GPT 1.05 - Tissue Renewal & Repair PDF

GENERAL PATHOLOGY Tissue Renewal and Repair Block 1 Jose Jasper L. Andal, MD | August 12, 2024...

GENERAL PATHOLOGY Tissue Renewal and Repair Block 1 Jose Jasper L. Andal, MD | August 12, 2024 Trans 1.05 m OVERVIEW I. Overview of tissue repair III. Clinical context Involved in the process that restore function and A. Mechanisms of tissue A. Healing of skin structures of organs repair wounds Focuses on two things: Regeneration IV. Factors that influence ○ Tissue and cell regeneration Scar formation tissue repair ○ Scar formation B. Determinants of V. Abnormalities in tissue The fate of the damaged tissue will depend on: tissue repair repair ○ Nature of injury Cell proliferation A. Chronic wounds Ex. trauma, surgery performed Tissue B. Wound Dehiscence proliferative C. Excessive scar tissue ○ Cell proliferation capacity deposition ○ Integrity of the Extracellular Matrix (ECM) ECM and growth ECM is the connective tissue that binds our factors cells together II. Wound healing in the skin A. Repair by connective Regeneration tissue deposition ​Some tissues are able to replace the damaged Steps in Scar components and essentially return to a normal state Formation May occur through proliferation of differentiated cells ○ Ex. Hepatocytes may undergo either: ⭐ LEGEND : Important information Gray Text or : Book information Simple replication of the existing differentiated cells, or Replication of the stem cells → differentiation 💬: Good-to-know info from lecturer : Processes or Clinical Correlation into the different component cells of the lost tissue BOOK CHAPTERS Robbins & Cotran Pathologic Basis of Disease (10th edition), Scar Formation Chapter 3, pp. 103-112 Tissue repair that involves: ○ Connective tissue deposition ABBREVIATIONS ○ Fibrosis (collagen deposition) ECM Extracellular Matrix Important mechanism that fills in gaps, maintain VEGF Vascular Endothelial Growth Factor structure, blocks pathogens, and prevent further FGF Fibroblast Growth Factor bleeding from an open area [Batch 2027] PDGF Platelet Derived Growth Factor To seal off the wound and enable wound contraction TGF Transforming Growth Factor ○ Ex. burn patient who has formed contractures I. OVERVIEW OF TISSUE REPAIR B. Determinants of Tissue Repair A. Mechanisms of Tissue Repair Cell Proliferation Figure 2. Cell cycle shows how cells proliferate to go back into the ⭐ Key determinant of tissue repair cell cycle from G0 ○ Cells that are not able to proliferate cannot undergo regeneration Normally maintains tissue homeostasis Most of the cell in the adult phase are in the G0 Figure 1. Mechanisms of tissue repair: regeneration and scar phase formation. Following mild injury, which damages the epithelium but not the underlying tissue, resolution occurs by regeneration, but Some cells have the ability to cycle rapidly after more severe injury with damage to the connective tissue, repair ○ Ex. bone marrow cells to replace blood elements is by scar formation Page 1 of 14 | TH: ARANZANSO | GPT TG 7 | ACUÑA, DELMONTE, GOMEZ, SALVADOR, A. Checked and verified: ARCEO, J., GONZALES, H., ILAGAN, C..  GPT 1.05 Tissue Renewal and Repair Embryonic Stem Cell Potential source of cells to replace injured tissues In-vivo (in-utero): different cell lines mature into every possible tissue in an infant In-vitro (in laboratory): can be cultured and given growth factors to become different cell lines Pluripotent stem cells (PSC) ○ Cells that can differentiate into various cell lineages through the help of growth factors ○ → Multipotent stem cells → Lineage committed stem cells → Endoderm: ex. Foregut, Upper respiratory tract Mesoderm: ex. Connective tissue Ectoderm: ex. Skin ○ → Cell culture → Embryonic stem cell → Pancreatic islet cells (for type 1 diabetics) Hematopoietic cells (for bone marrow Figure 5. Mechanisms regulating cell population transplant) Cardiomyocytes (for MI patients) Neurons (for patients with neurodegenerative Skin Epithelium disease) Hepatocytes (for Liver diseases) Figure 6. Skin epithelium Figure 3. In vivo (totipotent stem cells) and in vitro (embryonic stem cells) differentiate into different cell types The differentiated keratinocytes are seen on the surface layer Tissue/Adult Stem Cells The basal cells present in the stratum basale layer of A small population of adult stem cells are present in the epidermis will replicate continuously and will specific niches undergo differentiation to replace the lost Explains why these organs (skin, intestines, liver) keratinocytes have the capacity to regenerate Tissue Proliferative Capacity Labile Tissues Proliferate throughout adult life or postnatal life ⭐Examples: ○ Bone marrow RBCs die with a lifespan of about 3 months Continuously produced to maintain adequate Figure 4. Stem cell niches levels of RBCs and WBCs in the peripheral A. Skin: Bulge stem cells beside hair follicles blood B. Intestine: Stems cells lodged in the crypts Produce different lines of hematopoietic cells C. Liver: Oval cells present in the canals of Hering [Batch 2027] ○ Surface epithelium of the skin, GI tract, and Mechanisms Regulating Cell Populations urinary tract Stem cells maintain the pool of regenerating cells [Batch ○ Cells of tissues lining cavities 2027] “Why do we not run out of these stem cells?” ○ If we run out of stem cells, we lose the ability to Stable Tissues (Quiescent) replace the tissues in the body Maintained in the G0 stage in the cell cycle ○ Some of the stem cells will replicate into baseline Low level of replication cell population → proliferate and differentiate (ex: Can proliferate under stimuli (ex: injury) into the stratum corneum layer of the skin) Examples: bone and liver ○ Some will replicate back into the stem cells to maintain the stem cell pool Stem cells regenerate themselves Page 2 of 14 | TH: ARANZANSO | GPT TG 7 | ACUÑA, DELMONTE, GOMEZ, SALVADOR, A.  