Tissue_Repair2023_James Lyons.pptx

Tissue Repair Dr. James Lyons Associate Dean of Medical Education Professor of Pathology & Family Medicine Objectives • • • • • • • • • • • • • Describe the cardinal features of inflammation Define healing, repair, regeneration, scarring Describe factors that can impair tissue repair Understand t...

Tissue Repair Dr. James Lyons Associate Dean of Medical Education Professor of Pathology & Family Medicine Objectives • • • • • • • • • • • • • Describe the cardinal features of inflammation Define healing, repair, regeneration, scarring Describe factors that can impair tissue repair Understand the phases of the cell cycle Describe the three types of cells based on proliferative capacity Describe the conditions required for healing by scar formation (fibrosis) and healing by regeneration Discuss some key growth factors involved in healing Describe the ECM and the types of collagen Describe the sequence of events involved in repair by scarring Describe the sequence of events in cutaneous wound healing Discuss cutaneous wound healing by first intention and second intention Describe embryonic and adult stem cells Describe adult stem cell niches Inflammation • Cardinal features • • • • • RUBOR TUMOR CALOR DOLOR FUNCTIO redness vasodilation, stasis swelling edema, cellular infiltration warmth vasodilation, fever pain bradykinin, prostaglandins LAESA loss of function (may be permanent) • Reaction in tissue that consists mainly of responses of blood vessels and leukocytes • Designed to help get rid of foreign invaders and damaged or necrotic tissues Tissue Damage • May be the result of inflammatory process • Example: A bacterial infection with abscess formation and necrosis • May be the result of a non-inflammatory process • Example: Diphtheria toxin in an immunosuppressed person (cannot mount an inflammatory response due to immunosuppression) Overview TISSUE DAMAGE Terms • Healing • An outcome of tissue damage that is characterized by regeneration and/or scarring • Repair • A term often used as a synonym for "healing” • Others use the term to refer exclusively to scarring, e.g. "repair by scar formation", "repair by fibrosis " Terms • Regeneration • Replacement of injured cells by healthy cells of same type • Full reconstitution of architecture and function • Regrowth of amputated limb in amphibian • Scarring • Replacement of injured cells by fibrous connective tissue • This process ultimately leads to formation of a scar • Occurs in tissue not capable of healing by regeneration Healing/Repair • Healing of an injured tissue may occur solely by either regeneration or scarring; in most cases both processes are involved, with one process usually predominating over the other Factors That Can Impair Tissue Repair • Persistent infection • Diabetes • Malnutrition • Corticosteroids • Mechanical factors such as local pressure • Poor blood flow • Foreign bodies Review of Cell Cycle • Cell cycle represents phases that a cell experiences while it prepares for mitosis. • S = cell is actively synthesizing DNA. • G1= presynthetic phase • G2 = postsynthetic (premitotic) phase • G0 = intermitotic (resting) phase. Cell is "out" of the cell cycle Cells of Body Are Divided Into Three Groups Based On Proliferative Capacity • Labile cells • Stable (quiescent) cells • Permanent (non-dividing) cells Labile Cells Short G0 Continually dividing throughout life Examples of tissues with labile cells Bone marrow (contains stem cells which have an unlimited capacity to proliferate) epithelium (skin, oral cavity, vagina, cervix, urinary tract) mucosa of gastrointestinal tract Stable Cells • Stable (Quiescent) Cells • Normally have a low level of replication • Prolonged G0 (sometimes for years) • They re-enter cell cycle only under certain conditions, such as healing • Include: • parenchymal cells of liver, pancreas, kidney • fibroblasts, smooth muscle cells • vascular endothelial cells Permanent Cells • Permanent (Non-dividing) Cells • Have left cell cycle forever • Cannot undergo mitosis postnatally • Examples: the bulk of cardiac muscle cells, skeletal muscle cells, neurons Healing By Regeneration • Can occur only if 3 conditions are met: • Tissue is composed of labile or stable cells • Area of injury must contain some surviving (viable) cells that are capable of undergoing cell division • The connective tissue framework, to which the tissue's cells are attached, must be intact --> this framework (basement membrane) serves