Pocket Companion to Robbins and Cotran Pathologic Basis of Disease PDF

POCKET COMPANION TO Robbins and Cotran Pathologic Basis of Disease Eighth Edition Richard N. Mitchell, MD, PhD Associate Professor, Department of Pathology Harvard Medical School and Health Sciences and Technology Director, Human Pathology...

POCKET COMPANION TO Robbins and Cotran Pathologic Basis of Disease Eighth Edition Richard N. Mitchell, MD, PhD Associate Professor, Department of Pathology Harvard Medical School and Health Sciences and Technology Director, Human Pathology Harvard-MIT Division of Health Sciences and Technology Staff Pathologist, Brigham and Women’s Hospital Boston, Massachusetts Vinay Kumar, MBBS, MD, FRCPath Alice Hogge and Arthur Baer Professor Chairman, Department of Pathology Executive Vice Dean, Division of Biologic Sciences and The Pritzker School of Medicine The University of Chicago Chicago, Illinois Abul K. Abbas, MBBS Professor and Chairman, Department of Pathology University of California, San Francisco San Francisco, California Nelson Fausto, MD Professor and Chairman, Department of Pathology University of Washington School of Medicine Seattle, Washington Jon C. Aster, MD, PhD Professor of Pathology Harvard Medical School Brigham and Women’s Hospital Boston, Massachusetts With Illustrations by James A. Perkins, MS, MFA 1600 John F. Kennedy Blvd. Ste 1800 Philadelphia, PA 19103-2899 POCKET COMPANION TO ROBBINS AND COTRAN PATHOLOGIC BASIS OF DISEASE ISBN: 978-1-4160-5454-2 Copyright © 2012, 2006, 1999, 1995, 1991 by Saunders, an imprint of Elsevier Inc. All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Details on how to seek permission, further information about the Publisher’s permissions policies and our arrangements with organizations such as the Copyright Clearance Center and the Copyright Licensing Agency, can be found at our website: www.elsevier.com/permissions. This book and the individual contributions contained in it are protected under copyright by the Publisher (other than as may be noted herein). Notices Knowledge and best practice in this field are constantly changing. As new research and experience broaden our understanding, changes in research methods, professional practices, or medical treatment may become necessary. Practitioners and researchers must always rely on their own experience and knowledge in evaluating and using any information, methods, compounds, or experiments described herein. In using such information or methods they should be mindful of their own safety and the safety of others, including parties for whom they have a professional responsibility. 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ISBN: 978-1-4160-5454-2 Executive Editor: William Schmitt Managing Editor: Rebecca Gruliow Publishing Services Manager: Julie Eddy Senior Project Manager: Laura Loveall Design Direction: Ellen Zanolle Printed in the United States Last digit is the print number: 9 8 7 6 5 4 3 2 1  Contributors Charles E. Alpers, MD Professor of Pathology, Adjunct Professor of Medicine, University of Washington School of Medicine; Pathologist, University of Washington Medical Center, Seattle, WA The Kidney Douglas C. Anthony, MD, PhD Professor and Chair, Department of Pathology and Anatomical Sciences, University of Missouri, Columbia, MO Peripheral Nerve and Skeletal Muscle; The Central Nervous System James M. Crawford, MD, PhD Senior Vice President for Laboratory Services; Chair, Department of Pathology and Laboratory Medicine, North Shore–Long Island Jewish Health System, Manhasset, NY Liver and Biliary Tract Umberto De Girolami, MD Professor of Pathology, Harvard Medical School; Director of Neuropathology, Brigham and Women’s Hospital, Boston, MA Peripheral Nerve and Skeletal Muscle; The Central Nervous System Lora Hedrick Ellenson, MD Weill Medical College of Cornell University, Professor of Pathology and Laboratory Medicine; Attending Pathologist, New York Presbyterian Hospital, New York, NY The Female Genital Tract Jonathan I. Epstein, MD Professor of Pathology, Urology, and Oncology; The Reinhard Professor of Urologic Pathology, The Johns Hopkins University School of Medicine; Director of Surgical Pathology, The Johns Hopkins Hospital, Baltimore, MD The Lower Urinary Tract and Male Genital System Robert Folberg, MD Dean, Oakland University William Beaumont School of Medicine, Rochester, MI; Chief Academic Officer, Beaumont Hospitals, Royal Oak, MI The Eye Matthew P. Frosch, MD, PhD Associate Professor of Pathology, Harvard Medical School; Director, C.S. Kubik Laboratory for Neuropathology, Massachusetts General Hospital, Boston, MA Peripheral Nerve and Skeletal Muscle; The Central Nervous System Ralph H. Hruban, MD Professor of Pathology and Oncology, The Sol Goldman Pancreatic Cancer Research Center, The Johns Hopkins University School of Medicine, Baltimore, MD The Pancreas iii iv Contributors Aliya N. Husain, MBBS Professor, Department of Pathology, Pritzker School of Medicine, The University of Chicago, Chicago, IL The Lung Christine A. Iacobuzio-Donahue, MD, PhD Associate Professor of Pathology and Oncology, The Sol Goldman Pancreatic Cancer Research Center, The Johns Hopkins University School of Medicine, Baltimore, MD The Pancreas Alexander J.F. Lazar, MD, PhD Assistant Professor, Department of Pathology and Dermatology, Sections of Dermatopathology and Soft Tissue Sarcoma Pathology, Faculty of Sarcoma Research Center, University of Texas M.D. Anderson Cancer Center, Houston, TX The Skin Susan C. Lester, MD, PhD Assistant Professor of Pathology, Harvard Medical School; Chief, Breast Pathology, Brigham and Women’s Hospital, Boston, MA The Breast Mark W. Lingen, DDS, PhD Associate Professor, Department of Pathology, Pritzker School of Medicine, The University of Chicago, Chicago, IL Head and Neck Chen Liu, MD, PhD Associate Professor of Pathology, Immunology and Laboratory Medicine; Director, Gastrointestinal and Liver Pathology, The University of Florida College of Medicine, Gainesville, FL Liver and Biliary Tract Anirban Maitra, MBBS Associate Professor of Pathology and Oncology, The Johns Hopkins University School of Medicine; Pathologist, The Johns Hopkins Hospital, Baltimore, MD Diseases of Infancy and Childhood; The Endocrine System Alexander J. McAdam, MD, PhD Assistant Professor of Pathology, Harvard Medical School; Medical Director, Infectious Diseases Diagnostic Laboratory, Children’s Hospital Boston, Boston, MA Infectious Diseases Richard N. Mitchell, MD, PhD Associate Professor, Department of Pathology, Harvard Medical School; Director, Human Pathology, Harvard-MIT Division of Health Sciences and Technology, Harvard Medical School; Staff Pathologist, Brigham and Women’s Hospital, Boston, MA Hemodynamic Disorders, Thromboembolic Disease, and Shock; Blood Vessels; The Heart George F. Murphy, MD Professor of Pathology, Harvard Medical School; Director of Dermatopathology, Brigham and Women’s Hospital, Boston, MA The Skin  Contributors v Edyta C. Pirog, MD Associate Professor of Clinical Pathology and Laboratory Medicine, New York Presbyterian Hospital-Weil Medical College of Cornell University; Associate Attending Pathologist, New York Presbyterian Hospital, New York, NY The Female Genital Tract Andrew E. Rosenberg, MD Professor, Department of Pathology, Harvard Medical School; Pathologist, Massachusetts General Hospital, Boston, MA Bones, Joints, and Soft-Tissue Tumors Frederick J. Schoen, MD, PhD Professor of Pathology and Health Sciences and Technology, Harvard Medical School; Director, Cardiac Pathology and Executive Vice Chairman, Department of Pathology, Brigham and Women’s Hospital, Boston, MA Blood Vessels; The Heart Arlene H. Sharpe, MD, PhD Professor of Pathology, Harvard Medical School; Chief, Immunology Research Division, Department of Pathology, Brigham and Women’s Hospital, Boston, MA Infectious Diseases Thomas Stricker, MD, PhD Orthopedic Pathology Fellow, Department of Pathology, Pritzker School of Medicine, The University of Chicago, Chicago, IL Neoplasia Jerrold R. Turner, MD, PhD Professor and Associate Chair, Department of Pathology, Pritzker School of Medicine, The University of Chicago, Chicago, IL The Gastrointestinal Tract This page intentionally left blank  Preface Robbins and Cotran Pathologic Basis of Disease (AKA, the Big Book) has long been a fundamental text for students of medicine around the world; with publication of the eighth edition, it entered its Golden Jubilee 50th year—vibrant and vigorous in its illumination of the “molecular basis of human disease with clinical correlations.” While the Pocket Companion may not (yet) have the same storied history, it nevertheless provides an important and useful adjunct to the parent volume. Initially an offspring of the fourth edition in 1991 (identified then as just Robbins Patho- logic Basis of Disease), the Pocket Companion was born of the recog- nition that the immense wealth of information about human disease somehow needed to be succinctly organized and made accessible for the overwhelmed medical student and harried house officer. This edition of the Pocket Companion carries on that