, blood vessels
IX Cartilage, vitreous
Basement Membrane Collagens
IV Basement membranes
Function of collagen • Collagen synthesis (by fibroblast) is necessary
for the healing wound to become strong and
mechanically stable.
• Collagen synthesis begins early in the process
of healing (day 3-5) and continue for several
weeks.
• There is a shift from type III collagen early in
repair to the more resilient type I collagen.
aa
Pathways of theReparative Response
Injury
Inflammation
No necrosis Necrosis
Normalstructure
(RESOLUTION)
Regeneration
SCARRING
Exudateresolved
Exudateorganised
Frameworkintact
Frameworkdestroyed
Tissue ofstable or
labile cells
Tissue ofpermanent
cells
Thank you