• Repair (healing) means restoration of the normal structure and function of tissue after injury.
• Inflammation removes microbes and damaged tissue and also starts the process of tissue repair.
• Damaged tissues are repaired by two main processes: regeneration and scar formation (Fig. 2.20).
• Regeneration means damaged tissue is replaced by new cells, allowing the tissue to return almost to its normal state.
• Regeneration occurs when surviving cells multiply and are able to produce the mature cells of that tissue.
• These regenerating cells may be:
  • Mature differentiated cells, or
  • More commonly, tissue stem cells.
• Scar formation occurs when complete regeneration is not possible or when the supporting tissue structure is damaged.
• In this process, damaged tissue is replaced by connective (fibrous) tissue, producing a scar.
• The fibrous scar provides enough structural strength and stability for the injured tissue to continue functioning.
• When excessive scarring due to chronic inflammation occurs in organs such as the lungs, liver, or kidneys, it is called fibrosis.
• In many common injuries, both regeneration and scar formation occur together, but their contribution varies depending on the type and severity of injury.

KEY CONCEPT

Tissue injury → inflammation → repair

Repair occurs by:

Regeneration → damaged cells replaced → near-normal tissue

OR

Scar formation → fibrous connective tissue deposited → structural stability

• Many injuries heal by a combination of both processes.

CONCEPTUAL EXAMPLES

• If damaged cells can multiply and the tissue framework is preserved → regeneration is more likely.
• If tissue damage is severe or its supporting structure is destroyed → scar formation is more likely.
• Long-term scarring in organs such as the liver, lungs, or kidneys → fibrosis.

FIG. 2.20 — Tissue Repair: Regeneration vs Scar Formation

1️⃣ Top: HEALTHY

• 🩷 Peach/pink cells = normal surface epithelium
• 🔵 Blue-purple dots = nuclei
• 🟣 Dark purple line underneath = supporting basement membrane / tissue framework
• 🌸 Light pink area below = connective tissue

Everything is intact and normally organized.

🟨 Yellow branching arrow = “What happens after injury?”

It gives 2 possible pathways:

2️⃣ LEFT — Mild, superficial injury

Only the surface epithelial cells are damaged.

✅ Most important point: the underlying connective-tissue framework remains intact.

Mild injury → framework preserved → surviving cells multiply → wound closes

⬇️ Yellow arrow

3️⃣ REGENERATION

New epithelial cells replace the lost cells.

➡️ The tissue becomes almost exactly like normal again.

Regeneration = damaged cells replaced by the SAME type of cells.

4️⃣ RIGHT — Severe injury

The injury goes deep.

🔴 Dark red area = severely damaged/necrotic tissue + wound/clot
🟣/🔵 cells in the wound = inflammatory cells coming to clean the damage.

Now both are damaged:

Epithelium + underlying connective-tissue framework

So simple regeneration is not enough.

⬇️ Yellow arrow

5️⃣ SCAR FORMATION

🟡/cream wavy fibers = newly deposited collagen/fibrous connective tissue.

Severe injury → framework destroyed → collagen deposited → scar

The wound becomes strong, but the original architecture is not perfectly restored.

🧠 Easiest memory

SUPERFICIAL = SAME again → REGENERATION

SEVERE = SCAR

⭐ Whole figure in ONE line

Mild injury + intact framework → REGENERATION → normal tissue
Severe injury + destroyed framework → COLLAGEN → SCAR

🎯 Key exam point

The main deciding factor is whether the connective-tissue framework is intact or destroyed.

Cell and Tissue Regeneration

• The ability of a tissue to repair itself depends partly on its natural capacity for cell proliferation.
• In some tissues, cells are continuously lost and replaced by new cells derived from:
  • Tissue stem cells
  • Remaining mature tissue cells
• Examples include:
  • Hematopoietic cells in bone marrow
  • Basal cells of skin squamous epithelium
  • Columnar epithelium of the gastrointestinal tract
• These tissues can regenerate easily after injury if their stem cell pool remains preserved.
• Other tissues contain cells that normally remain in the G0 stage of the cell cycle, meaning they are usually not dividing.
• However, these cells can start dividing after injury or loss of tissue mass.
• Examples include the parenchymal cells of most solid organs such as:
  • Liver
  • Kidney
  • Pancreas
• Endothelial cells, fibroblasts, and smooth muscle cells are also normally inactive but can proliferate when stimulated by growth factors.
• Their proliferation is especially important in wound healing.
• Some tissues contain terminally differentiated cells that cannot divide, including most:
  • Neurons
  • Cardiac muscle cells
• Injury to these cells is usually irreversible, so healing mainly occurs by scar formation rather than regeneration.