tradi- tion and, as before, is intended to be much more than a simple topical outline. In assembling this update, four major objectives have guided the writing: Make the detailed expositions in Robbins and Cotran Pathologic Basis of Disease easier to digest by providing a condensed overview. Facilitate the use of the Big Book by providing the relevant cross-referenced page numbers. Help readers identify the core material that requires their pri- mary attention. Serve as a handy tool for quick review of a large body of information. In the age of Wikipedia and other online data compendiums, it is obviously not difficult to just find information; to be sure, the Pocket Companion is also available in a readily searchable digital format. However, what the 21st century student of pathology needs is an organized, pithy, and easy-to-digest synopsis of the pertinent concepts and facts with specific links to the definitive material in a more expansive volume. This eighth edition of the Pocket Companion hopefully accomplishes that end. It has been completely rewritten, reflecting all the innovations and new knowledge encompassed in the parent tome. Illustrative tables and figures have also been included wher- ever possible to reduce the verbiage. Although as before, the beau- tiful gross and histologic images of the Big Book are not reproduced. Pains have also been taken to present all the material with the same stylistic voice; the organization of the material and level of detail is considerably more uniform between chapters than in previous editions. In doing so, we hope that the Pocket Compan- ion retains the flavor and excitement of the Big Book—just in a more bite-size format—and truly is a suitable “companion.” vii viii Preface In closing, the authors specifically wish to acknowledge the invaluable assistance and editing skills (and infinite patience) of Rebecca Mitchell and Becca Gruliow; without their help and col- laboration, this edition of the Pocket Companion might still be in gestation. Rick Mitchell Vinay Kumar Abul Abbas Nelson Fausto Jon Aster  Contents General Pathology 1 Cellular Responses to Stress and Toxic Insults: Adaptation, Injury, and Death, 3 2 Acute and Chronic Inflammation, 24 3 Tissue Renewal, Regeneration, and Repair, 49 4 Hemodynamic Disorders, Thromboembolic Disease, and Shock, 66 5 Genetic Disorders, 84 6 Diseases of the Immune System, 109 7 Neoplasia, 145 8 Infectious Diseases, 177 9 Environmental and Nutritional Diseases, 208 10 Diseases of Infancy and Childhood, 236 Systemic Pathology: Diseases of Organ Systems 11 Blood Vessels, 259 12 The Heart, 282 13 Diseases of White Blood Cells, Lymph Nodes, Spleen, and Thymus, 309 14 Red Blood Cells and Bleeding Disorders, 343 15 The Lung, 363 16 Head and Neck, 391 17 The Gastrointestinal Tract, 400 ix x Contents 18 Liver and Biliary Tract, 437 19 The Pancreas, 466 20 The Kidney, 472 21 The Lower Urinary Tract and Male Genital System, 501 22 The Female Genital Tract, 517 23 The Breast, 539 24 The Endocrine System, 552 25 The Skin, 590 26 Bones, Joints, and Soft-Tissue Tumors, 612 27 Peripheral Nerve and Skeletal Muscle, 642 28 The Central Nervous System, 657 29 The Eye, 690  General Pathology This page intentionally left blank  1 Cellular Responses to Stress and Toxic Insults: Adaptation, Injury, and Death Introduction (p. 4) Pathology is the study of the structural and functional causes of human disease. The four aspects of a disease process that form the core of pathology are: The cause of a disease (etiology) The mechanism(s) of disease development (pathogenesis) The structural alterations induced in cells and tissues by the disease (morphologic change) The functional consequences of the morphologic changes (clinical significance) Overview (p. 5) Normal cell function requires a balance between physiologic demands and the constraints of cell structure and metabolic capac- ity; the result is a steady state, or homeostasis. Cells can alter their functional state in response to modest stress to maintain the steady state. More excessive physiologic stresses, or adverse pathologic stimuli (injury), result in (1) adaption, (2) reversible injury, or (3) irreversible injury and cell death (Table 1-1). These responses may be considered a continuum of progressive impairment of cell structure and function. Adaptation occurs when physiologic or pathologic stressors induce a new state that changes the cell but otherwise preserves its viability in the face of the exogenous stimuli. These changes include: Hypertrophy (increased cell mass, p. 7) Hyperplasia (increased cell number, p. 8) Atrophy (decreased cell mass, p. 9) Metaplasia (change from one mature cell type to another, p. 10) Reversible injury denotes pathologic cell changes that can be restored to normalcy if the stimulus is removed or if the cause of injury is mild. Irreversible injury occurs when stressors exceed the capacity of the cell to adapt (beyond a point of no return) and denotes per- manent pathologic changes that cause cell death. Cell death occurs primarily through two morphologic and mech- anistic patterns denoted necrosis and apoptosis (Table 1-2). 3 4 General Pathology TABLE 1-1 Cellular Responses to Injury Nature of Injurious Stimulus Cellular Response Altered physiological stimuli; some Cellular adaptations nonlethal, injurious stimuli Increased demand, increased Hyperplasia, hypertrophy stimulation (e.g., by growth factors, hormones) Decreased nutrients, decreased Atrophy stimulation Chronic irritation (physical Metaplasia or chemical) Reduced oxygen supply; chemical Cell injury injury; microbial infection Acute and transient Acute reversible injury Cellular swelling fatty change Irreversible injury ! cell death Necrosis Apoptosis Metabolic alterations, genetic Intracellular accumulations; or acquired; chronic injury calcification Cumulative, sublethal injury over Cellular aging long life span TABLE 1-2 Features of Necrosis and Apoptosis Feature Necrosis Apoptosis Cell size Enlarged (swelling) Reduced (shrinkage) Nucleus Pyknosis ! Fragmentation into karyorrhexis ! nucleosome-size karyolysis fragments Plasma Disrupted Intact; altered membrane structure, especially orientation of lipids Cellular contents Enzymatic digestion; Intact; may be released may leak out of cell in apoptotic bodies Adjacent Frequent No inflammation Physiologic or Invariable pathologic Often physiologic, pathologic (culmination of means of eliminating role irreversible cell injury) unwanted cells; may be pathologic after some forms of cell injury, especially DNA damage Although necrosis always represents a pathologic process, apo- ptosis may also serve a number of normal functions (e.g., in embryogenesis) and is not necessarily associated with cell injury. Necrosis is the more common type of cell death, involving severe cell swelling, denaturation and coagulation of proteins, breakdown of cellular organelles, and cell rupture. Usually, a large number of  Cellular Responses to Stress and Toxic Insults 5 cells in the adjoining tissue are affected, and an inflammatory infil- trate is recruited. Apoptosis occurs when a cell dies by activation of an internal “suicide” program, involving an orchestrated disassembly of cellu- lar components; there is minimal disruption of the surrounding tissue and there is minimal, if any, inflammation. Morphologically, there is chromatin condensation and fragmentation. Autophagy is an adaptive response of cells to nutrient deprivation; it is essentially a self-cannibalization to maintain viability. However, it can also culminate in cell death and is invoked as a cause for cell loss in degenerative disorders of muscle and nervous system (p. 32). Causes of Cell Injury (p. 11) Oxygen deprivation (hypoxia) affects aerobic respiration and therefore ability to generate adenosine triphosphate (ATP). This extremely important and common cause of cell injury and death occurs as a result of: Ischemia (loss of blood supply) Inadequate oxygenation (e.g., cardiorespiratory failure) Loss of oxygen-carrying capacity of the blood (e.g., anemia, carbon monoxide poisoning) Physical agents, including trauma, heat, cold, radiation, and electric shock (Chapter 9) Chemical agents and drugs, including therapeutic drugs, poisons, environmental pollutants, and “social stimuli” (alcohol and narcotics) Infectious agents, including viruses, bacteria, fungi, and parasites (Chapter 8) Immunologic reactions, including autoimmune diseases (Chapter 6) and cell injury following responses to infection (Chapter 2) Genetic derangements, such as chromosomal alterations and specific gene mutations (Chapter 5) Nutritional imbalances, including protein–calorie deficiency or lack of specific vitamins, as well as nutritional excesses (Chapter 9) Morphologic Alternations in Cell Injury (p. 12) Injury leads to loss of cell function long before damage is morpho- logically recognizable. Morphologic changes become apparent only some time after a critical biochemical system within the cell has been deranged; the interval between injury and morphologic change depends on the method of detection (Fig. 1-1). However, once