GPT 1.05 Tissue Renewal and Repair Figure 7. (L) Intramedullary cavity in the middle of the bone and a normal lower cortex. Upper cortex presents with a callus after a trauma/fracture. (R) Trichrome stain of cirrhotic liver. ○ Bone Has a very low level of replication and very slow turnover rate Fracture due to blunt force trauma or vehicular Figure 8. ECM implications and Functions accidents → osteocytes become active again to help heal the fractured bone → callus [Batch 2027] Liver Regeneration in Post-Hepatectomy Callus will form as a healing response to an Hepatocytes are composed of parenchymal cells and injury/fracture rest on a scaffold of connective tissue on the ECM ○ Liver [Batch 2027] Alcohol-induced liver injury, liver resection After a hepatectomy, the remaining liver parenchyma (hepatectomy), or any injurious stimuli can has the ability to proliferate and regenerate from induce liver regeneration existing progenitor cells Two mechanisms are used to regenerate normal Permanent Tissues liver tissue in post-hepatectomy patients: [Batch 2027] Non-proliferative in postnatal life ○ Proliferation of remaining hepatocytes Terminally differentiated ○ Liver regeneration from progenitor cells Have a very limited capacity to repair themselves and a very small stem cell pool → limited ability to proliferate Examples: ○ Neurons ○ Skeletal muscles Satellite cells attached to the endomysial sheath provide some regenerative capacity for muscle [Robbins & Cotran, 10th ed., pg. 104] ○ Cardiac muscles Px who experienced myocardial infarction (MI) or part of the cardiac myocytes that undergo necrosis → difficult to replace necrotic tissue with viable myocytes due to permanence of Figure 9. Liver regeneration by hepatocyte proliferation with the aid tissue and have very limited capacity to of growth factors. proliferate in postnatal life [Batch 2027] Restoration of normal tissue structure can occur only if the residual tissue is structurally intact ECM & Growth Factors (GF) ECM is the architectural structure or scaffold of the ⭐ Even if the hepatocytes can regenerate, an intact reticulin network or ECM is required for cells to tissue [Batch 2027] proliferate onto Also have key roles in: ○ Cell proliferation by elaborating growth factors ○ Responding to injury and trauma ○ Proliferating blood vessels for the oxygen and nutrient supply of cells Growth factors are typically produced by cells (macrophages and epithelial and stromal cells) near the site of damage [Robbins & Cotran, 10th ed., pg. 104] ○ All growth factors activate signaling pathways that stimulate DNA replication Figure 10. Intact reticulin network in the liver parenchyma - Central Vein in the middle portion surrounded by sinusoids Page 3 of 14 | TH: ARANZANSO | GPT TG 7 | ACUÑA, DELMONTE, GOMEZ, SALVADOR, A.  GPT 1.05 Tissue Renewal and Repair The following conditions disrupt the normal reticulin TG Note: In Dr. Andal’s lecture, the steps in scar network and produces scarring and fibrosis: formation are (1) Inflammation, (2) Granulation Tissue ○ Chronic tissue injury Formation, and (3) Connective Tissue Deposition. ○ Liver cirrhosis from alcohol injury (scarring) However, in Robbins & Cotran’s Pathologic Basis of ○ Hepatitis B or Hepatitis C infections Disease (10th edition), the steps in scar formation are (1) Inflammation, (2) Cell Proliferation, (3) Granulation Tissue Formation, and (4) Connective Tissue Deposition. A. Steps in Scar Formation 1. Inflammation Initiating Factor: tissue injury Enables the migration of inflammatory cells to the area of injury Allows the formation of a temporary scab or plug ○ Recruits more inflammatory cells and sets the stage for cell proliferation Hemostatic Plug Formation[Robbins & Cotran, 10th ed., pg. 105] ○ Composed of platelets ○ Stops bleeding and provides a scaffold for the Figure 11. Disorganized reticular network in a cirrhotic liver deposition of fibrin parenchyma with trichrome stain. Figure 11 Chemotactic Agents[Robbins & Cotran, 10th ed., pg. 105] ○ Trichrome Stain: highlights the fibrous tissue in ○ Recruit neutrophils and monocytes over the next 6 blue to 48 hours ○ Red-Colored Tissue: normal hepatocytes ○ Eliminate offending agents and clear the debris ○ Blue-Stained Tissue: deposited fibrous tissue ○ Examples: ○ Cirrhotic liver is multinodular Breakdown products of complement activation Due to nodules regenerating in the liver’s Chemokines released from activated platelets fibrous tissue and destroyed ECM matrix Other mediators produced at the site of injury ○ As the injurious agents and necrotic cells are Table 1. Intact vs disrupted reticulin network cleared, the inflammation resolves DESCRIPTION Hepatocytes can proliferate and TG Note: Dr. Andal refers to the second step as the Intact Reticulin Formation of Granulation Tissue. However, his regenerate to maintain liver Network description of this step aligns with the text provided architecture Disrupted ECM under Cell Proliferation in Robbins & Cotran’s Pathologic Generating hepatocytes can grow Basis of Disease (10th ed.). For this trans, we will and Reticulin but in a disarray combine and place both information provided under Cell Network Proliferation. II. WOUND HEALING IN THE SKIN 2. Cell Proliferation Takes up to 10 days Several cell types proliferate and migrate to close the wound Allows for blood vessel to grow in the area of injury and supply nutrients to the proliferating cells Table 2. Cells in cell proliferation CELL TYPES DESCRIPTION Respond to locally produced Figure 12. Steps in wound healing and scar formation in the skin. growth factors Epithelial Cells Migrate over the wound to cover it Replacement of Injured Cells with Connective up Tissue[Robbins & Cotran, 10th ed., pg. 105] Proliferate to form new blood ○ Occurs when repair by regeneration is not possible Endothelial Cells and Pericytes ⭐️ vessels → angiogenesis Pericytes: supporting cells that hold new blood vessels together ○ Leads to: Formation of a scar; or Proliferate and migrate into the Combination of regeneration of some residual cells and scar formation ⭐️ Fibroblasts site of injury Lay down collagen fibers that form Scar Formation: “patches” rather than restores the the scar tissue[Robbins & Cotran, 10th ed., pg. 105] Angiogenesis Formation of new blood vessels Page 4 of 14 | TH: ARANZANSO | GPT TG 7 | ACUÑA, DELMONTE, GOMEZ, SALVADOR, A.  