KEY CONCEPT

• Continuously dividing tissues → regenerate easily
• G0/quiescent tissues → can divide after injury
• Terminally differentiated tissues → cannot regenerate → scar formation

CONCEPTUAL EXAMPLES

• Skin or intestinal epithelium injured → stem cells survive → rapid regeneration
• Liver tissue lost → resting cells enter the cell cycle → proliferation occurs
• Cardiac muscle or neurons severely injured → cells cannot divide → scar forms

Cell and Tissue Regeneration

• Cell proliferation is stimulated mainly by signals from:
  • Growth factors
  • Extracellular matrix (ECM)
• Different growth factors may act on many cell types or only on specific cells (Table 2.10).
• Growth factors are usually produced by cells near the site of tissue injury.
• The most important source is macrophages activated by tissue damage, while epithelial and stromal cells also produce growth factors.
• Some growth factors bind to ECM proteins, so they become concentrated at the injured area.
• Growth factors activate signaling pathways that cause cell division and proliferation.
• Cells also attach to ECM proteins through integrins, and integrin signals can further stimulate cell proliferation.
• During regeneration, surviving cells proliferate, while stem cells also produce new mature cells.
• Embryonic stem cells (ES cells) can:
  • Self-renew
  • Produce all mature cell lineages, called totipotential ability
• Tissue stem cells are present in most adult tissues and usually produce the mature cells of the same tissue in which they live.
• Tissue stem cells have a more limited self-renewal capacity than embryonic stem cells.
• Stem cells undergo asymmetric cell division:
  • One daughter cell remains a stem cell → maintains self-renewal.
  • The other daughter cell differentiates → becomes a mature cell.
• Tissue stem cells live in specialized areas called stem cell niches.
• Injury stimulates these stem cells to proliferate and differentiate, helping to replace damaged cells.
• Stem cells are especially important when surviving mature cells have little or no ability to divide.
• The amount of regeneration depends on the type of tissue and severity of injury.
• In the intestinal epithelium and skin, damaged cells are rapidly replaced if the basement membrane remains intact.
• Repair occurs through:
  • Proliferation of surviving cells
  • Differentiation of tissue stem cells
• Parenchymal organs can regenerate if their mature cells can divide, but regeneration is usually limited except in the liver.
• The pancreas, adrenal, thyroid, and lung have some regenerative capacity.
• After surgical removal of one kidney, the remaining kidney undergoes a compensatory response:
  • Hypertrophy → cells become larger
  • Hyperplasia → proximal duct cells increase in number
• The liver has an exceptionally strong regenerative capacity and is an important model for studying regeneration.
• Complete restoration of normal tissue structure occurs only when the remaining tissue framework is intact, such as after partial surgical removal of the liver.
• If both the cells and their supporting framework are destroyed by infection or inflammation, regeneration becomes incomplete and scar formation occurs.
• For example, extensive liver destruction with collapse of the reticulin framework, as in a liver abscess, causes scarring even though surviving liver cells can regenerate.

KEY CONCEPT

• Growth factors + ECM signals → cell proliferation
• Stem cells → self-renew + produce mature cells
• Intact supporting framework → effective regeneration
• Destroyed supporting framework → incomplete regeneration + scar
• Liver → greatest regenerative capacity among major parenchymal organs

CONCEPTUAL EXAMPLES

• Skin injury + intact basement membrane → rapid regeneration
• Partial liver removal + preserved framework → liver cells regenerate
• Liver abscess + destroyed reticulin framework → regeneration cannot fully restore architecture → scar forms
• One kidney removed → remaining kidney enlarges by hypertrophy + hyperplasia

