• Angiogenesis means formation of new blood vessels from existing vessels.
• It is important for:
  • Healing at sites of injury
  • Formation of collateral circulation in ischemic tissues
  • Tumor growth
• Angiogenesis may be therapeutically:
  • Increased to improve blood flow in ischemic heart disease
  • Inhibited to reduce tumor growth or abnormal vessels, such as in wet macular degeneration
• New vessels develop by sprouting from existing vessels.
• Many factors regulate angiogenesis, but the most important is vascular endothelial growth factor (VEGF).
• VEGF → endothelial cell migration + proliferation
• At an injured site:Hypoxia → ↑ HIF → ↑ VEGF production
• HIF (hypoxia-inducible factor) is a transcription factor that increases VEGF production during hypoxia.
• VEGF causes nearby intact vessels to:
  • Dilate
  • Become more permeable
• Matrix metalloproteinases digest the basement membrane, allowing a new vascular sprout to form.
• Endothelial cells at the tip of the sprout migrate toward the injured tissue.
• Endothelial cells behind the tip proliferate and organize into vascular tubes.
• Pericytes are recruited around capillaries, while smooth muscle cells are recruited around larger vessels, producing a mature blood vessel.
• As healing continues, many newly formed vessels regress.
• Therefore, highly vascular granulation tissue gradually becomes a pale, largely avascular scar.

KEY CONCEPT

Injury → hypoxia → HIF ↑ → VEGF ↑ → endothelial migration + proliferation → new vascular sprouts → vascular tubes → mature vessels

Granulation tissue → vessel regression → pale scar

CONCEPTUAL EXAMPLES

• Injured tissue becomes hypoxic → VEGF increases → new vessels grow toward the wound
• Early granulation tissue → many vessels → pink/red appearance → healing progresses → vessels decrease → pale scar

Activation of Fibroblasts and Deposition of Connective Tissue

• Connective tissue deposition occurs in two main steps (Fig. 2.22):
  1. Fibroblasts migrate into the injured area and proliferate
  2. Fibroblasts produce and deposit ECM proteins
• These processes are stimulated by locally produced cytokines and growth factors, especially:
  • PDGF
  • FGF-2
  • TGF-β
• Their major sources are inflammatory cells, especially macrophages at the injury site.
• Scar tissue mainly consists of connective tissue.
• Connective tissue contains:
  • Fibroblasts
  • Extracellular matrix (ECM)
• ECM mainly contains collagen and other glycoproteins.
• Cell–ECM interactions are essential for healing, normal tissue development, and maintenance of tissue architecture (Fig. 2.23).
• Major functions of ECM include:
  • Providing mechanical support for cell attachment and migration
  • Maintaining cell polarity
  • Regulating cell proliferation
  • Binding and storing growth factors
  • Providing a scaffold for tissue regeneration
• An intact basement membrane or stromal scaffold is therefore essential for normal tissue regeneration.
• ECM exists in two major forms:
  • Interstitial matrix → found between connective tissue cells and between epithelium and underlying supporting tissues.
  • It contains mainly collagens, fibronectin, elastin, proteoglycans, and hyaluronate.
  • Basement membrane → highly organized ECM surrounding epithelial, endothelial, and smooth muscle cells.
  • Its major components are type IV collagen and laminin.

FIG. 2.22 — How Persistent Injury Causes Fibrosis

Think of the whole figure as one simple chain:

Persistent injury → chronic inflammation → fibroblasts activated → collagen/ECM deposited → SCAR / FIBROSIS

🔝 1. Persistent tissue injury

🟧 Peach cells = tissue cells.
Some are damaged and breaking apart.

⬇️ Yellow arrow = persistent damage keeps stimulating inflammation.

🔥 2. Inflammation

Two important inflammatory cells appear:

• 🩷 Pink round cells = T lymphocytes
• 🟠 Large orange cell = macrophage

These cells release signals that tell the tissue to start repair.