GPT 1.05 Tissue Renewal and Repair Allows healing and repair in the injured tissues Migration and deposition of the Involves signaling pathways, cell-cell interactions, basement membrane Suppression of ECM proteins, and tissue enzymes[Robbins & Cotran, 10th ed., pg. Once the new blood vessels are endothelial 106] formed, a negative feedback proliferation Critical in the development of collateral circulations at mechanism suppresses the sites of ischemia and tumor growth proliferation ○ Growing tumors will require new blood vessels to allow cancer cells to grow Signaling Factors in Angiogenesis Inhibiting the new blood vessels can Table 4. Signaling factors in angiogenesis potentially help stop the growth of the tumor SIGNALING DESCRIPTION FACTORS Steps in Angiogenesis Mainly VEGF-A [Robbins & Cotran, 10th ed., pg. Vascular 106] Endothelial Promotes vasodilation Growth Factor ○ Nitric oxide (NO) production (VEGF) Stimulates migration and proliferation of endothelial cells Mainly FGF-2[Robbins & Cotran, 10th ed., pg. 106] Stimulates the proliferation of Fibroblast ⭐️ endothelial cells Promotes the migration of macrophages and fibroblasts to the Growth Factors damaged area (FGF) Stimulates epithelial cell migration to cover epidermal wounds Fibroblasts: form fibrous tissue ○ Deposit collagen → fibrosis Figure 13. Steps in angiogenesis Table 3. Steps in angiogenesis STEPS DESCRIPTION In response to nitric oxide and increased permeability induced by the VEGFs Vasodilation To allow for more blood flow and the migration of inflammatory cells Partly due to the inflammation that occurs in the area [Batch 2027] Pericytes separate from the Figure 14. Fibroblasts in varying appearance abluminal surface Breakdown of basement (A) Quiescent fibroblasts appear as spindle (more Separation of membrane by matrix elongated) or stellate-shaped cells (star-shaped) Pericytes metalloproteinases (MMPs) ○ Nucleus: has fine chromatin pattern and distinct Allows the formation of a vessel nucleoli sprout or “leading tip” cells (B) Active fibroblasts appear more plump [Batch 2027] Endothelial cells from the existing (C) Fibroblasts may also appear as stellate- or Migration of blood vessel migrate to the new spindle-shaped on tissue culture Endothelial Cells blood vessels Stimulated by the FGFs Platelet Participates in the stabilization of Proliferation of Behind the leading front (“tip”) of Derived Growth new blood vessels Endothelial Cells migrating cells Factor (PDGF) Recruits smooth muscle cells The endothelial cells branch off Remodeling into Specifically TGF-β and meet each other Capillary Tubes Supported by the ECM and pericytes/smooth muscle cells ⭐️ Trans- forming Growth Suppresses endothelial proliferation and migration Enhances production of ECM Recruitment of ○ Pericytes for small capillaries Periendothelial Factor (TGF) proteins ○ Smooth muscle cells for larger Cells Serves as the negative feedback vessels loop once the repair is done[Trans 2025] Page 5 of 14 | TH: ARANZANSO | GPT TG 7 | ACUÑA, DELMONTE, GOMEZ, SALVADOR, A.  GPT 1.05 Tissue Renewal and Repair 3. Formation of Granulation Tissue [Robbins & Cotran, 10th ed., pg. 105] ⭐️ TGF-β Granulation Tissue Formation From Dr. Andal ○ Migration and proliferation of fibroblasts “The cytokine that I want you to remember, and this ○ Deposition of loose connective tissue will most likely come out in the exam, is TGF-β.” ○ Vessels and interspersed mononuclear leukocytes Histologic Appearance: One of the most important cytokines for the synthesis ○ Proliferation of fibroblasts and new thin-walled, and deposition of connective tissue proteins delicate capillaries in a loose ECM Produced by macrophages and other cells of the ○ Often mixed with inflammatory cells (mainly granulation tissue macrophages) Functions: Granulation tissue progressively fills the site of injury ○ Stimulates fibroblast migration and proliferation ○ Amount of tissue formed depends on the size of Elaboration of TGF-β in the area where scar the tissue defect and intensity of inflammation tissue deposition is needed because fibroblasts and connective tissue are wanted in that area ○ Increases synthesis of collagen and fibronectin ○ Decreases degradation of ECM by inhibiting metalloproteinases Part of the feedback loop and regulatory processes: the presence of metalloproteinases that degrade the ECM. At the early stages: decrease of ECM degradation is wanted At later stages: promotion of ECM degradation is wanted for remodeling scar tissue area Figure 15. Chronic inflammation and granulation tissue Figure 15 ○ On the surface: fibrin and inflammatory cells ○ Deeper layers: new blood vessels Figure 16. Fibrosis in granulation tissue. (A) H and E section: cells Clear spaces are the lumen of the vessels are stained pink or purple (B) Trichrome stain highlights collagen/fibrosis as blue-stained tissues. ○ White areas: loose connective tissue Figure 16 From 2026 Trans: ○ A: An area of young or early stages of granulation New blood vessels = key feature of granulation tissue tissue formation Macrophages = key feature in the latter phase of healing Areas full of blood cells: Blood vessels More inflammatory cells found in areas of infection