Introduction to Pathology: Tissue Repair and Regeneration (PDF)

Introduction to Pathology Tissue Repair and Regeneration Kenneth W. Hunter, Sc. D. Objectives Following this lecture, students should be able to: 1. Explain how the inflammatory response to injured tissues sets into motion the process of repair 2. Describe the process of regeneration and explain the...

Introduction to Pathology Tissue Repair and Regeneration Kenneth W. Hunter, Sc. D. Objectives Following this lecture, students should be able to: 1. Explain how the inflammatory response to injured tissues sets into motion the process of repair 2. Describe the process of regeneration and explain the roles of cell proliferation and stem cells 3. Compare and contrast the structure and function of labile, stabile, and permanent tissues, and provide examples of each 4. List the major growth factors and discuss their role in tissue repair (e.g. TGF-b) 5. Identify the basic biochemical components of the extracellular matrix (e.g., collagen), and summarize the importance of ECM in tissue repair 6. Compare and contrast regeneration with repair by connective tissue deposition (scar formation) 7. Discuss the timing and role of the following components of repair by connective tissue deposition: inflammation, formation of granulation tissue, recruitment and activation of fibroblasts, and remodeling 8. Summarize angiogenesis and identify the major growth factors that initiate the process 9. Explain the process of tissue remodeling, including the role of matrix metalloproteinases 10. List the major endogenous and exogenous factors that can influence tissue repair (e.g., infection) 11. Outline the process and time course of healing by first intention, and discuss the development of tensile strength in a healing surgical wound 12. Describe healing by second intention, and explain the role of myofibroblasts in wound contraction 13. Explain the process of fibrosis in parenchymal organs 14. Describe the formation and morphology of a hypertrophic scar or keloid Repair of Damaged Tissues Repair of damaged tissues is critical to the survival of an organism The inflammatory response to microbes and injured tissues sets into motion the process of repair Repair or healing refers to the restoration of tissue architecture and function after an injury It occurs by two types of reactions: 1) regeneration of the injured tissue (mild superficial injury), and 2) scar formation by the deposition of connective tissue (severe injury) Role of Inflammation in Repair Without inflammation, damaged tissues would never heal Chemotactic agents produced by M1 macrophages at the site of injury (e.g., CXCL8) recruit neutrophils and then monocytes over the next 6 to 48 hours Neutrophils and macrophages clear microbes and necrotic tissue Macrophages produce growth factors that stimulate the proliferation of many cell types in the next stage of repair As the injurious agents and necrotic cells are cleared, the inflammation resolves Acute inflammation (skin wound) Resolved inflammation (skin healed) A Brief Primer on the Cell Cycle Non-dividing cells are in cell cycle arrest in the G1 phase or have exited the cycle and are in the G0 phase Growth factors stimulate cells to transition from G0 into the G1 phase and beyond into DNA synthesis (S), G2, and mitosis (M) phases Progression is regulated by cyclins, whose activity is controlled by cyclindependent kinases The cell cycle has checkpoints where DNA damage is sensed and repaired; if the damage is too extensive, the cell dies rather than risking malignant transformation A Brief Review of Stem Cells Stem cells have two important properties: selfrenewal capacity and asymmetric replication Embryonic stem cells (ES cells) are the most undifferentiated stem cells. They are present in the inner cell mass of the blastocyst and have extensive cell renewal capacity Adult stem cells, also called tissue stem cells, are less undifferentiated than ES cells and are found among differentiated cells within an organ or tissue (stem cell niches, below) In tissues with terminally differentiated cells, there is a homeostatic equilibrium between mature cell death and replication, selfrenewal, and differentiation of stem cells Proliferative Capacities of Tissues The tissues of the body are divided into three groups based intrinsic proliferative capacity: A Labile (continuously dividing) tissues: Cells continuously lost and replaced by maturation from stem cells and by proliferation of mature cells (e.g. hematopoietic cells in the bone marrow [A] and the majority of surface epithelia) Stable tissues: Cells quiescent, but capable of proliferating in response to injury or loss of tissue mass (e.g., liver, kidney, and pancreas). Except for liver (B), stable tissues have a limited capacity to regenerate after injury Permanent tissues: Cells