developed, reversible injury and irreversible injury (necrosis) have characteristic features. Reversible Injury (p. 12) Cell swelling appears whenever cells cannot maintain ionic and fluid homeostasis (largely due to loss of activity in plasma mem- brane energy-dependent ion pumps). Fatty change is manifested by cytoplasmic lipid vacuoles, princi- pally encountered in cells involved in or dependent on fat metabolism (e.g., hepatocytes and myocardial cells). Necrosis (p. 14) Necrosis is the sum of the morphologic changes that follow cell death in living tissue or organs. Two processes underlie the basic morphologic changes: 6 General Pathology Reversible Irreversible cell injury cell injury Ultrastructural Light changes microscopic Biochemical changes Cell alterations function cell death Effect Gross morphologic changes Duration of injury FIGURE 1-1 Timing of biochemical and morphologic changes in cell injury. Denaturation of proteins Enzymatic digestion of organelles and other cytosolic components There are several distinctive features: necrotic cells are more eosinophilic (pink) than viable cells by standard hematoxylin and eosin (H&E) staining. They appear “glassy” owing to glycogen loss and may be vacuolated; cell membranes are fragmented. Necrotic cells may attract calcium salts; this is particularly true of necrotic fat cells (forming fatty soaps). Nuclear changes include pyknosis (small, dense nucleus), karyolysis (faint, dissolved nucleus), and karyorrhexis (fragmented nucleus). General tissue patterns of necrosis include the following: Coagulative necrosis (p. 15) is the most common pattern, pre- dominated by protein denaturation with preservation of the cell and tissue framework. This pattern is characteristic of hyp- oxic death in all tissues except the brain. Necrotic tissue undergoes either heterolysis (digestion by lysosomal enzymes of invading leukocytes) or autolysis (digestion by its own lysosomal enzymes). Liquefactive necrosis (p. 15) occurs when autolysis or heterolysis predominates over protein denaturation. The necrotic area is soft and filled with fluid. This type of necrosis is most frequently seen in localized bacterial infections (abscesses) and in the brain. Gangrenous necrosis (p.15) is not a specific pattern but is rather just coagulative necrosis as applied to an ischemic limb; superimposed bacterial infection makes for a more liquefactive pattern called wet gangrene. Caseous necrosis (p. 16) is characteristic of tuberculous lesions; it appears grossly as soft, friable, “cheesy” material and microscop- ically as amorphous eosinophilic material with cell debris. Fat necrosis (p. 16) is seen in adipose tissue; lipase activation (e.g., from injured pancreatic cells or macrophages) releases fatty  Cellular Responses to Stress and Toxic Insults 7 acids from triglycerides, which then complex with calcium to create soaps. Grossly, these are white, chalky areas (fat saponifi- cation); histologically, there are vague cell outlines and calcium deposition. Fibrinoid necrosis (p. 16 and Chapter 6) is a pathologic pattern resulting from antigen-antibody (immune complex) deposition in blood vessels. Microscopically there is bright-pink amorphous material (protein deposition) in arterial walls, often with asso- ciated inflammation and thrombosis. Mechanisms of Cell Injury (p. 17) The biochemical pathways in cell injury can be organized around a few general principles: Responses to injurious stimuli depend on the type of injury, duration, and severity. The consequences of injury depend on the type, state, and adaptability of the injured cell. Cell injury results from perturbations in any of five essential cel- lular elements: ATP production (mostly through effects on mitochondrial aero- bic respiration) Mitochondrial integrity independent of ATP production Plasma membrane integrity, responsible for ionic and osmotic homeostasis Protein synthesis, folding, degradation, and refolding Integrity of the genetic apparatus The intracellular mechanisms of cell injury fall into one of six general pathways (Fig. 1-2). Structural and biochemical elements of the cell are so closely interrelated that regardless of the locus of initial injury, secondary effects rapidly propagate through other elements. Depletion of ATP (p. 17) Decreased ATP synthesis and ATP depletion are common consequences of both ischemic and toxic injury. ATP is generated through glycolysis (anaerobic, inefficient) and oxidative phosphorylation in the mitochondria (aerobic, efficient). Hypoxia leads to increased anaer- obic glycolysis with glycogen depletion, increased lactic acid pro- duction, and intracellular acidosis. ATP is critical for membrane transport, maintenance of ionic gradients (particularly Naþ, Kþ, and Ca2þ), and protein synthesis; reduced ATP synthesis dramati- cally affects those pathways. Mitochondrial Damage (p. 18) Mitochondrial damage can occur directly due to hypoxia or toxins or as a consequence of increased cytosolic Ca2þ, oxidative stress, or phospholipid breakdown. Damage results in formation of a high- conductance channel (mitochondrial permeability transition pore) that leaks protons and dissipates the electromotive potential that drives oxidative phosphorylation. Damaged mitochondria also leak cytochrome c, which can trigger apoptosis (see later discussion). Influx of Calcium and Loss of Calcium Homeostasis (p. 19) Cytosolic calcium is maintained at extremely low levels by energy- dependent transport; ischemia and toxins can cause Ca2þ influx across the plasma membrane and release of Ca2þ from mitochondria 8 General Pathology MITOCHONDRIAL ATP DAMAGE Multiple Leakage of downstream pro-apoptotic effects proteins ENTRY ROS OF Ca2+ Ca Ca Ca Mitochondrial Activation Damage permeability of multiple to lipids, cellular proteins, enzymes DNA MEMBRANE PROTEIN DAMAGE MISFOLDING, DNA DAMAGE Plasma Lysosomal membrane membrane Enzymatic Activation of Loss of digestion pro-apoptotic cellular of cellular proteins components components FIGURE 1-2 Cellular and biochemical sites of damage in cell injury. ATP, Adenosine triphosphate; ROS, reactive oxygen species. and endoplasmic reticulum (ER). Increased cytosolic calcium activates phospholipases that degrade membrane phospholipids; proteases that break down membrane and cytoskeletal proteins; ATPases that hasten ATP depletion; and endonucleases that cause chromatin fragmentation.  Cellular Responses to Stress and Toxic Insults 9 Accumulation of Oxygen-Derived Free Radicals (Oxidative Stress) (p. 20) Free radicals are unstable, partially reduced molecules with unpaired electrons in outer orbitals that make them particularly reactive with other molecules. Although other elements can have free radical forms, O2-derived free radicals (also called reactive oxygen species or ROS) are the most common in biological systems. The major forms are superoxide anion (O , 2 one extra electron), hydrogen peroxide (H2O2 , two extra electrons), hydroxyl ions (OH, three extra electrons) and peroxynitrate ion (ONOO-; formed by interactions of nitric oxide [NO] and O2 ). Free radicals readily propagate additional free radical formation with other molecules in an autocatalytic chain reaction that often breaks chemical bonds. Thus, free radicals damage lipids (peroxidizing double bonds and causing chain breakage), proteins (oxidizing and fragmenting peptide bonds), and nucleic acids (causing single strand breaks). Free radical generation occurs by: Normal metabolic processes such as the reduction of oxygen to water during respiration; the sequential addition of four electrons leads to small numbers ROS intermediates. Absorption of radiant energy; ionizing radiation (e.g., ultraviolet light and x-rays) can hydrolyze water into hydroxyl (OH) and hydrogen (H) free radicals. Production by leukocytes during inflammation to sterilize sites of infection (Chapter 2). Enzymatic metabolism of exogenous chemicals or drugs (e.g., acetaminophen). Transition metals (e.g., iron and copper) can catalyze free radical formation. Nitric oxide (NO), an important chemical mediator (Chapter 2), can act directly as a free radical or be converted to other highly reactive forms. Fortunately, free radicals are inherently unstable and generally decay spontaneously. In addition, several systems contribute to free radical inactivation: Antioxidants either block the initiation of free radical formation or scavenge free radicals; these include vitamins E and A, ascorbic acid, and glutathione. The levels of transition metals that can participate in free radical formation are minimized by binding to storage and transport proteins (e.g., transferrin, ferritin, lactoferrin, and ceruloplasmin). Free radical scavenging enzyme systems catabolize hydrogen peroxide (catalase, glutathione peroxidase) and superoxide anion (superoxide dismutase). Defects in Membrane Permeability (p. 22) Membranes can be damaged directly by toxins, physical and chemi- cal agents, lytic complement components, and perforins, or indi- rectly as described by the preceding events (e.g., ROS, Ca2þ activation of phospholipases). Increased plasma membrane perme- ability affects intracellular osmolarity as well as enzymatic activity; increased mitochondrial membrane permeability reduces ATP synthesis and can drive apoptosis; altered lysosomal integrity unleashes extremely potent acid hydrolases that can digest proteins, nucleic acids, lipids, and glycogen. 