Small dots: inflammatory cells Appearance of granulation tissue varies depending on Blue squiggly lines: areas of collagen phase of granulation tissue formation ○ Acute phase = more loose, and less fibrosis deposition ○ Latter phase = less blood vessels, more fibrosis Only small amount of fibrous tissue in the early stages of collagen/fibrous tissue deposition 4. Deposition of Connective Tissue ○ B: More blue due to larger amount of fibrous Steps in deposition of connective tissue: tissue deposited in the scar tissue ○ Migration and proliferation of fibroblasts into the site of injury From Dr. Andal (If an exam question were to be asked:) ○ Deposition of ECM proteins “Which of these two micrographs represents later The fibroblasts migrate into the base of the injured phases of scar tissue deposition?” area and deposit ECM proteins (e.g. fibrous tissue) ○ Answer: the one on the right. and other matrix proteins. More blue = more fibrosis in the tissue ○ Results in the formation of a collagenous scar Has a lot less vessels: because the tissue is ○ Collagen: a fibrous tissue being remodeled, as part of the reparative Deposition of collagen is from the fibroblasts that process have migrated from onto that area. Less blood vessel formation and less ○ Critical for the development of strength in a inflammation, but more collagen deposition, healing wound site [Robbins & Cotran, 10th ed., pg. 107] in the latter phases “Which stain is used to highlight fibrous tissue?” ○ Answer: Trichrome stain Page 6 of 14 | TH: ARANZANSO | GPT TG 7 | ACUÑA, DELMONTE, GOMEZ, SALVADOR, A.  GPT 1.05 Tissue Renewal and Repair Fibroblasts-Myofibroblasts FIBROBLASTS Elaborate and increase ECM synthesis Acquire features of smooth muscle cells (Actin filament) Marked ability to differentiate and form myofibroblasts. Figure 18. Healing of skin wounds by first intention (L) and second intention (R). In the latter, note the large amount of granulation Figure 17. Two-stage model of myofibroblast differentiation. Under tissue and wound contraction mechanical stress, fibroblasts will differentiate into myofibroblast Healing by First Intention MYOFIBROBLASTS Also referred to as: epithelial generation, primary Myofibroblast= fibroblast with actin filament [Batch 2027] union. Continue to Scar contraction The principal mechanism of repairing an injury that ○ Because ends of wound need to be apposed involves only the epithelial layer together to heal ○ Example: surgical scars, clean and small ○ Ex. Wound with large surgical defects. Scar tissue lacerations will be needed to contract the edges of the Repair has three connected processes: surgical scar to go near each other[Batch 2027] ○ Inflammation Remodeling of Connective Tissue ○ Proliferation After deposition, the connective tissue in the scar ○ Maturation continues to be modified and remodeled [Batch 2027] Wounding rapidly activates the coagulase pathway. Negative feedback loop is needed once scar As a result, there is formation of blood clots on the formation is done and healing is in the latter phases. wound surface. [Robbins & Cotran, 10th ed., pg. 108] The collagen and fibrous tissue that have been ○ Clot: deposited in the area need to be degraded for Stops the bleeding remodeling. Supports migrating cells, which are attracted ○ This will allow the reconstitution of the normal by growth factors, cytokines and chemokines cells that were present before the injury occurred. released in the area. Greater amount of the following enzymes will help remodel the collagen scar, prevent excess collagen TG Note: The contents of Tables 5 and 6 were not deposition or excessive fibrosis, and help the area go discussed to us by Doc. However, they were included back to pre-injury state [Batch 2027] because Doc Andal seemed to have tackled a few of the ○ Matrix metalloproteinases (MMPs) specified events in his discussion from last year. ○ Collagenases ○ Tissue inhibitors of metalloproteinases (TIMPs) Table 5. Events in healing by first intention [Robbins and Cotran, p. 108; Trans 2027] III. CLINICAL CONTEXT Timeline Events A. Healing of Skin Wounds Sutured wounds are approximated by the There are two types of healing in the context of skin sutures wounds: Scab formation ○ Healing by First intention/primary union Hemostasis occurs immediately ○ Healing by Second Intention/secondary union Neutrophils, inflammatory cells, fibroblasts are recruited to the fibrin clot forming 24 Hours granulation tissue After Basal cells at the edge of the incision begin Surgery to show mitotic activity Within 24–48 hours, epithelial cells from both edges have begun to migrate and proliferate along the dermis, yielding a thin but continuous epithelial layer that closes the wound. Page 7 of 14 | TH: ARANZANSO | GPT TG 7 | ACUÑA, DELMONTE, GOMEZ, SALVADOR, A.  GPT 1.05 Tissue Renewal and Repair Variable amount of fibrosis Accumulation of ECM Less amount of new blood vessel formation Through myofibroblastic action, wound contraction Wound is starting to be covered up by new helps to close the wound. skin cells ○ Decreases the gap between its dermal edges Still some scab formation at the top ○ Reduces the wound surface area Macrophages in the area ○ Important feature in healing by secondary union Neutrophils have been largely ○ Wound contraction involves a network of replaced by macrophages. myofibroblasts, modified fibroblasts which have ○ The macrophages clear contractile properties extracellular debris, fibrin, Table 6. Events in healing by second intention [Robbins and Cotran, p. 109] and other foreign Timeline Events Day 3 material; promote 24 Hours Provisional matrix containing fibrin, angiogenesis; and ECM After plasma, fibronectin, and type III deposition. Surgery collagen is formed. 