are considered to be terminally differentiated and nonproliferative in postnatal life (e.g., most neurons [C] and cardiac muscle cells). In permanent tissues, repair is typically dominated by scar formation Most mature tissues contain variable proportions of three cell types: continuously dividing cells, quiescent cells that can return to the cell cycle, and cells that have lost replicative ability B C USMLE QUESTION A 37-year-old truck driver is involved in a collision. He incurs blunt force chest trauma. In response to this injury, cells in tissues of the chest are stimulated to enter the G1 phase of the cell cycle from the G0 phase. Which of the following cell types is most likely to remain in G0 following this injury? A. Smooth muscle B. Endothelium C. Cardiac muscle D. Fibroblast E. Hepatocyte Growth Factors Proteins that stimulate the survival and proliferation of particular cells, and may also promote other physiological processes (see Table next slide) Bind to specific receptors that affect the expression of genes involved in cell proliferation Promote entry of cells into the cell cycle, relieve blocks on cell cycle progression (thus promoting replication), prevent apoptosis, and enhance the synthesis of cellular proteins in preparation for mitosis Many growth factors involved in repair are produced by macrophages and lymphocytes recruited to the site of injury Other growth factors are produced by parenchymal cells or stromal (connective tissue) cells in response to cell injury Examples of Growth Factors Involved in Tissue Repair Growth Factor Sources Functions Epidermal growth factor (EGF) Activated macrophages, salivary Mitogenic for keratinocytes and glands, keratinocytes, and many other fibroblasts; stimulates keratinocyte cells migration; stimulates formation of granulation tissue Transforming growth factor-α (TGF-α) Activated macrophages, keratinocytes, many other cell types Hepatocyte growth factor (HGF) (scatter factor) Fibroblasts, stromal cells in the liver, Enhances proliferation of hepatocytes endothelial cells and other epithelial cells; increases cell motility Vascular endothelial growth factor (VEGF) Mesenchymal cells Platelet-derived growth factor (PDGF) Platelets, macrophages, endothelial cells, smooth muscle cells, keratinocytes Transforming growth factor-β (TGF-β) Platelets, T lymphocytes, macrophages, endothelial cells, keratinocytes, smooth muscle cells, fibroblasts Fibroblasts Stimulates proliferation of hepatocytes and many other epithelial cells Stimulates proliferation of endothelial cells; increases vascular permeability Chemotactic for neutrophils, macrophages, fibroblasts, and smooth muscle cells; activates and stimulates proliferation of fibroblasts, endothelial, and other cells; stimulates ECM protein synthesis Macrophages, mast cells, endothelial Chemotactic and mitogenic for Fibroblast growth factors (FGFs), fibroblasts; stimulates angiogenesis and including acidic (FGF-1) and basic (FGF-2) cells, many other cell types ECM protein synthesis Keratinocyte growth factor (KGF) DO NOT MEMORIZE THIS TABLE Chemotactic for leukocytes and fibroblasts; stimulates ECM protein synthesis; suppresses acute inflammation Stimulates keratinocyte migration, proliferation, and differentiation The Role of Regeneration in Liver Repair Regeneration is the typical response to injury in the rapidly dividing epithelia of the skin and intestines Some stabile parenchymal tissues like the liver (see below) are able to replace damaged cells and essentially return to a normal functional and structural state (resolution) It involves proliferation of residual (uninjured) cells that retain the capacity to divide, and replacement from tissue stem cells Hepatocyte hyperplasia in the regenerating liver is triggered by cytokines and growth factors USMLE QUESTION An 87-year-old woman has had a cough productive of yellowish sputum for the past 2 days. On examination her temperature is 37° C. A chest radiograph shows bilateral patchy infiltrates. Her peripheral blood shows leukocytosis. A week later she is afebrile. Which of the following is the most likely outcome of her pulmonary disease? A. Chronic inflammation B. Fibrous scarring C. Neoplasia D. Resolution E. Ulceration USMLE QUESTION A 3-year-old child has been diagnosed with ornithine transcarbamylase deficiency and has developed hepatic failure. The left lobe of an adult donor liver is used as an orthotopic transplant. A year later, the size of each liver in donor and recipient is greater than at the time of transplantation. Which of the following cellular alterations is most likely to explain this phenomenon? A. Metaplasia B. Dysplasia C. Hyperplasia D. Anaplasia E. Neoplasia Extracellular Matrix (ECM) and Tissue Repair Tissue repair also depends on interactions between cells and ECM Interstitial matrix: in the spaces between