10 General Pathology Damage to DNA and Proteins (p. 23) Damage to DNA that exceeds normal repair capacity (e.g., due to ROS, radiation, or drugs) leads to activation of apoptosis. Simi- larly, accumulation of large amounts of improperly folded proteins (e.g., due to ROS or heritable mutations) leads to a stress response that also triggers apoptotic pathways. Within limits, all the changes of cell injury described previously can be offset, and cells can return to normal after injury abates (reversible injury). However, persistent or excessive injury causes cells to pass a threshold into irreversible injury associated with extensive cell membrane damage, lysosomal swelling, and mito- chondrial vacuolization with deficient ATP synthesis. Extracellular calcium enters the cell and intracellular calcium stores are released, leading to activation of enzymes that catabolize membranes, proteins, ATP, and nucleic acids. Proteins, essential coenzymes, and RNAs are lost from hyperpermeable plasma membranes, and cells leak metabolites vital for the reconstitution of ATP. The transition from reversible to irreversible injury is difficult to identify, although two phenomena consistently characterize irreversibility: Inability to reverse mitochondrial dysfunction (lack of ATP gener- ation) even after resolution of the original injury Development of profound disturbances in membrane function Leakage of intracellular enzymes or proteins across abnormally permeable plasma membranes into the bloodstream provides important clinical markers of cell death. Cardiac muscle contains a specific isoform of the enzyme creatine kinase and of the contrac- tile protein troponin; hepatocytes contain transaminases, and hepatic bile duct epithelium contains a temperature-resistant iso- form of alkaline phosphatase. Irreversible injury in these tissues is consequently reflected by increased circulating levels of such proteins in the blood. Examples of Cell Injury and Necrosis (p. 23) Ischemic and Hypoxic Injury (p. 23) Ischemia and hypoxic injury are the most common forms of cell injury in clinical medicine. Hypoxia is reduced O2-carrying capac- ity; ischemia, which also clearly causes hypoxia, is due to reduced blood flow. Hypoxia alone allows continued delivery of substrates for glycolysis and removal of accumulated wastes (e.g., lactic acid); ischemia does neither and therefore tends to injure tissues faster than hypoxia alone. Hypoxia leads to loss of ATP generation by mitochondria; ATP depletion has multiple, initially reversible effects (Fig. 1-3): Failure of Naþ/Kþ-ATPase membrane transport causes sodium to enter the cell and potassium to exit; there is also increased Ca2þ influx as well as release of Ca2þ from intracellular stores. The net gain of solute is accompanied by isosmotic gain of water, cell swelling, and ER dilation. Cell swelling is also increased owing to the osmotic load from accumulation of metabolic breakdown products. Cellular energy metabolism is altered. With hypoxia, cells use anaer- obic glycolysis for energy production (metabolism of glucose derived from glycogen). Consequently, glycogen stores are rapidly depleted along with lactic acid accumulation and reduced intracellular pH. Reduced protein synthesis results from detachment of ribosomes from rough ER.  Cellular Responses to Stress and Toxic Insults 11 REVERSIBLE IRREVERSIBLE INJURY INJURY (Cell death) Membrane injury Ischemia Loss of phospholipids Cytoskeletal alterations Free radicals Lipid breakdown Others Leakage of enzymes Ca2+ (CK, LDH) influx Mitochondria Oxidative phosphorylation Cellular swelling Loss of Influx of Ca2+ Na microvilli H2O, and Na+ Blebs pump Efflux of K+ ER swelling Myelin figures ATP Other Glycolysis effects Clumping of nuclear chromatin Intracellular release Detachment pH and activation of of lysosomal enzymes Glycogen ribosomes Protein Basophilia ( RNP) synthesis Nuclear changes Protein digestion Lipid deposition FIGURE 1-3 Sequence of events in reversible and irreversible ischemic cell injury. Although reduced adenosine triphosphate (ATP) levels play a central role, ischemia can also cause direct membrane damage. CK, Creatine kinase; ER, endoplasmic reticulum; LDH, lactate dehydrogenase; RNP, ribonucleoprotein. All the aforementioned changes are reversible if oxygenation is restored. If ischemia persists, irreversible injury ensues, a transition largely dependent upon the extent of ATP depletion and membrane dysfunction, particularly mitochondrial membranes. ATP depletion induces the pore transition change in the mito- chondrial membrane; pore formation results in reduced mem- brane potential and diffusion of solutes. ATP depletion also releases cytochrome c, a soluble component of the electron transport chain that is a key regulator in driving apoptosis (see later discussion). Increased cytosolic calcium activates membrane phospholipases, leading to progressive loss of phospholipids and membrane dam- age; decreased ATP also leads to diminished phospholipid synthesis. 12 General Pathology Increased cytosolic calcium activates intracellular proteases, caus- ing degradation of intermediate cytoskeletal elements, rendering the cell membrane susceptible to stretching and rupture, partic- ularly in the setting of cell swelling. Free fatty acids and lysophospholipids accumulate in ischemic cells as a result of phospholipid degradation; these are directly toxic to membranes. Ischemia-Reperfusion Injury (p. 24) Restoration of blood flow to ischemic tissues can result in recovery of reversibly injured cells or may not affect the outcome if irrevers- ible damage has occurred. However, depending on the intensity and duration of the ischemic insult, additional cells may die after blood flow resumes, involving either necrosis or apoptosis. The process is characteristically associated with neutrophilic infiltrates. The additional damage is designated reperfusion injury and is clini- cally important in myocardial infarction, acute renal failure, and stroke. Several mechanisms potentially underlie reperfusion injury: New damage may occur during reoxygenation by increased gen- eration of ROS from parenchymal and endothelial cells, as well as from infiltrating leukocytes. Superoxide anions produced in reperfused tissue result from incomplete reduction of O2 by damaged mitochondria or because of the normal action of oxidases from tissue cells or invading inflammatory cells. Anti- oxidant defense mechanisms may also be compromised, favoring radical accumulation. Ischemic injury recruits circulating inflammatory cells (Chapter 2) through enhanced cytokine and adhesion molecule expression by hypoxic parenchymal and endothelial cells. The ensuing inflamma- tion causes additional injury. By restoring blood flow, reperfusion may actually increase local inflammatory cell infiltration. Complement activation (normally involved in host defense; Chapter 2) may also contribute. Immunoglobulin M (IgM) antibodies can deposit in ischemic tissues; when blood flow is resumed, complement proteins are activated by binding to the antibodies, resulting in further cell injury and inflammation. Chemical (Toxic) Injury (p. 24) Chemical injury occurs by two general mechanisms: Directly, by binding to some critical molecular component (e.g., mercuric chloride binds to cell membrane protein sulfhydryl groups, inhibiting ATPase-dependent transport and causing increased permeability) Indirectly, by conversion to reactive toxic metabolites (e.g., car- bon tetrachloride and acetaminophen); toxic metabolites, in turn, cause cellular injury either by direct covalent binding to membrane protein and lipids or, more commonly, by the forma- tion of reactive free radicals Apoptosis (p. 25) Programmed cell death (apoptosis) occurs when a cell dies through activation of a tightly regulated internal suicide program. The func- tion of apoptosis is to eliminate unwanted cells selectively, with minimal disturbance to surrounding cells and the host. The cell’s plasma membrane remains intact, but its structure is altered so that the apoptotic cell fragments and becomes an avid target for phago- cytosis. The dead cell is rapidly cleared before its contents have leaked out; therefore cell death by this pathway does not elicit an inflammatory reaction in the host. Thus, apoptosis is fundamentally  Cellular Responses to Stress and Toxic Insults 13 different from necrosis, which is characterized by loss of membrane integrity, enzymatic digestion of cells, and frequently a host reaction (see Table 1-2). Nevertheless, apoptosis and necrosis sometimes coexist and may share some common features and mechanisms. Causes of Apoptosis (p. 25) Apoptosis can be physiologic or pathologic. Physiologic Causes Programmed destruction of cells during embryogenesis Hormone-dependent involution of tissues (e.g., endometrium, prostate) in