3–7 Days Granulation tissue progressively Matrix becomes mostly composed of invades the incision space. type I collagen. Neovascularization reaches its Original granulation tissue scaffold is peak as granulation tissue fills 2 Weeks converted into a pale, avascular scar. the incisional space. Dermal appendages that have been The new vessels are leaky, destroyed in the line of the incision are allowing the passage of plasma permanently lost. proteins and fluid into the By the Scar is made up of acellular connective Day 5 extravascular space. End of the tissue devoid of inflammatory infiltrate. ○ New granulation tissue is 1st Month ○ Covered by intact epidermis often edematous. Large skin defects may be reduced to Fibroblasts progressively Within 6 5% to 10% of their original size by migrate into the granulation Weeks wound contraction of myofibroblasts. tissue, where they proliferate and lay down collagen and ECM. B. Surgical Wound Strength Skin is fully re-epithelialized When doing surgery on a patient, a small surgical Collagen deposition is remodeled incision is done. After the surgery, this incision is Structural integrity is back to normal sutured up. Continued collagen Healing of the incision is facilitated by sutures. accumulation and fibroblast In the area where the wound occurs, collagen 2nd proliferation. synthesis should happen. Week Leukocyte infiltrate, edema, and ○ Deposition of fibrous collagen promotes and increased vascularity are imparts tensile strength to the wound. Weeks substantially diminished. Sutured wounds are 70% of normal skin strength by Scar comprises a cellular ~3 months connective tissue. ○ Devoid of inflammatory IV. FACTORS THAT INFLUENCE TISSUE End of the First cells REPAIR ○ Covered by a normal Month TG Note: Some of the descriptions as listed on the succeeding epidermis. The wound’s tensile strength table were lifted from Robbins and Cotran (10th ed., p. 108) increases with time. since Doc did not specify on most of them. Healing by Second Intention Tissue repair can be influenced by several factors, Also referred to as secondary union thereby altering the reparative process. There are Form that is delayed in one way or another systemic and local, as well as extrinsic or local ○ E.g., When you want to delay healing variables, that modify healing. The type of healing of skin wounds will also determine the types and different physio-changes in Table 7. ⭐ Factors that influence tissue repair Factors Description the patient. Clinically one of the most important Happens when cell or tissue loss is more extensive causes of delayed healing Inflammatory reaction is more intense Promotes continuous inflammation in a ○ Large tissue defects have more necrotic debris, specific area exudate, and fibrin that must be removed → large ○ E.g., A surgeon’s failure to use defects have a greater potential for secondary, Infection antiseptic systems allow inflammation-mediated injury microbes to come in → infection More granulation tissue development → affecting tissue repair → ○ To fill in a bigger gap due to the larger area of delays healing deficit Potentially increases the local tissue ○ Greater volume of granulation tissue → greater injury mass of scar tissue → large scar formation Page 8 of 14 | TH: ARANZANSO | GPT TG 7 | ACUÑA, DELMONTE, GOMEZ, SALVADOR, A.  GPT 1.05 Tissue Renewal and Repair Metabolic disease that compromises Once fibrosis is organized in body tissue repair. cavities, function will be affected. Diabetes One of the most important systemic ○ E.g., Pneumonia causes of abnormal wound healing Infection can spill over the Profound effects on repair pleural cavities organizing Nutritional Protein and vitamin C deficiency: fibrosis → affect the patient’s status ○ Inhibit collagen synthesis breathing ○ Retard healing Have antiinflammatory effects V. ABNORMALITIES IN TISSUE REPAIR Administration may result in weakness A. Chronic Wounds of the scar due to: ○ Inhibition of TGF-β production ○ Diminished fibrosis Glucocorti- However, these above-mentioned coids effects of glucocorticoids are desirable (steroids) in certain cases. ○ E.g., Corneal infections - glucocorticoids + antibiotics are prescribed to reduce the likelihood of opacity that may Figure 19. Chronic wounds illustrating defects in wound healing (A) Venous leg ulcer (B) Arterial ulcer (C) Diabetic ulcer, and (D) result from scarring Pressure sore Eg., Increased local pressure or torsion May cause wounds to pull apart or dehisce E.g., Fractured ends of the bone Mechanical continues to move factors ○ Callus, reparative tissue will not set in ○ Important to immobilize for a while Figure 20. L: Ulcer crater taken from site of venous leg ulcer Impairs healing due to its effect on R: Middle part of the ulcer zoomed in angiogenesis ○ Poor blood supply → delay Venous Leg Ulcers tissue repair Common in elderly with chronic venous hypertension Poor May be due to: Caused by severe varicose veins or congestive heart perfusion ○ Peripheral vascular disease failure ○ Arteriosclerosis Commonly accompanied by chronic inflammation and ○ Diabetes deposits of hemosiderin (iron pigment) from red cell ○ Obstructed venous drainage breakdown (e.g., in varicose veins) Poor venous circulation can affect wound healing due May result in inflammation, delaying to poor oxygen delivery to the site of ulcer wound healing Fig. 20 (L) Foreign ○ The edges of the ulcer (arrows) will show more Fragments of steel, glass, or bone bodies impede healing by perpetuating chronic inflammation Fig. 20 (R) components ○ Blood vessel formation ⭐ hanging stratified squamous epithelium of the skin Stable and labile cells → complete Type and ○ White Areas: edema with variable amounts of restoration extent of inflammatory cell Permanent cells → scarring, loss of tissue injury ○ Elongated spindle cells function Inflammation arising in tissue spaces Arterial Ulcers develops extensive exudates Develop in individuals with atherosclerosis of the Repair may occur by digestion of peripheral arteries especially associated with exudate. diabetes ○ This is initiated by proteolytic Ischemia results in atrophy and necrosis of the skin enzymes of leukocytes + and underlying tissue resorption of the liquefied Lesions are painful Location of exudate → resolution the injury In the absence of cellular necrosis, normal tissue architecture is ⭐Diabetic Ulcers May also be referred as diabetic foot generally restored Affects the lower extremities, particularly the feet In the case of larger accumulations, Presence of tissue necrosis and failure to heal as a granulation tissue grows into the result of vascular disease exudate → formation of a fibrous scar → organization Page 9 of 14 | TH: ARANZANSO | GPT TG 7 | ACUÑA, DELMONTE, GOMEZ, SALVADOR, A.  