cells in connective tissue Basement membrane: highly organized form of ECM around epithelial cells, endothelial cells, and smooth muscle cells The ECM is constantly being remodeled Disruption of the ECM (basement membrane or stromal scaffold) results in a failure of the tissues to regenerate and leads to repair by scar formation Repair by Connective Tissue Deposition: Scar Formation If injured tissues cannot regenerate, or if supporting structures are severely damaged, repair occurs by the laying down of connective tissue (fibrous scar, A) Fibrous scars cannot replace the function of lost parenchymal cells; may provide enough structural stability for the injured tissue to function A B Fibrosis is most often described as the extensive deposition of collagen that occurs in parenchymal organs as a result of chronic inflammation (e.g. liver cirrhosis, B) If fibrosis develops in a tissue space occupied by an inflammatory exudate, it is called organization (organizing pneumonia affecting the lung, C) Both regeneration and scar formation contribute in varying degrees in many injuries C Stages in Scar Formation Inflammation: chemotactic agents recruit neutrophils and then monocytes during the next 6 to 48 hours; Macrophages are the central cellular players in the repair process Formation of Granulation Tissue: over the next 10 days, endothelial and other vascular cells (angiogenesis) and fibroblasts proliferate and migrate to form granulation tissue Recruitment and Activation of Fibroblasts: Fibroblasts lay down collagen fibers that form the scar (fibrosis) Remodeling: Collagen is reorganized to produce the stable fibrous scar; process begins 2 to 3 weeks after injury and may continue for months or years Formation of Granulation Tissue Granulation tissue is composed of proliferating fibroblasts, loose connective tissue, new blood vessels, and scattered chronic inflammatory cells This term derives from its pink, soft, granular gross appearance (photos) Formation of new blood vessels (angiogenesis) is critical to healing Fibroblasts produce components of connective tissue (e.g. collagen) Angiogenesis Formation of new blood vessels, primarily venules, that supply nutrients and oxygen needed to support the repair process (A) Critical in healing at sites of injury Involves a variety of growth factors, cell-cell interactions, interactions with ECM proteins, and tissue enzymes The most important are vascular endothelial growth factor (VEGF) and basic fibroblast growth factor (FGF-2) Recruitment and Activation of Fibroblasts and Deposition of Connective Tissue Deposition of connective tissue in the scar occurs in two steps: 1) migration and proliferation of fibroblasts, and 2) deposition of connective tissue proteins by these cells (A) Fibroblast recruitment and activation is driven by many growth factors produced mainly by M2 macrophages (e.g., PDGF, FGF-2, and TGF-b) Collagen synthesis and deposition is critical to developing tensile strength in a scar Ultimately, the granulation tissue evolves into an avascular scar composed of spindle-shaped fibroblasts, dense collagen, fragments of elastic tissue, and other ECM components (B) A B Collagen is stained blue by the trichrome stain Remodeling of Connective Tissue After its synthesis and deposition, the connective tissue in the scar can be modified to approximate original tissue form (remodeling) Degradation of collagens is accomplished by a family of matrix metalloproteinases (MMPs) MMPs are produced by a variety of cell types (fibroblasts, macrophages, neutrophils, synovial cells, and some epithelial cells) Their synthesis and secretion are regulated by growth factors, cytokines, and other agents Remodeling is controlled by the ratio of collagen synthesis and degradation Factors that Influence Tissue Repair Tissue repair may be altered by a variety of influences, frequently reducing the quality or adequacy of the reparative process. Examples include: Infection is clinically the most important cause of delay in healing; it prolongs inflammation and potentially increases the local tissue injury Nutrition has profound effects on repair; protein deficiency, for example, and especially vitamin C deficiency inhibit collagen synthesis and retard healing Glucocorticoids (steroids) have well-documented anti-inflammatory effects, and their administration may result in weakness of the scar because of inhibition of TGF-β production and diminished fibrosis Mechanical variables such as increased local pressure or torsion may cause wounds to pull apart (dehiscence) Poor perfusion, due either to arteriosclerosis and diabetes or to obstructed venous drainage (e.g., in varicose veins), also impairs healing Foreign bodies such as fragments of rocks, steel, glass, or even bone impede healing USMLE QUESTION A 54-year-old man undergoes laparoscopic hernia repair. In