the adult Cell deletion in proliferating cell populations (e.g., intestinal epithelium) to maintain a constant cell number Death of cells that have served their useful purpose (e.g., neutrophils following an acute inflammatory response) Deletion of potentially harmful self-reactive lymphocytes Pathologic Causes DNA damage (e.g., due to hypoxia, radiation, or cytotoxic drugs). If repair mechanisms cannot cope with the damage caused, cells will undergo apoptosis rather than risk mutations that could result in malignant transformation. Relatively mild injury may induce apoptosis, whereas larger doses of the same stimuli result in necrosis. Accumulation of misfolded proteins (e.g., due to inherited defects or due to free radical damage). This may be the basis of cell loss in a number of neurodegenerative disorders. Cell death in certain viral infections (e.g., hepatitis), either caused directly by the infection or by cytotoxic T cells. Cytotoxic T cells may also be a cause of apoptotic cell death in tumors and in the rejection of transplanted tissues. Pathologic atrophy in parenchymal organs after duct obstruction (e.g., pancreas). Morphologic and Biochemical Changes in Apoptosis (p. 26) Morphologic features of apoptosis (see Table 1-2) include cell shrink- age, chromatin condensation and fragmentation, cellular blebbing and fragmentation into apoptotic bodies, and phagocytosis of apo- ptotic bodies by adjacent healthy cells or macrophages. Lack of inflammation makes it difficult to detect apoptosis histologically. Protein breakdown occurs through a family of proteases called caspases (so named because they have an active site cysteine and cleave at aspartate residues). Internucleosomal cleavage of DNA into fragments 180 to 200 base pairs in size gives rise to a characteristic ladder pattern of DNA bands on agarose gel electrophoresis. Plasma membrane alterations (e.g., flipping of phosphati- dylserine from the inner to the outer leaf of the plasma mem- brane) allow recognition of apoptotic cells for phagocytosis. Mechanisms of Apoptosis (p. 27) (Fig. 1-4) Apoptosis is a cascade of molecular events that can be initiated by a variety of triggers. The process of apoptosis is divided into an initi- ation phase, when caspases become active, and an execution phase, when the enzymes cause cell death. Initiation of apoptosis occurs through two distinct but convergent pathways: the intrinsic mito- chondrial pathway and the extrinsic death receptor–mediated pathway.  MITOCHONDRIAL (INTRINSIC) DEATH RECEPTOR (EXTRINSIC) PATHWAY PATHWAY 14 Cell injury Receptor-ligand interactions Growth factor Fas Cytochrome c General Pathology withdrawal Mitochondria TNF receptor and other DNA damage pro-apoptotic Adapter proteins (by radiation, proteins toxins, free radicals) Bcl-2 family Initiator Phagocyte Initiator caspases Protein effectors (Bax, Bak) caspases misfolding Regulators Executioner (ER stress) Bcl-2 (Bcl-2, Bcl-x) caspases family sensors Endonuclease Breakdown of activation cytoskeleton DNA fragmentation Ligands for phagocytic cell receptors Membrane bleb Apoptotic body FIGURE 1-4 Mechanisms of apoptosis. Some of the major inducers of apoptosis include specific death ligands (tumor necrosis factor [TNF] and Fas ligand [Fas L]), withdrawal of growth factors or hormones, and injurious agents (e.g., radiation). Some stimuli (such as cytotoxic cells) directly activate initiator caspases (right). Others act by way of mitochondrial events involving cytochrome c and other pro-apoptotic proteins. The Bcl-2 family of proteins regulate apoptosis by modulating this mitochondrial release. Initator caspases cleave and activate executioner caspases, that, in turn, activate latent cytoplasmic endonucleases and proteases that catabolize nuclear and cytoskeletal proteins. This results in a cascade of intracellular degradation, including fragmentation of nuclear chromatin and breakdown of the cytoskeleton. The end result is formation of apoptotic bodies containing intracellular organelles and other cytosolic components; these bodies also express new ligands for binding and uptake by phagocytic cells. ER, Endoplasmic reticulum.  Cellular Responses to Stress and Toxic Insults 15 Intrinsic (Mitochondrial) Pathway (p. 28) When mitochondrial permeability is increased, cytochrome c, as well as other pro-apoptotic molecules, is released into the cytoplasm; death receptors are not involved. Mitochondrial permeability is regulated by more than 20 proteins of the Bcl family. Bcl-2 and Bcl-x are the two predominant anti-apoptotic proteins responsible for reducing mitochondrial leakiness. On the other hand, cellular stress (e.g., misfolded proteins, DNA damage) or loss of survival signals is sensed by other Bcl members (e.g., Bim, Bid, and Bad). These molecules then activate two critical pro-apoptotic proteins—Bax and Bak—that form oligomers that insert into the mitochondrial membrane and create permeability channels. With concomitantly decreasing Bcl-2/Bcl-x levels, mitochondrial membrane permeability increases, leaking out several proteins that can activate caspases. Thus, released cytochrome c binds to apoptosis activating factor-1 (Apaf-1) to form a large multimeric apoptosome complex that triggers caspase-9 (an initiator caspase) activation. The essence of the intrinsic pathway is a balance between pro-apoptotic and anti-apoptotic molecules that regulate mitochondrial permeability. Extrinsic (Death Receptor–Initiated) Pathway (p. 29) Death receptors are members of the tumor necrosis factor (TNF) receptor family (e.g., type 1 TNF receptor and Fas). They have a cytoplasmic death domain involved in protein-protein interactions. Cross-linking these receptors by external ligands, such as TNF or Fas ligand (FasL), causes them to trimerize to form binding sites for adapter proteins that serve to bring multi- ple inactive caspase-8 molecules into close proximity. Low-level enzymatic activity of these pro-caspases eventually cleaves and activates one of the assembled group, rapidly leading to a down- stream cascade of caspase activation. This enzymatic pathway can be inhibited by a blocking protein called FLIP; viruses and normal cells can produce FLIP to protect themselves against Fas-mediated death. Execution Phase (p. 30) Caspases occur as inactive pro-enzymes that are activated through proteolytic cleavage; the cleavage sites can be hydrolyzed by other caspases or autocatalytically. Initiator caspases (e.g., caspase- 8 and -9) are activated early in the sequence and induce the cleav- age of the executioner caspases (e.g., caspase-3 and -6) that do the bulk of the intracellular proteolytic degradation. Once an initiator caspase is activated, the death program is set in motion by rapid and sequential activation of other caspases. Executioner caspases act on many cell components; they cleave cytoskeletal and nuclear matrix proteins, disrupting the cytoskeleton and leading to nuclear breakdown. In the nucleus, caspases cleave proteins involved in transcription, DNA replication, and DNA repair; in particular, caspase-3 activates a cytoplasmic DNAase resulting in the charac- teristic internucleosomal cleavage. Apoptosis in Health and Disease (p. 30) Growth Factor Deprivation Examples include hormone-sensitive cells deprived of the relevant hormone, lymphocytes not stimulated by antigens or cytokines, and neurons deprived of nerve growth factor. Apoptosis is trig- gered by the intrinsic (mitochondrial) pathway due to a relative excess of pro-apoptotic versus anti-apoptotic members of the Bcl family. 16 General Pathology DNA Damage DNA damage by any means (e.g., radiation or chemotherapeutic agents) induces apoptosis through accumulation of the tumor- suppressor protein p53. This results in cell cycle arrest at G1 puta- tively to allow time for DNA repair (Chapter 7). If repair cannot take place, p53 then induces apoptosis by increasing the transcrip- tion of several pro-apoptotic members of the Bcl family. Absent or mutated p53 (i.e., in certain cancers) reduces apoptosis and favors cell survival even in the presence significant DNA damage. Protein Misfolding Accumulation of misfolded proteins—due to oxidative stress, hyp- oxia, or genetic mutations—leads to the unfolded protein response, increasingly recognized as a feature of several neurodegenerative disorders. This response induces increased production of chaperones and increased proteasomal degradation with decreased protein synthesis. If the adaptive responses cannot keep pace with the accumulating misfolded proteins, caspases are activated and apopto- sis results (Fig. 1-5). TNF Family Receptors Apoptosis induced by Fas-FasL interactions (see preceding discus- sion) are important for eliminating lymphocytes that recognize self-antigens; mutations in Fas or FasL result in autoimmune diseases (Chapter 6). TNF is an important mediator of the inflammatory reaction (Chapter 2), but can also induce apoptosis (see preceding discus- sion). The major physiologic functions of TNF are mediated through activation of the transcription factor nuclear factor-kB (NF-kB), which in turn promotes cell