🩷 T lymphocytes → Cytokines

⬇️ Yellow arrow

T cells release cytokines, e.g. IL-13.

IL-13 → attracts/activates fibroblasts → promotes fibrosis.

🟠 Macrophage → TGF-β + MMPs

⬇️ Yellow arrow

TGF-β = MAIN fibrosis signal.

It tells fibroblasts:

“Multiply and make collagen.”

MMPs = enzymes that break down/remodel old extracellular matrix, allowing the scar tissue to be reorganized.

🟧 Orange arrow — Fibroblast recruitment & differentiation

Fibroblasts are brought into the damaged area and become activated.

➡️ Some become myofibroblasts.

Myofibroblast = fibroblast that makes lots of collagen + can contract the wound.

🟤 Beige spindle-shaped cells

= Fibroblasts / myofibroblasts

They produce the material needed for the scar.

🔵 Blue wavy lines

= Extracellular matrix (ECM), especially collagen

More fibroblast activity
→ more collagen/ECM
→ normal tissue gets replaced by fibrous tissue.

⬇️ Final yellow arrow

➡️ SCARRING / FIBROSIS

Fibrosis = excessive connective tissue/collagen replacing normal tissue.

🧠 Fastest exam memory

Persistent injury → Macrophage → TGF-β → Fibroblast → Collagen → FIBROSIS

⭐ Most important mediator of fibrosis = TGF-β.

Collagen

• Collagen is the major ECM protein of the interstitial matrix and scar tissue.
• More than 30 collagen types exist, and each collagen molecule contains three polypeptide chains arranged in a triple helix.
• Important fibrillar collagens are types I, II, III, and V.
• They are important in scar tissue, tendons, bone, and skin.
• Collagen gains tensile strength by cross-linking of its triple helices.
• This cross-linking requires vitamin C as a cofactor.
• Therefore:Vitamin C deficiency → defective collagen → poor wound healing + easy bleeding

Elastin

• Elastin allows tissues to stretch and return to their original shape.
• It is especially important in:
  • Cardiac valves
  • Large blood vessels
  • Uterus
  • Skin
  • Ligaments

Proteoglycans and Hyaluronan

• Proteoglycans form highly hydrated gels that resist compressive forces.
• In joint cartilage, they also provide lubrication between bony surfaces.
• They can store growth factors such as FGF and HGF.
• Some proteoglycans also help regulate cell proliferation, migration, and adhesion.

Adhesive Glycoproteins and Adhesion Receptors

• These molecules help interactions between:
  • Cell ↔ cell
  • Cell ↔ ECM
  • ECM ↔ ECM
• Two important adhesive glycoproteins are:
  • Fibronectin → major component of interstitial ECM
  • Laminin → major component of basement membrane
• Fibronectin is produced by fibroblasts, monocytes, endothelial cells, and other cells.
• In healing wounds, fibronectin provides a scaffold for:
  • Further ECM deposition
  • Angiogenesis
  • Reepithelialization
• Laminin is the most abundant glycoprotein of the basement membrane.
• It helps in:
  • Cell attachment
  • Cell proliferation
  • Differentiation
  • Cell movement
• Cells attach to laminin and fibronectin through integrins (see Table 2.3).

