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