GPT 1.05 Tissue Renewal and Repair Histologically characterized by: ○ Epithelial ulceration (Fig. 20L) ⭐ Keloid Results from the inability to remodel the fibrous ○ Extensive granulation tissue in the dermis (bed of tissue. the ulcer) which is formed by: Scar tissue grows beyond the boundaries of the New blood vessels original wound and does not regress. Loose edematous connective tissue Common in African Americans and people with ↪ White spaces in between new blood darker skin color, can also occur in some Asians. vessels correspond to the presence of Keloid will form exuberant fibrous tissue covered by edema fluid hyperpigmented skin Inflammation Histologically, fibrous tissue will appear differently based on the staining: Pressure Sores ○ H&E stain: pink Happens when the patient lies in a particular position ○ Trichrome stain: blue for a long amount of time (Figure 21C) Stratified squamous epithelium is on top ○ E.g., Bedridden patients (black arrow), while bundles of dense collagen Areas of skin ulceration and necrosis of underlying bands correspond to fibrous tissue that was not tissues caused by prolonged compression of tissue remodeled (red arrow) against a bone ○ Important to remove and remodel scar tissue to Lesions are caused by mechanical pressure in a prevent this. specific area and local ischemia. ○ Pressure sores can be produced by cutting off the Under the microscope, hypertrophic scars and keloids blood supply can appear the same since they are both characterized by B. Wound Dehiscence collagen bundles. However, in hypertrophic scars, Happens when a surgical incision reopens internally collagen deposition does not grow outside the boundary or externally of that tissue. In keloid, collagen production is abundant, Risk factors: which is why it overhangs the injured area. ○ Obesity ○ Malnutrition SUMMARY/ KEY POINTS FROM DR. ANDAL ○ Infections ○ Vascular insufficiency STEP DESCRIPTION, KEY POINTS The suture strength holding the abdominal wound Inflammatory cells: chemokines, together can be affected by increased pressure neutrophils, and macrophages to Inflammation ○ Vomiting, coughing, and other physical factors remove the debris and microbes present in the area of injury C. Excessive Scar Tissue Deposition Granulation tissue Presence of new blood vessels and formation the process of angiogenesis Fibrous collagen deposited by Connective tissue fibroblasts deposition Scar contraction due to myofibroblasts DETERMINANT DESCRIPTION E.g. whether the injury is a small Nature of the injury incision or laceration or large Figure 21. (A) Hypertrophic scar (B) Keloid (C) Microscopic image of abrasion a keloid ⭐ Hypertrophic Scars Cell proliferation Whether there are stem cells present in the tissue that will allow Scar deposition (stem cell function) for faster regeneration of the tissues Normally: in the area ○ Scar formation occurs → once scar is deposited, E.g. As seen in the skin and liver there is a need to remodel that scar → remove Once the ECM is disrupted in a deposited connective tissue to allow cells and very significant fashion, there can tissues (that were present before the injury) to Integrity of the be scar formation, but the ability replace the scar tissue ECM (scar to remodel and degrade the scar Hypertrophic scar: formation, is also a key factor in how the ○ Results from the inability to remodel the fibrous remodeling) patient heals tissue ○ E.g. from burns or large This pathology is also found in keloids. abrasions ○ Excessive scar deposition Accumulation of excessive amounts of collagen Main Components of Connective Tissue Repair Often grow rapidly and has abundant myofibroblasts Angiogenesis Tend to regress over several months Fibroblast migration and proliferation Generally develop after thermal or traumatic injury Collagen synthesis involving deep layers of the dermis Connective tissue remodeling Fibrosis formed is within the boundaries of the wound Page 10 of 14 | TH: ARANZANSO | GPT TG 7 | ACUÑA, DELMONTE, GOMEZ, SALVADOR, A.  GPT 1.05 Tissue Renewal and Repair Repair by Connective Tissue Deposition 5. The histological appearance of a tissue you’re Granulation formation examining presents with fibroblasts, new Fibrosis - remodeling thin-walled capillaries, a loose ECM, and Multiple factors affecting healing (Refer to Table 7) macrophages. Which of the following is FALSE? Infection A. The tissue is in the stage of granulation tissue Diabetes formation Nutritional status B. The next step involves recruiting neutrophils Glucocorticoids (steroids) and the formation of a platelet plug Mechanical factors C. The macrophages indicate that the tissue is in Poor perfusion the latter phase of healing Foreign bodies D. The fibroblasts deposit collagen, leading to the Type and extent of tissue injury fibrosis of the tissue Location of the injury Deposition of Connective Tissue REFERENCES 6. The following are all functions of TGF-β except… Jose Jasper L. Andal MD. Notes from Tissue Renewal A. Increases synthesis of collagen and fibronectin and Repair B. Stimulates migration and proliferation of BOOKS endothelial cells Kumar, V., Abbas, A. and Aster, J. (2020) Robbins and C. Decreases degradation of ECM Cotran Pathologic Basis of Diseases. 