• Integrins link the intracellular cytoskeleton to the ECM and can also mediate cell–cell adhesion.
• Integrin signaling influences:
  • Cell migration
  • Proliferation
  • Shape
  • Differentiation
• During scar formation, fibroblasts migrate from the wound edges toward the center in response to growth factors.
• Some fibroblasts become myofibroblasts.
• Myofibroblasts contain smooth muscle actin and contract strongly.
• Therefore:Myofibroblast contraction → wound margins pulled toward center → wound closes
• Activated fibroblasts and myofibroblasts produce connective tissue proteins, especially collagen and other ECM proteins.
• TGF-β is the most important cytokine for connective tissue synthesis and deposition.
• TGF-β is produced by many cells in granulation tissue, including activated macrophages.
• TGF-β causes:
  • ↑ Fibroblast migration
  • ↑ Fibroblast proliferation
  • ↑ Collagen synthesis
  • ↑ Fibronectin synthesis
  • ↓ ECM degradation by inhibiting metalloproteinases
• Therefore:TGF-β → more ECM production + less ECM breakdown → scar formation
• TGF-β also contributes to fibrosis of the lung, liver, and kidneys during chronic inflammation.
• It also has an anti-inflammatory effect by inhibiting lymphocyte proliferation and reducing the activity of other leukocytes.
• As healing progresses, fibroblasts produce progressively more ECM.
• Collagen synthesis begins early, around days 3–5, and may continue for several weeks.
• Increasing collagen strengthens the healing wound.
• Final collagen accumulation depends on:
  • Increased collagen synthesis
  • Decreased collagen degradation

KEY CONCEPT

Macrophages → PDGF + FGF-2 + TGF-β → fibroblast migration/proliferation → collagen + ECM deposition → scar

TGF-β = major cytokine for fibrosis and scar formation

Fibroblast → myofibroblast → contraction → wound edges pulled together

Vitamin C → collagen cross-linking → strong wound

CONCEPTUAL EXAMPLES

• Macrophages enter wound → release growth factors → fibroblasts enter → collagen deposited → scar forms
• Myofibroblasts contract → wound edges move toward each other → wound becomes smaller
• Vitamin C deficiency → weak collagen → poor wound healing
• Chronic inflammation → persistent TGF-β activity → excess collagen deposition → fibrosis

FIG. 2.23 — Extracellular Matrix (ECM)

Easiest concept: ECM = the supporting “scaffold” around cells

The whole image shows 2 types of ECM:

1. Basement membrane = thin sheet directly under cells
2. Interstitial matrix = loose 3-D network between cells

🔝 1️⃣ Top blue cells = EPITHELIUM

🔵 Blue rectangular cells = epithelial cells
🟣 Oval inside = nucleus

They sit on the basement membrane.

🟣 Small purple structures under the cells = INTEGRINS

Think:

Cell → integrin → ECM

Integrins are cell-surface attachment proteins.

They:

• anchor the cell to ECM
• also transmit signals between ECM ↔ cell

🟥 2️⃣ BASEMENT MEMBRANE

The thin red/pink horizontal layer directly below the epithelium represents the basement membrane.

Main components:

🔵 Type IV collagen

Forms a sheet-like supporting network.

🟣 Laminin

Acts like a connecting/adhesive protein.

It helps connect:

integrins on cells ↔ basement membrane

🟢 Proteoglycans

Highly hydrated molecules that:

• fill space
• bind water
• help filtration/support

🧠 Basement membrane memory:

“4-L-P”

Type IV collagen + Laminin + Proteoglycan

🔍 LEFT CIRCLE = Basement membrane magnified

This is a zoomed-in view.

• 🔵 Wavy network = Type IV collagen
• 🟣 connecting structures = laminin
• 🟢 branched structures = proteoglycans

Together they make a thin but strong supporting sheet.

Simple idea:

Epithelial cell
⬇️ integrin
Laminin
⬇️
Type IV collagen network + proteoglycan

🩸 3️⃣ CENTRAL RED TUBE = CAPILLARY

🔴 Inside = blood
🔴 Red round cells = red blood cells

The vessel wall is lined by:

⚪ Endothelial cells

Even endothelial cells have their own basement membrane outside them.

So basement membrane is found under:

• epithelium
• endothelium

🟧 4️⃣ Orange spindle-shaped cells = FIBROBLASTS

Fibroblasts are the main ECM-producing cells of connective tissue.

Think:

Fibroblast = ECM factory

They produce:

• collagen
• elastin
• proteoglycans
• adhesive glycoproteins

🕸️ 5️⃣ INTERSTITIAL MATRIX

The loose fibers filling the space around fibroblasts and vessels are the interstitial ECM.