as scaffolding for the replicating cells Healing By Scarring (Fibrosis) • Occurs in injured tissues incapable of regeneration, either because • The tissue consists of permanent cells AND/OR • No surviving tissue cells remain AND/OR • Connective tissue framework is disrupted/destroyed Tissue Repair Mechanisms of tissue repair: regeneration and scar formation. Following a mild injury, which damaged the epithelium but not the underlying tissue, repair occurs by regeneration, but after more severe injury with damage to the connective tissue, repair is by scar formation. Healing By Scarring (Fibrosis) • Consists of multiple components • Inflammation is often involved • Formation of new blood vessels (angiogenesis; neovascularization) • Migration and proliferation of fibroblasts • Deposition and organization of fibrous tissue into a mature scar (remodeling) Growth Factors • Polypeptides • May promote cell proliferation by recruiting G0 cells into the cell cycle (mitogenic) • May inhibit cell proliferation by blocking cells from entering the cell cycle • They may influence other cell functions besides growth, e.g. cell differentiation, cell migration through tissues (chemotactic), angiogenesis Growth Factors Involved In Repair/Regeneration Table 1-1 Robbins Pathologic Basis of Disease Some Key Growth Factors • Vascular endothelial growth factors (VEGFs) • Important in angiogenesis • VEGF-A is generally referred to as VEGF and it is the major angiogenic factor • Stimulates proliferation of endothelial cells and promotes growth of new blood vessels (capillary sprouting), increases vascular permeability • Hypoxia is the most important inducer of VEGF production • Fibroblast growth factors (FGFs) • Important in wound healing • Stimulate proliferation and migration of fibroblasts to damaged area • Stimulate angiogenesis and ECM protein synthesis Some Key Growth Factors • Platelet derived growth factor (PDGF) • Stimulates fibroblast and endothelial cells proliferation and is chemotactic for neutrophils, macrophages, smooth muscle and fibroblasts, thereby promoting recruitment of these cells to sites of inflammation/injury • Stimulates ECM protein synthesis • Transforming growth factor-B (TGF-B) • Suppresses acute inflammation, inhibits lymphocyte proliferation • Chemotactic for leukocytes and fibroblasts, pleiotropic • Stimulates synthesis of collagen, fibronectin, proteoglycans • Involved in and drives scar formation and fibrosis Extracellular Matrix (ECM) • Tissues have two interacting components: cells and extracellular matrix (ECM). • The ECM consists of many kinds of macromolecules, most of which form complex structures, such as collagen fibrils. • The ECM supports the cells and contains the fluid transporting nutrients to the cells and carrying away their wastes and secretory products. • Important ECM components include collagen, elastin, fibronectin, laminin, integrins, and proteoglycans Khan Academy Video: https://www.youtube.com/watch?v=cMNx17 H3dRU Collagen Types • Type I – Bone, Skin, Tendon, Scar • Type II – Cartilage (Cartwolage) • Type III – Forms Reticulin, in skin, blood vessels, fetal tissue • Type IV – Basement Membrane Acronym: Be So Totally Cool Read Books Sequence of cellular / biochemical events involved in repair by scarring • After tissue injury, fibroblasts and vascular endothelial cells proliferate • This leads to the formation (usually beginning by 3 - 7 days after injury) of granulation tissue, so-called because of its pink, soft, granular appearance Histology of Granulation Tissue • Important features of granulation tissue: • Proliferation of newly formed small blood vessels (angiogenesis) • The new vessels originate, in part, from "budding" of preexisting blood vessels that are within the area of repair • The new vessels are immature and have leaky interendothelial junctions, allowing an exudate to form --> therefore granulation tissue is edematous • Angiogenesis involves several growth factors --> these factors stimulate endothelial mitosis and migration of the new blood vessels • Fibroblast proliferation • Synthesize Type III collagen (flexible) • Admixed inflammatory cells (mainly macrophages) • Myofibroblast proliferation • Important in wound contraction Granulation Tissue https://en.wikipedia.org/wiki/Granulation_tissue Granulation Tissue http://library.med.utah.edu/WebPath/TUTORIAL/MYOCARD/MI025. html Sequence of cellular / biochemical events involved in repair by scarring • Weeks to months