survival by increasing anti- apoptotic members of the Bcl family. Whether TNF induces cell death, promotes cell survival, or drives inflammatory responses depends on which of two TNF receptors it binds, as well as which adapter protein attaches to the receptor. Cytotoxic T Lymphocytes Cytotoxic T lymphocytes (CTLs) recognize foreign antigens on the surface of infected host cells (Chapter 6) and secrete perforin, a transmembrane pore-forming molecule. This allows entry of the CTL-derived serine protease granzyme B that in turn activates multiple caspases, thereby directly inducing the effector phase of apoptosis. CTLs also express FasL on their surfaces and can kill tar- get cells by Fas ligation (via FasL). Disorders Associated with Dysregulated Apoptosis (p. 32) Dysregulated (“too little or too much”) apoptosis underlies multi- ple disorders: Disorders with defective apoptosis and increased cell survival. Insuffi- cient apoptosis may prolong the survival or reduce the turnover of abnormal cells. Such accumulated cells may lead to (1) cancers, especially tumors with p53 mutations, or hormone-dependent tumors, such as breast, prostate, or ovarian cancers (Chapter 7), and (2) autoimmune disorders, when autoreactive lymphocytes are not eliminated (Chapter 6). Disorders with increased apoptosis and excessive cell death. Increased cell loss can cause (1) neurodegenerative diseases, with drop out of specific sets of neurons (Chapter 28); (2) ischemic injury (e.g., myocardial infarction, Chapter 12; stroke, Chapter 28); and (3) death of virus-infected cells (Chapter 8).  RESPONSES TO UNFOLDED PROTEINS Increased synthesis of Repair STRESS chaperones Cellular Responses to Stress and Toxic Insults (UV, heat, free radical UNFOLDED PROTEIN injury, etc.) RESPONSE (UPR) Decreased translation of proteins Ubiquitin Activation of the Protein ubiquitin-proteasome Accumulation of pathway misfolded proteins Degradation of Mutations Proteasome unfolded proteins Activation of caspases APOPTOSIS FIGURE 1-5 Misfolded proteins trigger an unfolded protein response that includes increased chaperone synthesis, decreased protein translation, and activation of proteasome degradation pathways. If these are ineffective in reducing the burden of misfolded proteins, caspase activation and apoptosis ensue. 17 18 General Pathology Intracellular Accumulations (p. 32) Cells may accumulate abnormal amounts of various substances. A normal endogenous substance (water, protein, carbohydrate, lipid) is produced at a normal (or even increased) rate, with the metabolic rate inadequate to remove it (e.g., fat accumulation in liver cells). An abnormal endogenous substance (product of a mutated gene) accumulates because of defective folding or transport, and inade- quate degradation (e.g., a1-antitrypsin disease, Chapter 18). A normal substance accumulates because of genetic or acquired defects in its metabolism (e.g., lysososmal storage diseases, Chapter 5). Abnormal exogenous substances may accumulate in normal cells because they lack the machinery to degrade such substances (e.g., macrophages laden with environmental carbon). Lipids (p. 33) Triglycerides (the most common), cholesterol and cholesterol esters, and phospholipids can accumulate in cells. Steatosis (Fatty Change) (p. 33) The terms describe an abnormal accumulation of triglycerides within parenchymal cells either due to excessive entry or defective metabolism and export. It can occur in heart, muscle, and kidney, but it is most common in the liver. Fatty change is typically revers- ible, but it can lead to inflammation and fibrosis. Hepatic causes include alcohol abuse (most common in the United States), protein malnutrition, diabetes mellitus, obesity, toxins, and anoxia. Grossly, fatty livers are enlarged, yellow, and greasy; microscopically, there are small, intracytoplasmic droplets or large vacuoles of fat. The condition is caused by excessive entry or defective metabolism or export of lipids (Fig. 1-6): Increased fatty acids entering the liver (starvation, corticosteroids) Decreased fatty acid oxidation (hypoxia) Increased triglyceride formation (alcohol) Decreased apoprotein synthesis (carbon tetrachloride poisoning, starvation) Impaired lipoprotein secretion from the liver (alcohol) Cholesterol and Cholesterol Esters (p. 34) Cholesterol is normally required for cell membrane or lipid-soluble hormone synthesis; production is tightly regulated, but accumula- tion (seen as intracellular cytoplasmic vacuoles) can be present in a variety of pathologic states: Atherosclerosis: Cholesterol and cholesterol esters accumulate in arterial wall smooth muscle cells and macrophages (Chapter 11). Extracellular accumulations appear microscopically as cleftlike cavities formed when cholesterol crystals are dissolved during normal histologic processing. Xanthomas: In acquired and hereditary hyperlipidemias, lipids accumulate in clusters of “foamy” macrophages and mesenchy- mal cells. Cholesterolosis: Focal accumulations of cholesterol-laden macrophages occur in the lamina propria of gallbladders. Niemann-Pick disease, type C: This type of lysosomal storage dis- ease is due to mutation of an enzyme involved in cholesterol catabolism.  Cellular Responses to Stress and Toxic Insults 19 Free fatty acids Acetate Oxidation to ketone bodies, CO2 Fatty acids Phospholipids α-Glycero- phosphate CATABOLISM Cholesterol esters Triglycerides Apoprotein Lipoproteins Lipid accumulation FIGURE 1-6 Schematic diagram of the mechanisms leading to fat accumulation in hepatic steatosis. Defects in any of uptake, catabolism, or secretion can result in lipid overload. Proteins (p. 35) Intracellular protein accumulation may be due to excessive synthe- sis, absorption, or defects in cellular transport. Morphologically visible accumulations appear as rounded, eosinophilic cytoplasmic droplets. In some disorders (e.g., amyloidosis, Chapter 6), abnor- mal proteins deposit primarily in the extracellular space. Reabsorption droplets of proteins accumulate in proximal renal tubules in the setting of chronic proteinuria. The process is reversible; the droplets are metabolized and clear if the protein- uria resolves. Normally secreted proteins can accumulate if produced in exces- sive amounts, e.g., immunoglobulin within plasma cells. In that case, the ER becomes grossly distended with eosinophilic inclusions called Russell bodies. Defective intracellular transport and secretion, e.g., a1-antitrypsin deficiency, where partially folded intermediates of mutated proteins accumulate in hepatocyte ER. In many cases, pathology results not only from the unfolded protein response and apopto- sis (see preceding discussion) but also from loss of protein function. Thus reduction in secreted a1-antitrypsin also leads to emphysema (Chapter 15). Accumulated cytoskeletal proteins. Excess intermediate filaments (e.g., keratin or certain neurofilaments) are hallmarks of cell injury; thus, keratin intermediate filaments coalesce into cytoplasmic eosin- ophilic inclusions called alcoholic hyaline (Chapter 18), and the neurofibrillary tangle in Alzheimer’s disease contains neurofilaments (Chapter 28). Aggregates of abnormal proteins. Aggregation of abnormally folded proteins (e.g., genetic mutations, aging, and so on), either intracellular and/or extracellular, can cause pathologic change; extracellular amyloid is an example. 20 General Pathology Hyaline Change (p. 36) Hyaline change refers to any deposit that imparts a homogeneous, glassy pink appearance in H&E-stained histologic sections. Examples of intracellular hyaline change include proximal tubule epithelial protein droplets, Russell bodies, viral inclusions, and alcoholic hyaline. Extracellular hyaline change occurs, for example, in damaged arterioles (e.g., due to chronic hypertension), presum- ably due to extravasated proteins. Glycogen (p. 36) Glycogen is commonly stored within cells as a ready energy source. Excessive intracellular deposits (seen as clear vacuoles) are seen with abnormalities of glycogen storage (so-called glycogenoses, Chapter 5) and glucose metabolism (diabetes mellitus). Pigments (p. 36) Pigments are colored substances that can be exogenous (e.g., coal dust) or endogenous, such as melanin or hemosiderin. Exogenous pigments include carbon or coal dust (most com- mon); when visibly accumulated within pulmonary macrophages and lymph nodes, these deposits are called anthracosis. Pigments from tattooing are taken up by macrophages and persist for the life of the cell. Endogenous pigments include: Lipofuscin, the so-called wear-and-tear pigment, is usually associated with cellular and tissue atrophy (brown atrophy). This is seen microscopically as fine, yellow-brown intracytoplasmic granules. The pigment is composed of complex lipids, phospholipids, and protein, probably derived from cell membrane peroxidation. Melanin is a normal, endogenous, brown-black pigment formed by enzymatic oxidation of tyrosine to dihydroxyphenylalanine in melanocytes. Homogentisic acid is a black pigment formed in patients with alkaptonuria (lacking homogentisic oxidase) that deposits in skin and