It contains:

🧵 Fibrillar collagens

Give tensile strength.

🌀 Elastin

Allows stretch + recoil.

🌿 Proteoglycans + hyaluronan

Bind water and create a hydrated gel around cells.

🔍 RIGHT CIRCLE = Interstitial matrix magnified

This circle shows how a fibroblast attaches to the ECM.

🟧 Fibroblast

The orange cell.

🟣 Integrins

Integrins on the fibroblast membrane connect the cell to ECM.

🔗 Adhesive glycoproteins

Act like bridges between:

Integrin ↔ ECM proteins

🧵 Cross-linked collagen triple helices

Collagen molecules are organized and cross-linked.

This gives tissue strength.

🌿 Proteoglycans

Fill spaces, hold water and support the collagen network.

⭐ MAIN DIFFERENCE

Basement membrane	Interstitial matrix
Thin sheet under cells	Network between cells
Type IV collagen	Fibrillar collagen
Laminin	Elastin
Proteoglycans	Proteoglycans + hyaluronan

🧠 Whole figure in ONE FLOW

Cells
→ attach through integrins
→ to ECM

ECM has:

Basement membrane
→ Type IV collagen + laminin + proteoglycan

OR

Interstitial matrix
→ fibrillar collagen + elastin + proteoglycan/hyaluronan

🎯 Most important exam points

Basement membrane → Type IV collagen + laminin

Interstitial ECM → fibrillar collagen + elastin

Fibroblast → main producer of connective-tissue ECM

Integrins → connect cells to ECM and transmit signals.

Remodeling of Connective Tissue

• After a scar forms, continued remodeling makes it stronger and smaller.
• Scar strength increases because:
  • Collagen fibers become cross-linked
  • Collagen fibers become larger
  • Early type III collagen is gradually replaced by stronger type I collagen
• In a well-sutured skin wound, tensile strength reaches about 70–80% of normal skin by 3 months.
• Over time, the scar becomes smaller because matrix metalloproteinases (MMPs) break down ECM components.
• MMPs require metal ions such as zinc for their activity.
• MMPs are produced by several cells, including:
  • Fibroblasts
  • Macrophages
  • Neutrophils
  • Synovial cells
  • Some epithelial cells
• Their production is controlled by growth factors, cytokines, and other signals.
• Important MMP groups include:
  • Interstitial collagenases → break down fibrillar collagen
  • Gelatinases → degrade amorphous collagen and fibronectin
  • Stromelysins → degrade several ECM components, including proteoglycans, laminin, fibronectin, and amorphous collagen
• TIMPs (tissue inhibitors of metalloproteinases) inhibit MMP activity.
• Therefore, the balance between MMPs and TIMPs determines the final size and composition of the scar.

KEY CONCEPT

Scar formed → collagen cross-linking + type III → type I collagen → scar becomes stronger

MMPs → break down ECM → scar becomes smaller

TIMPs → inhibit MMPs

MMP–TIMP balance → determines final scar size and structure

CONCEPTUAL EXAMPLES

• Early wound → more type III collagen → remodeling → stronger type I collagen
• Well-sutured skin wound → about 70–80% normal strength by 3 months
• More MMP activity → more ECM breakdown → smaller scar
• More TIMP activity → less MMP action → less ECM breakdown

MORPHOLOGY

• Granulation tissue is made of:
  • Proliferating fibroblasts
  • Many thin-walled, delicate capillaries
  • Loose extracellular matrix (ECM)
  • Inflammatory cells, mainly macrophages (Fig. 2.24A)
• Granulation tissue is gradually deposited at the site of injury.
• The amount formed depends mainly on:
  • Size of the tissue defect
  • Intensity of inflammation
• A scar or fibrosis is composed of:
  • Inactive, spindle-shaped fibroblasts
  • Dense bundles of collagen
  • Other ECM components (Fig. 2.24B)
• Special stains can identify different components of scars and fibrotic tissue.
• Trichrome stain detects collagen fibers.
  • Red blood cells → orange
  • Muscle → red
  • Collagen → blue
• Elastin stain identifies delicate elastin fibers, which are the main component of pliable elastic tissue.
• Reticulin is another ECM protein present in:
  • Connective tissue stroma of normal organs
  • Early scars
• Reticulin is composed of type III collagen and can also be identified by a special stain.