10th Edition, D. Stimulates fibroblast migration and proliferation Elsevier 7. In the early part of scar tissue deposition, ECM REVIEW QUESTIONS degradation is promoted. This is then followed by scar contraction by myofibroblasts. Mechanisms of Tissue Repair A. Only the first statement is true. B. Only the second statement is true. 1. What are the two main mechanisms of tissue repair? C. Both statements are true. A. Regeneration and inflammation D. Both statements are false. B. Regeneration and scar formation C. Scar formation and cell death Healing of Skin Wounds D. D) Inflammation and fibrosis 8. What is the first step in the repair process of healing Tissue Proliferative Capacity by first intention? A. Maturation 2. Which of the following is an example of a labile B. Inflammation tissue? C. Proliferation A. Bone D. Remodeling B. Liver C. Neuron Factors that Influence Tissue Repair D. Skin Epithelium 9. Which of the following statements is true in relation ECM and Growth Factors to the factors that influence tissue repair? A. Deficiency in protein and vitamin D inhibit 3. Which of the following statements about the ECM is collagen synthesis, thereby retarding the healing correct? process. A. The ECM has no role in cell proliferation B. Permanent cells will result in complete B. The ECM provides structural support but does restoration, whereas stable and labile cells will not influence growth factors result in loss of function. C. The ECM elaborates growth factors that are C. Diabetes may impede healing due to poor blood crucial for cell proliferation supply. D. The ECM is not required for liver regeneration D. In healing, glucocorticoids should always be avoided due to inhibition of TGF-β production Steps in Scar Formation and diminished fibrosis. 4. Which of the following is NOT a function of Abnormalities in Tissue Repair fibroblast growth factors? A. Stimulates the proliferation of endothelial cells 10. Keloid and arterial ulcers appear similar B. Promotes the migration of macrophages and histologically because both have collagen bundles. In fibroblasts to the damaged area terms of excessive scar tissue deposition, C. Promotes the breakdown of the basement hypertrophic scars usually regress because they membrane grow beyond the boundary of the wound. D. Stimulates epithelial cell migration to cover A. Only the first statement is true. epidermal wounds B. Only the second statement is true. Page 11 of 14 | TH: ARANZANSO | GPT TG 7 | ACUÑA, DELMONTE, GOMEZ, SALVADOR, A.  GPT 1.05 Tissue Renewal and Repair C. Both statements are true. 8. B - Recall D. Both statements are false. 9. C- Protein deficiency and vitamin C deficiency ANSWERS/RATIONALE Stable and labile cells → complete restoration. Permanent cells → scarring, loss of function. 1. B - Recall 2. D - Recall Effects of glucocorticoids are desirable in certain 3. C - Recall cases. 4. C - Recall. The breakdown of the basement E.g., Corneal infections membrane is stimulated by matrix 10. D - metalloproteinases Hypertrophic scars and keloids can appear the 5. B - Recruiting neutrophils and the formation of a same since they are both characterized by platelet plug occurs in the first step, prior to the collagen bundles formation of the granulation tissue. 11. Fibrosis formed is within the boundaries of the 6. B - Recall wound 7. D - In the early stage of scar tissue deposition, decrease of ECM degradation is promoted. ECM degradation occurs in the later stage of scar tissue deposition SUMMARY 2 PROCESSES OF TISSUE REPAIR Tissues can replace damaged components and essentially return to normal state Repairs through: Regeneration ○ Proliferation of differentiated cells ○ Differentiation and proliferation of stem cells Injured tissue is incapable of regeneration Supporting structures are too severely damaged and cannot support regeneration Scar Formation Repairs through: ○ Fibrosis ○ Collagen deposition DETERMINANTS OF TISSUE REPAIR Nature of the injury Give rise to various differentiated tissues In vivo: different cell lines mature into every possible tissue in an Embryonic Stem cells infant Cell proliferation In vitro: can be cultured and given growth factors to become different cell lines Only have the capacity to replace damaged cells and maintain cell Adult Stem cells populations within the tissues where they reside ECM is the architectural structure or scaffold of the tissue Also have key roles in: ○ Cell proliferation by elaborating growth factors ECM and Growth Factors ○ Responding to injury and trauma ○ Proliferating blood vessels for the oxygen and nutrient supply of cells STEPS IN SCAR FORMATION Tissue injury acts as the initiating factor Platelet plug-hemostasis ○ Plugs blood vessels and lost tissue to prevent further blood loss and promote hemostasis Inflammation Chemokines attract neutrophils Inflammation removes microbes, debris ○ Removed by macrophages and neutrophils ○ Inflammation resolved when cleared Occurs after inflammation is resolved Cells proliferate and migrate to close the clean wound ○ Epithelial cells Cell proliferation respond to locally produced growth factors and migrate over the wound to cover it up ○ Pericytes and Endothelial Cells proliferate to form new blood vessels, a process known as Angiogenesis. Page 12 of 14 | TH: ARANZANSO | GPT TG 7 | ACUÑA, DELMONTE, GOMEZ, SALVADOR, A.  