after onset of injury, granulation tissue is transformed into a mature scar • Amount of collagen, as well as other extracellular matrix proteins increases (note: the extracellular matrix is the non-cellular component present within all tissues and organs) • Type I collagen (less flexible) replaces Type III collagen (more flexible) • Number of fibroblasts, as well as vascularity, decreases • Fully mature scar is practically avascular, has very few cells, and has much dense collagen --> it has been remodeled Steps In Scar Formation Steps in repair by scar formation. Injury to a tissue, such as muscle (which has limited regenerative capacity), first induces inflammation, which clears dead cells and microbes, if any. This is followed by the formation of vascularized granulation tissue and then the deposition of extracellular matrix to form the scar. Cutaneous Wound Healing Healing by first intention  small wounds such as a cut where the predominant healing is by regeneration, wound edges are close together Cutaneous Wound Healing • Scalpel blade injures a limited number of epidermal cells, and also injures the underlying dermal connective tissue • The incisional space quickly fills with blood and hemostasis occurs • Vasoconstriction (short-lived, 5 to 10 minutes), platelet plug, fibrin clot • Within 24 hours, cellular evidence of acute inflammation (neutrophils) appears • Involves increased vascular permeability, chemotaxis, vasodilation Cutaneous Wound Healing • By day 3, the acute inflammatory response is replaced by chronic inflammation (macrophages) • Also at 3 days, granulation tissue usually begins forming, courtesy of growth factors secreted by platelets, macrophages and endothelial cells. • By day 7, granulation tissue has typically filled the incisional space Cutaneous Wound Healing • During the second week collagen accumulates while the leukocytes and increased vascularity begin to disappear • By the end of the first month, the connective tissue scar is formed and the epidermis is fully regenerated • Healing by regeneration and scarring is complete Cutaneous Wound Healing Healing by second intention  larger wounds such as a pressure ulcer (bedsore) where there is a greater degree of healing by scarring and less healing is regeneration. Wound edges are farther apart, the wound often contracts as it heals. Stem Cells • Progenitor cells • First identified in embryos (embryonic stem cells) • Unlimited proliferative capacity • Asymmetric replication • In every division one cell retains self-renewing capacity and the other cell enters a differentiation pathway Embryonic Stem Cells • Pluripotent , can give rise to all tissues of the body • Can be isolated from blastocysts (structures formed at about the 32-cell stage of development) • Can be maintained in culture as undifferentiated cells or may be induced to differentiate • Have made possible the production of knockout mice • a specific gene is inactivated or deleted from cultured embryonic stem cells • these cells are injected into blastocysts which are implanted into uterus of surrogate Embryonic Stem Cells • Potential for repopulating damaged organs -> therapeutic cloning Adult Stem Cells (AKA Tissue Stem Cells) • Have a more restricted differentiation capacity • Generally Lineage Specific • Adult stem cells in a tissue can generally only produce cells that are normal constituents of that tissue • However stem cells with broad differentiation potential have been found in adult bone marrow • Located in “niches” • crypts in the small and large intestine • hair follicle bulge, sebaceous glands, interfollicular lower epidermis • limbus of cornea • canals of Hering in liver Stem Cell Niches Stem Cell Niches Stem Cell Niches Prometheus Prometheus is a character from Greek mythology who —in defiance of Zeus—stole fire from Olympus and gave it to mankind. As punishment Zeus had Prometheus chained to a rock where an eagle tore out his liver. Each night his liver grew back, and each day the eagle returned to have his meal… Circa 560 - 550 BC. References • 1. Kumar V, Abbas A, Aster A, Robbins and Cotran Pathologic Basis of Disease, 10th edition, Elsevier Saunders, 2021 • 2. Kumar K, Abbas A, Fausto N., Mitchell R., Robbins Basic Pathology, 8th edition, Elsevier Saunders 2007 • 3. WebPath https://webpath.med.utah.edu/ • 4. Pathoma • 5. Access Medicine • Special thanks to Paul Murphy, M.D. - Department of Pathology, University of Kentucky, Lexington, KY Thank You! Questions?