KEY CONCEPT

Granulation tissue = fibroblasts + new capillaries + loose ECM + macrophages

Mature scar/fibrosis = inactive fibroblasts + dense collagen + ECM

Early repair → granulation tissue → increasing collagen → mature fibrous scar

CONCEPTUAL EXAMPLES

• Healing wound → many delicate new vessels + fibroblasts → granulation tissue
• Older healed wound → fewer active cells + dense collagen → scar
• Trichrome stain → collagen appears blue
• Early scar → reticulin/type III collagen is present

Factors That Interfere With Tissue Repair

• Factors that delay healing may be external or internal, and may act systemically or locally.
• Infection is one of the most important causes of delayed healing.
  • It prolongs inflammation
  • It may increase local tissue damage
• Diabetes is an important systemic cause of poor wound healing because it interferes with tissue repair in several ways.
• Poor nutrition delays healing.
  • Protein deficiency → reduced repair
  • Vitamin C deficiency → reduced collagen synthesis → delayed healing
• Glucocorticoids (steroids) suppress inflammation and inhibit TGF-β, which normally promotes collagen deposition.
• Therefore, after surgery: Glucocorticoids → ↓ TGF-β → ↓ collagen deposition → poor wound healing
• However, in some corneal infections, glucocorticoids may be given with antibiotics to reduce excessive collagen deposition and help prevent loss of vision.
• Mechanical stress, such as excessive pressure, twisting, or movement, may cause wound edges to pull apart.
• Poor blood perfusion also delays healing and may result from:
  • Arteriosclerosis
  • Diabetes
  • Poor venous drainage, such as in varicose veins
• Foreign bodies, such as:
  • Steel fragments
  • Glass
  • Bone fragments
    can interfere with normal healing.

KEY CONCEPT

Delayed wound healing = infection + diabetes + poor nutrition + steroids + mechanical stress + poor perfusion + foreign bodies

Vitamin C deficiency → ↓ collagen synthesis → poor healing

Steroids → ↓ TGF-β → ↓ collagen deposition → delayed healing

CONCEPTUAL EXAMPLES

• Infected wound → inflammation continues → healing becomes slow
• Vitamin C deficiency → weak collagen formation → delayed wound repair
• Poor blood flow → less support to injured tissue → poor healing
• Excess movement at wound → wound edges separate → healing is delayed

Clinical Examples of Abnormal Wound Healing and Scarring

• Abnormal tissue repair may result from:
  • Too little scar formation
  • Excessive repair tissue deposition
  • Contractures

Defects in Healing: Chronic Wounds

• Chronic wounds usually develop when local or systemic factors continuously interfere with healing.
• Venous leg ulcers (Fig. 2.25A):
  • Common in elderly people
  • Caused by chronic venous hypertension
  • May result from severe varicose veins or congestive heart failure
  • Poor venous circulation leads to reduced oxygen delivery → poor healing
• Arterial ulcers (Fig. 2.25B):
  • Occur with atherosclerosis of peripheral arteries
  • Especially associated with diabetes
  • Reduced arterial blood flow causes ischemia → poor repair + tissue necrosis
  • These ulcers are often painful
• Diabetic ulcers (Fig. 2.25C):
  • Mainly affect the lower limbs, especially the feet
  • Poor healing results from:
    • Small-vessel disease → ischemia
    • Neuropathy
    • Secondary infection
  • Microscopically, they show epidermal ulceration (Fig. 2.25E) with extensive granulation tissue in the dermis (Fig. 2.25F).
• Pressure sores (Fig. 2.25D):
  • Caused by prolonged compression of tissue against bone
  • Common in bedridden patients
  • Continuous pressure causes local ischemia → skin ulceration + underlying tissue necrosis
• Severe failure of healing may cause wound dehiscence, meaning the wound pulls apart or ruptures.
• Dehiscence occurs most commonly after abdominal surgery when vomiting, coughing, or ileus creates increased mechanical stress on the wound.