connective tissue; the pigmentation is called ochronosis. Hemosiderin is a hemoglobin-derived, golden–yellow-brown, granular intracellular pigment composed of aggregated ferritin. Accumulation can be localized (e.g., macrophage-mediated breakdown of blood in a bruise) or systemic, that is, resulting from increased dietary iron absorption (primary hemochro- matosis), impaired utilization (e.g., thalassemia), hemolysis, or chronic transfusions (Chapter 18). Pathologic Calcification (p. 38) Pathologic calcification—the abnormal tissue deposition of cal- cium salts—occurs in two forms: dystrophic calcification arises in nonviable tissues in the presence of normal calcium serum levels, and metastatic calcification happens in viable tissues in the setting of hypercalcemia. Dystrophic Calcification (p. 38) Although frequently only a marker of prior injury, it can also be a source of significant pathology. Dystrophic calcification occurs in arteries in atherosclerosis, in damaged heart valves, and in areas of necrosis (e.g., coagulative, caseous, and liquefactive). Calcium can be intracellular and extracellular. Deposition ultimately involves  Cellular Responses to Stress and Toxic Insults 21 precipitation of a crystalline calcium phosphate similar to bone hydroxyapatite: Initiation (nucleation) occurs extracellularly or intracellularly. Extracellular initiation occurs on membrane-bound vesicles from dead or dying cells that concentrate calcium due to their content of charged phospholipids; membrane-bound phos- phatases then generate phosphates that form calcium-phosphate complexes; the cycle of calcium and phosphate binding is repeated, eventually producing a deposit. Initiation of intracellu- lar calcification occurs in mitochondria of dead or dying cells. Propagation of crystal formation depends on the concentration of calcium and phosphates, the presence of inhibitors, and struc- tural components of the extracellular matrix. Metastatic Calcification (p. 38) These calcium deposits occur as amorphous basophilic densities that can be present widely throughout the body. Typically, they have no clinical sequelae, although massive deposition can cause renal and lung deficits. Metastatic calcification results from hyper- calcemia, which has four principal causes: Elevated parathyroid hormone (e.g., hyperparathyroidism due to parathyroid tumors or ectopic parathyroid hormone secreted by other neoplasms) Bone destruction, as in primary marrow malignancies (e.g., multiple myeloma) or by diffuse skeletal metastasis (e.g., breast cancer), by accelerated bone turnover (Paget’s disease), or immobilization Vitamin D–related disorders, including vitamin D intoxication and systemic sarcoidosis Renal failure, causing secondary hyperparathyroidism due to phosphate retention and the resulting hypocalcemia Cellular Aging (p. 39) With increasing age, degenerative changes impact the structure and physiologic function of all organ systems. The tempo and severity of such changes in any given individual are influenced by genetic factors, diet, social conditions, and the impact of other co- morbidities, such as atherosclerosis, diabetes, and osteoarthritis. Cellular aging—reflecting the progressive accumulation of suble- thal cellular and molecular damage due to both genetic and exoge- nous influences—leads to cell death and diminished capacity to respond to injury; it is a critical component of the aging of the entire organism (Fig. 1-7). Aging—at least in model systems—appears to be a regulated process influenced by a limited number of genes; this, in turn, implies that aging can potentially be parsed into definable mecha- nistic alterations: Cellular senescence refers to the concept that cells have a limited capacity for replication. Cells from children can have more rounds of replication than cells from geriatrics; in turn, cells from geriatrics replicate more than cells from patients with accelerated aging disorders (e.g., Werner syndrome, due to a DNA helicase mutation). Many changes in gene expression accompany cellular senescence, including those that inhibit cell cycle progression (e.g., increased p16INK4a). In particular, telomere shortening (i.e., incomplete replication of chromosome ends) is a major mechanism underlying cell senescence. Telomeres are short, repeated sequences of DNA that comprise the termini of chromosomes; they are important to ensure complete replication  22 Telomere Environmental DNA repair Abnormal growth shortening insults defects factor signaling General Pathology (e.g., insulin/IGF) Free DNA Activation of radicals damage sirtuins Mechanism? ? ? Replicative Damage to proteins Accumulation Calorie restriction senescence and organelles of mutations CELLULAR AGING FIGURE 1-7 Mechanisms of cellular aging. Genetic factors and environmental insults combine to produce the cellular abnormalities characteristic of aging. IGF, Insulin-like growth factor.  Cellular Responses to Stress and Toxic Insults 23 of chromosomes and protect chromosomal ends from fusion and degradation. When cells replicate, a small section of the telomere is not replicated. As cells repeatedly divide, telomeres become progressively shortened, ultimately signaling a growth checkpoint where cells become senescent. Telomerase is an RNA–protein enzyme complex that maintains telomere length by using its own RNA as a template to add nucleotides to the ends of chromosomes; it is present in germ cells and stem cells, but is usually undetectable in somatic cells. In cancer cells, telomerase is often reactivated, consistent with the notion that telomere elongation (or at least preservation) may confer cell immortality. Accumulated metabolic and genetic damage clearly contribute to cellular aging. Ideally, any harm accruing from metabolic events or exogenous injury is counterbalanced by injury repair mechanisms. However, too much injury or too little compensa- tory response will lead to damage that can affect cell function and viability. For example, ROS (discussed previously) covalently modify proteins, lipids, and nucleic acids and lead to a variety of breaks; the cumulative amount of oxidative damage increases with age. Increased ROS production (e.g., exposure to ionizing radiation or mitochondrial dysfunction) or diminished antioxidant defenses (glutathione peroxidase, superoxide dismutase) contributes to cel- lular senescence and correlates with a shortened life span of the organism. The recognition and repair of damaged DNA is also a critical counterbalance. Thus, in patients with Werner syndrome, there is premature aging due to defective DNA helicase with accelerated accumulation of chromosomal damage; this mimics the injury that normally accompanies aging. Genetic instability is also characteris- tic of other disorders associated with premature aging (e.g., ataxia- telangiectasia, due to defective repair of DNA double-strand breaks). Damaged organelles may also accumulate with age and contrib- ute to cellular senescence; this is attributable in part to diminished function of the proteasome. The most effective way to prolong the life span is caloric restric- tion, likely related to its ability to promote the activity of sirtuins, a family of proteins with histone deacetylase activity. Sirtuins increase the production of proteins that reduce apoptosis, stimu- late protein folding, increase metabolic activity and insulin sensi- tivity, and reduce ROS. Signaling through the insulin or insulin- like growth factor-1 (IGF-1) receptors can also influence life span; inactivating insulin receptor mutations increase longevity.  2 Acute and Chronic Inflammation Overview of Inflammation (p. 44) Inflammation is the response of vascularized living tissue to injury. It may be evoked by microbial infections, physical agents, chemicals, necrotic tissue, or immune reactions. Inflammation is intended to contain and isolate injury, to destroy invading microorganisms and inactivate toxins, and to prepare the tissue for healing and repair (Chapter 3). Inflammation is characterized by: Two main components—a vascular wall response and an inflam- matory cell response Effects mediated by circulating plasma proteins and by factors produced locally by the vessel wall or inflammatory cells Termination when the offending agent is eliminated and the secreted mediators are removed; active anti-inflammatory mechanisms are also involved Tight association with healing; even as inflammation destroys, dilutes, or otherwise contains injury it sets into motion events that ultimately lead to repair of the damage A fundamentally protective response; however, inflammation can also be harmful, for example, by causing life-threatening hypersensitivity reactions or relentless and progressive organ damage from chronic inflammation and subsequent fibrosis (e.g., rheumatoid arthritis, atherosclerosis) Acute and chronic patterns: Acute inflammation: Early onset (i.e., seconds to minutes), short duration (i.e., minutes to days), involving fluid exuda- tion (edema) and polymorphonuclear cell (neutrophil) emigration Chronic inflammation: Later onset (i.e., days) and longer duration (i.e., weeks