spite of the small size of the incisions, he has poor wound healing. Further history reveals that his usual diet has poor nutritional value and is deficient in vitamin C. Synthesis of which of the following extracellular matrix components is most affected by this deficiency? A. Collagen B. Elastin C. Fibronectin D. Integrin E. Laminin Healing by First Intention Healing of a clean, uninfected surgical incision approximated by surgical sutures (primary union) The narrow incisional space first fills with fibrinclotted blood (hemostasis) Within 24 hr, neutrophils are seen at the incision margin, and epithelial cells are migrating from both edges (re-epithelialization) By day 3, neutrophils have been largely replaced by macrophages, and granulation tissue invades the incision space Over the next week fibroblasts proliferate and collagen accumulates A small scar is formed, but there is minimal wound contraction; any dermal appendages are lost Wound (Tensile) Strength Carefully sutured wounds have around 70% of the strength of normal skin, largely because of the placement of sutures When sutures are removed, usually at 1 week, wound strength is approximately 10% of unwounded skin, but this increases rapidly over the next 4 weeks The recovery of tensile strength results from collagen synthesis exceeding degradation during the first 2 months, and from crosslinking and increased fiber size of collagen Wound strength reaches approximately 70% of normal by 3 months and usually does not improve substantially beyond that point 3 months USMLE QUESTION A 20-year-old woman sustains an injury to her right calf in a mountain biking accident. On physical examination she has a 5 cm long laceration on the right lateral aspect of her lower leg. This wound is closed with sutures. Wound healing proceeds over the next week. Which of the following factors will be most likely to aid and not inhibit wound healing in this patient? A. Commensal bacteria B. Decreased tissue perfusion C. Presence of sutures D. Corticosteroid therapy E. Hypoalbuminemia Healing by Secondary Intention Inflammation is more intense in large tissue defects that have a greater volume of necrotic debris, exudate, and fibrin that must be removed A greater volume of granulation tissue is needed to fill in the gap, resulting in a greater mass of scar tissue Secondary “union” involves wound contraction; within 6 weeks, for example, large skin defects may be reduced to 5% to 10% of their original size, largely by contraction Wound contraction involves myofibroblasts (modified fibroblasts act like contractile smooth muscle cells, A); can cause functional loss (B) A B Healing of Skin Ulcers Pressure ulcer of the skin, commonly found in diabetic patients (A) A skin ulcer with a large gap between the edges of the lesion (B) A thin layer of epidermal re-epithelialization, and extensive granulation tissue formation in the dermis (C) Continuing re-epithelialization of the epidermis and wound contraction mediated by myofibroblasts (D) Fibrosis in Parenchymal Organs The term fibrosis is used to denote the excessive deposition of collagen and other ECM components in a tissue The basic mechanisms of fibrosis seen in parenchyma are the same as those of scar formation during epithelia repair Fibrosis most often refers to the deposition of collagen induced by persistent injurious stimuli (e.g., infections, immunologic reactions, and other types of chronic tissue injury) Fibrosis seen in chronic diseases such as pulmonary fibrosis is often responsible for organ dysfunction and even organ failure (photos) Mechanisms of Fibrosis Persistent tissue injury leads to chronic inflammation and loss of tissue architecture Cytokines produced by macrophages and other leukocytes stimulate the migration and proliferation of fibroblasts and myofibroblasts Collagen and other extracellular matrix proteins are deposited The net result is replacement of normal tissue by fibrosis USMLE QUESTION A 58-year-old man had chest pain persisting for 4 hours. A radiographic imaging procedure showed an infarction involving a 4-cm area of the posterior left ventricular free wall. Laboratory findings showed serum creatine kinase of 600 U/L. Which of the following pathologic findings would most likely be seen in the left ventricular lesion 1 month later? A. Chronic inflammation B. Coagulative necrosis C. Complete resolution D. Fibrotic scar E. Nodular regeneration Keloid Formation Accumulation of exuberant amounts of collagen can give rise to a raised hypertrophic scar or keloid (A) May occur even in what begins as normal wound healing; appears to be heritable (more common in African Americans) Healing wounds may also generate exuberant granulation tissue that protrudes above the level of the surrounding skin and hinders reepithelialization (B) Such tissue is called “proud flesh” (C) C