KEY CONCEPT

Poor blood flow / ischemia + neuropathy + infection + mechanical pressure → chronic wound → failure of normal healing

Venous ulcer → venous hypertension

Arterial ulcer → arterial ischemia

Diabetic ulcer → ischemia + neuropathy + infection

Pressure sore → prolonged pressure + ischemia

CONCEPTUAL EXAMPLES

• Severe varicose veins → venous hypertension → poor oxygen delivery → venous ulcer
• Peripheral atherosclerosis → ischemia → painful arterial ulcer
• Diabetic foot → ischemia + neuropathy + infection → chronic ulcer
• Bedridden patient → prolonged pressure over bone → ischemia → pressure sore
• Abdominal surgery + severe coughing/vomiting → mechanical stress → wound dehiscence

Excessive Scarring

• Excessive repair tissue formation can produce hypertrophic scars and keloids.
• Hypertrophic scars:
  • Contain many myofibroblasts
  • Often grow rapidly
  • Usually regress over several months
  • Commonly develop after deep thermal or traumatic injury of the dermis
• A keloid forms when scar tissue:
  • Grows beyond the original wound boundaries
  • Does not regress (Fig. 2.26)
• Keloid formation appears to depend partly on individual predisposition.
• Normal wound healing includes wound contraction, which reduces wound size.
• Excessive contraction produces a contracture, causing deformity of the wound and surrounding tissues.
• Contractures commonly develop on:
  • Palms
  • Soles
  • Anterior chest
• They are especially common after severe burns and may restrict joint movement.

KEY CONCEPT

Hypertrophic scar → excessive scar within wound → may regress

Keloid → scar extends beyond wound → does not regress

Excessive wound contraction → contracture → deformity + limited movement

CONCEPTUAL EXAMPLES

• Deep burn → excessive scar tissue within wound → hypertrophic scar
• Scar grows beyond original wound → keloid
• Severe burn near joint → excessive contraction → contracture → reduced joint movement

Fibrosis in Parenchymal Organs

• Fibrosis means excessive deposition of collagen and other ECM components in tissues.
• Although scar and fibrosis are often used interchangeably, fibrosis usually refers to abnormal collagen deposition in internal organs during chronic disease.
• The basic mechanism of fibrosis is similar to scar formation in skin.
• Fibrosis is caused by persistent injury, especially:
  • Chronic infections
  • Chronic immune reactions
• Persistent injury → prolonged repair signals → excessive ECM and collagen deposition → organ fibrosis
• Fibrosis may cause severe organ dysfunction and even organ failure.
• The most important cytokine promoting fibrosis is TGF-β.
• IL-13, produced by T lymphocytes, may also contribute (see Fig. 2.22).
• Important fibrotic disorders include:
  • Liver cirrhosis
  • Systemic sclerosis (scleroderma)
  • Idiopathic pulmonary fibrosis
  • Pneumoconioses
  • Drug- and radiation-induced pulmonary fibrosis
  • End-stage kidney disease
  • Constrictive pericarditis
• Because fibrosis can severely impair organ function, there is major interest in developing antifibrotic drugs.

KEY CONCEPT

Persistent chronic injury → TGF-β ↑ → fibroblast activation → collagen + ECM deposition → fibrosis → organ dysfunction

CONCEPTUAL EXAMPLES

• Chronic liver injury → excess collagen → cirrhosis
• Chronic lung injury → fibrosis → reduced lung function
• Chronic kidney damage → fibrosis → end-stage kidney disease