GPT 1.05 Tissue Renewal and Repair ○ Fibroblasts lays down collagen fibers that form the scar Granulation Tissue Formation Formation of ○ Migration and proliferation of fibroblasts Granulation Tissue ○ Deposition of loose connective tissue ○ Vessels and interspersed mononuclear leukocytes Steps in deposition of connective tissue: ○ Migration and proliferation of fibroblasts into the site of injury ○ Deposition of ECM proteins TGF-β ○ One of the most important cytokines for the synthesis and deposition of connective tissue proteins ○ Stimulates fibroblast migration and proliferation ○ Increases synthesis of collagen and fibronectin ○ Decreases degradation of ECM by inhibiting metalloproteinases Fibroblasts Deposition of ○ Elaborate and increase ECM synthesis Connective Tissue ○ Acquire features of smooth muscle cells (Actin filament) ○ Marked ability to differentiate and form myofibroblasts. Myofibroblasts ○ Myofibroblast= fibroblast with actin filament ○ Continue to Scar contraction Remodeling of Connective Tissue amount of the following enzymes will help remodel the collagen scar, prevent excess collagen deposition or excessive fibrosis and help the area of back to pre-injury state: ○ Matrix metalloproteinases (MMPs) ○ Collagenases ○ Tissue inhibitors of metalloproteinases (TIMPs) CLINICAL CONTEXT HEALING OF SKIN WOUNDS Healing by First Intetion Healing by Second Intention When the injury involves only the epithelial layer, the principal When cell or tissue loss is more extensive mechanism of repair is epithelial regeneration or primary union Also called secondary union Repair consists of three connected processes: Inflammatory reaction is more intense ○ Inflammation Development of granulation tissue ○ Proliferation Accumulation of ECM and formation of a large scar ○ Maturation Would contraction by the action of myofibroblasts Surgical wound strength 70% of normal skin strength Collagen synthesis responsible for producing tensile strength to help close the wound Wound strength is 70 - 80% normal by 3 months FACTORS INFLUENCING TISSUE REPAIR Clinically one of the most important causes of delayed healing Promotes continuous inflammation in a specific area Infection ○ E.g., A surgeon’s failure to use antiseptic systems allow microbes to come in → infection → affecting tissue repair → delays healing Potentially increases the local tissue injury Metabolic disease that compromises tissue repair. Diabetes One of the most important systemic causes of abnormal wound healing. Profound effects on repair Protein deficiency and vitamin C deficiency: Nutritional status ○ Inhibit collagen synthesis ○ Retard healing Have antiinflammatory effects Administration may result in weakness of the scar due to: ○ Inhibition of TGF-β production Glucocorticoids ○ Diminished fibrosis (steroids) However, these above-mentioned effects of glucocorticoids are desirable in certain cases. ○ E.g., Corneal infections - glucocorticoids + antibiotics are prescribed to reduce the likelihood of opacity that may result from scarring Page 13 of 14 | TH: ARANZANSO | GPT TG 7 | ACUÑA, DELMONTE, GOMEZ, SALVADOR, A.  GPT 1.05 Tissue Renewal and Repair Example includes increased local pressure or torsion May cause wounds to pull apart or dehisce Mechanical factors E.g., Fractured ends of the bone continues to move ○ Callus, reparative tissue will not set in ○ Important to immobilize for a while. Impairs healing due to its effect on angiogenesis ○ Poor blood supply → delay tissue repair May be due to: Poor perfusion ○ Peripheral vascular disease ○ Arteriosclerosis ○ Diabetes ○ Obstructed venous drainage (e.g., in varicose veins) May result in inflammation, delaying wound healing. Foreign bodies Fragments of steel, glass, or bone impede healing by perpetuating chronic inflammation. Type and extent of Stable and labile cells → complete restoration. tissue injury Permanent cells → scarring, loss of function. Inflammation arising in tissue spaces develops extensive exudates. Repair may occur by digestion of exudate. ○ This is initiated by proteolytic enzymes of leukocytes + resorption of the liquefied exudate → resolution In the absence of cellular necrosis, normal tissue architecture is generally restored Location of the In the case of larger accumulations, granulation tissue grows into the exudate → formation of a injury fibrous scar → organization Once fibrosis is organized in body cavities, function will be affected. ○ E.g., Pneumonia Infection can spill over the pleural cavities organizing fibrosis → affect the patient’s breathing. ABNORMALITIES IN TISSUE REPAIR Venous Leg Ulcer Common in elderly with chronic venous hypertension Caused by severe varicose veins or congestive heart failure Arterial Ulcers Develop in individuals with atherosclerosis of the peripheral arteries especially associated with diabetes Ischemia results in atrophy and necrosis of the skin and underlying tissue Chronic wounds Diabetic ulcers Affects the lower extremities (particularly the feet) Presence of tissue necrosis and failure to heal as a result of vascular disease Pressure sores Common in patients who are bedridden for a long time Areas of skin ulceration and necrosis of underlying tissues caused by prolonged compression of tissue against a bone Lesions are caused by mechanical pressure and local ischemia When a surgical incision reopens internally or externally Wound Dehiscence Risk factors: obesity, malnutrition, infections, and vascular insufficiency The suture strength holding the abdominal wound together can be affected by increased pressure Hypertrophic scars Accumulation of excessive amounts of collagen Often grow rapidly and has abundant myofibroblasts Excessive Scar Fibrosis formed is within the boundaries of the wound Tissue Deposition Keloid Scar tissue grows beyond the boundaries of the original wound and does not regress Keloid will form exuberant fibrous tissue covered by hyperpigmented skin FREEDOM WALL Page 14 of 14 | TH: ARANZANSO | GPT TG 7 | ACUÑA, DELMONTE, GOMEZ, SALVADOR, A.