Liver Regeneration

• The liver has a remarkable ability to regenerate, especially after partial hepatectomy, which may be done for tumor removal or living-donor liver transplantation.
• Liver regeneration occurs by two main mechanisms:
  • Proliferation of remaining hepatocytes
  • Repopulation from stem cells
• Which mechanism is more important depends on the type of liver injury.
• After partial hepatectomy, the remaining hepatocytes can proliferate and regenerate up to 90% of the liver in humans.
• Hepatocyte proliferation is stimulated by the combined action of cytokines and polypeptide growth factors.
• First, cytokines such as IL-6, produced mainly by Kupffer cells, act on hepatocytes and make them ready to respond to growth-factor signals.
• Next, growth factors such as HGF and TGF-α, produced by several cell types (see Table 2.10), stimulate the prepared hepatocytes to proliferate.
• Therefore:Kupffer cells → IL-6 → hepatocytes become responsive → HGF + TGF-α → hepatocyte proliferation → liver regeneration
• If hepatocytes have a reduced ability to proliferate, such as during chronic liver injury or inflammation, liver stem cells help repopulate the tissue.
• Some liver stem cells are located in specialized niches called the canals of Hering, where bile canaliculi connect with larger bile ducts.

KEY CONCEPT

• Partial hepatectomy → remaining hepatocytes proliferate → liver regenerates
• IL-6 primes hepatocytes → HGF and TGF-α stimulate proliferation
• Chronic injury with poor hepatocyte proliferation → stem cells contribute to regeneration

CONCEPTUAL EXAMPLES

Part of liver surgically removed → remaining hepatocytes divide → liver mass is restoredChronic liver injury → hepatocytes cannot proliferate effectively → stem cells help replace liver cellsKupffer cell releases IL-6 → hepatocyte becomes ready → HGF/TGF-α stimulate cell division

Repair by Scarring

• When regeneration alone is not enough, injured cells are replaced by connective tissue, producing a scar.
• Scarring occurs especially when:
  • Injury is severe or chronic
  • Parenchymal cells, epithelium, and connective tissue framework are damaged
  • Nondividing cells are injured
• Regeneration restores tissue, whereas a scar mainly patches the damaged area.
• Scar formation can occur in the skin or in internal organs when parenchymal cells are replaced by collagen.
• Example: after myocardial infarction, damaged heart muscle is replaced by a collagen scar.

Steps in Scar Formation

• Within minutes after injury, platelets form a hemostatic plug that:
  • Stops bleeding
  • Provides a scaffold for inflammatory cells
  • Helps form a stable clot
• The later steps are shown in Fig. 2.21.
• Inflammation (6–48 hours):
  • Neutrophils arrive first, followed by monocytes.
  • They remove harmful agents and tissue debris.
  • Macrophages are the central cells in repair.
  • Macrophages clear microbes and necrotic tissue and release growth factors that stimulate later cell proliferation.
  • When harmful agents and dead cells are removed, inflammation resolves.
• Cell proliferation (up to 10 days):
  • Epithelial cells, endothelial/vascular cells, and fibroblasts proliferate and migrate into the cleaned wound.
  • Epithelial cells migrate to cover the wound.
  • Endothelial and other vascular cells form new blood vessels, called angiogenesis.
  • Fibroblasts migrate into the wound and produce collagen fibers, which form the scar.
• Granulation tissue consists of:
  • Proliferating fibroblasts
  • ECM
  • New blood vessels
• It has a characteristic pink, soft, granular appearance.
• Remodeling:
  • Deposited connective tissue is reorganized into a stable fibrous scar.
  • It begins about 2–3 weeks after injury.
  • It may continue for months or years.
• Skin wounds heal by two patterns:
  • First intention (primary union) → epithelial regeneration with minimal scarring, as in a well-apposed surgical incision.
  • Second intention (secondary union) → larger wounds heal by a combination of regeneration and scarring.
• The main healing events are the same in both types.

KEY CONCEPT

Injury → platelet plug → inflammation → cell proliferation → angiogenesis + fibroblasts + collagen → granulation tissue → remodeling → stable scar

• Regeneration → restores tissue
• Scarring → patches tissue
• First intention → minimal scar
• Second intention → larger wound + regeneration + scar

CONCEPTUAL EXAMPLES

• Clean surgical incision → first intention → epithelial regeneration → minimal scar
• Large open wound → second intention → regeneration + scar formation
• Myocardial infarction → dead cardiac muscle replaced by collagen → fibrous scar
• Granulation tissue → remodeling over time → stable fibrous scar