14 Healing and Regeneration

Learning Objectives and Healing Outcomes

• Labile, stable and permanent cells differ in their regenerative capacity and how they divide (asymmetrically or symmetrically).
• Major determining factors for injury healing are either regeneration or repair.
• Fibroplasia is summarized in a series of steps.
• Factors that delay healing are also summarized.
• The animal either dies from being overwhelmed, effectively copes and heals completely, or copes poorly, which causes a chronic inflammatory process after injury.

Healing and Regeneration

• Healing happens in response to tissue injury and reestablishes integrity.
• Ideally, healing occurs subsequent to inflammation and restores tissue and organ function.
• Complete regeneration may not be possible, therefore, repair may be a more realistic outcome.
• Inflammation and healing are processes that frequently overlap.
• Regeneration happens in organs with labile cells.
• Bone marrow, skin, and epithelium injuries may completely regenerate, leaving no residual evidence of damage.
• Perfect regeneration is rare with most organs typically showing post-damage evidence like a scar.

Cell Types and Regenerative Capacity

• Labile cells are continuously shed and regenerated such as intestinal epithelium and skin.
• Stable cells have regenerative potential when needed, such as hepatocytes.
• Permanent cells cannot divide or regenerate, like CNS neurons and cardiac muscle.
• Labile cells are constantly turned over in epithelial skin and gut populations, as well as hematopoietic cells in bone marrow.
• Labile cells have a short life span, die via apoptosis, and are rapidly replaced.
• Stem cells constantly supply the basal epidermis layer, intestinal gland crypts, and hematopoietic stem cells.
• Tissue stem cells divide asymmetrically, one daughter cell remains a stem cell, and the other differentiates.
• Stable cells are normally replaced slowly but can rapidly renew after tissue loss such as hepatocytes and kidney proximal convoluted tubule cells.
• Stable cells proliferate symmetrically, with both daughter cells differentiating.
• Long-lived and fully differentiated stable cells leave the cell cycle at G0 and exist in "quiescence".
• Stable cells can re-enter the cell cycle, divide, proliferate, and replace lost cells.
• Stable cells include parenchymal cells of solid organs like hepatocytes, pneumocytes and mesenchymal cells such as osteoblasts and fibroblasts.
• Permanent cells are terminally differentiated post-mitotic cells, i.e. cardiac myocytes, skeletal muscle and neurons.
• Permanent cells are unable to re-enter the cell cycle or divide.
• Injured permanent cells are replaced by scar tissue.

Supporting Structures and Repair

• If labile cells are destroyed but the stroma is intact with a good blood supply, then rapid regeneration occurs.
• Connective tissue stroma acts as a scaffold and includes cells such as fibroblasts, fibrocytes, chondrocytes, osteocytes.
• Damage to the stroma results in unsatisfactory regeneration, requiring repair, not regeneration.

Fibroplasia and Fibrosis

• Fibrosis, or collagen scar, is a type of repair that generally occurs in organs containing stable or permanent cells with limited regeneration capacity.
• Necrotic myocytes are replaced by a fibrotic scar, and surrounding myocytes may hypertrophy in compensation.

Fibroplasia Components

• Angiogenesis which is the formation of new blood vessels.
• Migration and proliferation of fibroblasts.
• Deposition of the extracellular matrix.
• Maturation and organization which is also known as remodeling.

Angiogenesis Details

• Pre-existing capillaries sprout new vessels when the basement membrane is broken down by proteolytic degradation.
• Endothelial cells move toward the injury area.
• Endothelial cells proliferate just behind the leading edge.
• Supporting cells surround newly formed endothelial cells in capillaries.
• Healing wounds are often edematous and ooze due to leaky young vessels.

Fibroblast Role

• Fibrin's presence in the extracellular matrix provides provisional stroma for fibroblasts and endothelial cells to migrate.
• The growth factors from platelets, inflammatory cells, and the endothelium cause fibroblasts to proliferate, increasing collagen and fibronectin synthesis.

Macrophage Types

• M1 macrophages are activated by TNF & IFN-γ, LPS and secrete IL-12 and are pro-inflammatory.
• M2 macrophages are promoted by IL-4 & IL-13, secrete anti-inflammatory cytokines such as IL-10, and are involved in healing and tissue repair.

Cytokines and Growth Factors

• Macrophage-derived growth factor (MDGF).
• Platelet-derived growth factor (PDGF).
• Endothelial growth factor (EGF).
• Fibroblast growth factor (FGF).
• Tissue growth factor-beta (TGF-β).

Extracellular Matrix Repair

• Major components are collagens, basement membranes (BM), elastic fibers, fibronectin, and proteoglycans.
• Type I collagen is found in skin, bone, and mature scars,
• Type II collagen is a major cartilage component.
• Type III collagen is abundant in embryonic/pliable tissues, and deposited in wound healing.
• Type IV collagen is exclusively in basement membranes but does not form fibers.
• Mature collagen is remodeled by collagenases produced by fibroblasts.
• Basement membranes (BM) separate epithelial and endothelial cells from the stroma and they are synthesized by the cells resting on them.
• Damage to the basement membrane influences the type of repair.
• BM contains type IV collagen, proteoglycans, laminin, and heparin sulfate.
• Fibronectin is an adhesive glycoprotein that increases at the site of wound healing, encouraging cell adhesion, leukocyte/endothelial cell/fibroblast migration.
• Fibronectin provides a primitive skeleton for collagen deposition and binds to collagens, proteoglycans, fibrinogen, fibrin, cell surfaces, bacteria & DNA.
• Proteoglycans, also known as glycosaminoglycans, are highly hydrophilic, forming hydrated gels at low concentrations.
• Cell-matrix interactions: the ECM helps align cells for proper wound healing.
• Integrins, or cell-surface receptors, allow the ECM to directly modify cell behavior, while adhesion molecules include immunoglobulin superfamily, selectins, etc.
• Matrix adhesion molecules include fibronectin, osteonectin, tenascin, laminin, vitronectin, fibrinogen, vWF, and collagen.

Granulation Tissue

• The new tissue is called granulation tissue and the disorganized fibrin meshwork that occurred as a product of inflammation is said to be undergoing organization.
• Some fibroblasts acquire contractile elements as in smooth muscle "myofibroblasts".

Tissue Remodeling

• Tissue remodeling involves some factors that stimulate collagen synthesis, activating metalloproteinase which degrades collagen.
• The balance between synthesis and degradation remodels the collagen framework.

Factors that Delay Healing

• Continued tissue damage from the original or secondary agent.
• Especially bacterial infections and movement.
• Poor nutrition, particularly protein, Vitamin A, Vitamin C, and Zn.
• Poor blood supply in the affected area.
• Metabolic disturbances, such as diabetes.
• Low temperatures that affect cytokines and mediators.
• Low immune status.

Wound Healing Summary

• Induction of an acute inflammatory response that includes fibrin mash and acute cells.
• Regeneration of parenchymal cells happens if they are labile.
• Migration and proliferation of both parenchymal and connective tissue cells, also known as fibroplasia.
• Synthesis of extracellular matrix proteins.
• Remodeling, collagenization, and maturation of the wound.