to years), involving lymphocytes and macrophages, with blood vessel proliferation and fibrosis (scarring) There are five classic clinical signs of inflammation (most prom- inent in acute inflammation): Warmth (Latin: calor) due to vascular dilation Erythema (Latin: rubor) due to vascular dilation and congestion Edema (Latin: tumor) due to increased vascular permeability Pain (Latin: dolor) due to mediator release Loss of function (Latin: functio laesa) due to pain, edema, tissue injury, and/or scar 24  Acute and Chronic Inflammation 25 Acute Inflammation (p. 45) Acute inflammation has three major components: Alterations in vascular caliber, leading to increased blood flow Structural changes in the microvasculature, permitting plasma proteins and leukocytes to leave the circulation to produce inflammatory exudates Leukocyte emigration from blood vessels and accumulation at the site of injury with activation Reactions of Blood Vessels in Acute Inflammation (p. 46) Normal fluid exchange in vascular beds depends on an intact endo- thelium and is modulated by two opposing forces: Hydrostatic pressure causes fluid to move out of the circulation. Plasma colloid osmotic pressure causes fluid to move into the capillaries. DEFINITIONS Edema is excess fluid in interstitial tissue or body cavities and can be either an exudate or a transudate. Exudate is an inflammatory, extravascular fluid with cellular debris and high protein concentration (specific gravity of 1.020 or more). Transudate is excess, extravascular fluid with low protein content (specific gravity of 1.012 or less); it is essentially an ultrafiltrate of blood plasma resulting from elevated fluid pressures or dimin- ished plasma osmotic forces. Pus is a purulent inflammatory exudate rich in neutrophils and cell debris. Changes in Vascular Flow and Caliber (p. 46) Beginning immediately after injury, the vascular wall develops changes in caliber and permeability that affect flow. The changes develop at various rates depending on the nature of the injury and its severity. Vasodilation causes increased flow into areas of injury, thereby increasing hydrostatic pressure. Increased vascular permeability causes exudation of protein-rich fluid (see later discussion). The combination of vascular dilation and fluid loss leads to increased blood viscosity and increased concentration of red blood cells (RBCs). Slow movement of erythrocytes (stasis) grossly manifests as vascular congestion (erythema). With stasis, leukocytes—mostly neutrophils—accumulate along the endothelium (marginate) and are activated by mediators to increase adhesion molecule expression and migrate through the vessel wall. Increased Vascular Permeability (p. 47) Increased vascular permeability can be induced by several different pathways: Contraction of venule endothelium to form intercellular gaps: Most common mechanism of increased permeability Elicited by chemical mediators (e.g., histamine, bradykinin, leukotrienes, etc.) Occurs rapidly after injury and is reversible and transient (i.e., 15 to 30 minutes), hence the term immediate-transient response A similar response can occur with mild injury (e.g., sunburn) or inflammatory cytokines but is delayed (i.e., 2 to 12 hours) and protracted (i.e., 24 hours or more) 26 General Pathology Direct endothelial injury: Severe necrotizing injury (e.g., burns) causes endothelial cell necrosis and detachment that affects venules, capillaries, and arterioles Recruited neutrophils may contribute to the injury (e.g., through reactive oxygen species) Immediate and sustained endothelial leakage Increased transcytosis: Transendothelial channels form by interconnection of vesicles derived from the vesiculovacuolar organelle Vascular endothelial growth factor (VEGF) and other factors can induce vascular leakage by increasing the number of these channels Leakage from new blood vessels: Endothelial proliferation and capillary sprouting (angiogenesis) result in leaky vessels Increased permeability persists until the endothelium matures and intercellular junctions form Responses of Lymphatic Vessels (p. 47) Lymphatics and lymph nodes filter and “police” extravascular fluids. With the mononuclear phagocyte system, they represent a secondary line of defense when local inflammatory responses cannot contain an infection. In inflammation, lymphatic flow is increased to drain edema fluid, leukocytes, and cell debris from the extravascular space. In severe injuries, drainage may also transport the offending agent; lymphatics may become inflamed (lymphangitis, manifest grossly as red streaks), as may the draining lymph nodes (lymphadenitis, manifest as enlarged, painful nodes). The nodal enlargement is usually due to lymphoid follicle and sinusoidal phagocyte hyper- plasia (termed reactive lymphadenitis, Chapter 13). Reactions of Leukocytes in Inflammation (p. 48) A critical function of inflammation is to deliver leukocytes to sites of injury, especially those cells capable of phagocytosing microbes and necrotic debris (e.g., neutrophils and macrophages). After recruitment, the cells must recognize microbes and dead material and effect their removal. The type of leukocyte that ultimately migrates into a site of injury depends on the age of the inflammatory response and the original stimulus. In most forms of acute inflammation, neutrophils predominate during the first 6 to 24 hours and are then replaced by monocytes after 24 to 48 hours. There are several reasons for this sequence: neutrophils are more numerous in blood than monocytes, they respond more rapidly to chemokines, and they attach more firmly to the particular adhesion molecules that are induced on endothelial cells at early time points. After migration, neutrophils are also short-lived; they undergo apoptosis after 24 to 48 hours, whereas monocytes survive longer. The process of getting cells from vessel lumen to tissue interstitium is called extravasation and is divided into three steps (Fig. 2-1): Margination, rolling, and adhesion of leukocytes to the endothelium Transmigration across the endothelium Migration in interstitial tissues toward a chemotactic stimulus  Acute and Chronic Inflammation 27 Integrin Migration activation by through chemokines endothelium Rolling Stable adhesion Leukocyte Sialyl-Lewis X−modified glycoprotein Integrin (low-affinity state) Integrin (high- affinity state) PECAM-1 (CD31) P-selectin Proteoglycan Integrin ligand E-selectin (ICAM-1) Cytokines Chemokines (TNF, IL-1) Macrophage Fibrin and fibronectin with microbes (extracellular matrix) FIGURE 2-1 The multistep process of leukocyte migration through blood vessels, shown here for neutrophils. The leukocytes first roll (are loosely adherent with intermittent attachment and detachment of receptors), then (in sequence) become activated and firmly adhere to endothelium, transmigrate across the endothelium, pierce the basement membrane, and migrate toward chemoattractants emanating from the source of injury. Different molecules play predominant roles in different steps of this process—selectins in rolling, chemokines in activating the neutrophils to increase avidity of integrins, integrins in firm adhesion, and CD31 (PECAM-1) in transmigration. ICAM-1, Intercellular adhesion molecule-1; IL-1, interleukin-1; PECAM-1, platelet-endothelial cell adhesion molecule-1; TNF, tumor necrosis factor. Leukocyte Adhesion to Endothelium (p. 48) With progressive stasis of blood flow, leukocytes become increas- ingly distributed along the vessel periphery (margination), followed by rolling and then firm adhesion, before finally crossing the vascular wall. Rolling, adhesion, and transmigration occur by interactions between complementary adhesion molecules on leukocytes and endothelium. Expression of these adhesion molecules is enhanced by secreted proteins called cytokines. The major adhesion molecule pairs are listed in Table 2-1: Selectins (E, P, and L) bind via lectin (sugar-binding) domains to oligosaccharides (e.g., sialylated Lewis X) on cell surface glycoproteins. These interactions mediate rolling. Immunoglobulin family molecules on endothelial cells include intercellular adhesion molecule 1 (ICAM-1) and vascular cell adhe- sion molecule 1 (VCAM-1); these bind integrins on leukocytes and mediate firm adhesion. Integrins are a-b heterodimers (protein pairs) on leukocyte surfaces that bind to members of the immunoglobulin family molecules and to the extracellular matrix. The principal integrins that bind ICAM-1 are b2 integrins LFA-1 and Mac-1 (also called CD11a/CD18 and CD11b/CD18); the principal integrin that binds to VCAM-1 is the b1 integrin VLA4. Chemoattractants (chemokines) and cytokines affect adhesion and transmigration by modulating the surface expression or 28 General Pathology TABLE 2-1 Endothelial-Leukocyte Adhesion Molecules Endothelial Leukocyte Molecule Molecule Major Role P-selectin Sialyl-Lewis Rolling (neutrophils, X–modified monocytes, proteins T lymphocytes) E-selectin Sialyl-Lewis Rolling and adhesion X–modified (neutrophils, monocytes, proteins T lymphocytes) GlyCam-1, CD34 L-selectin* Rolling (neutrophils, monocytes) ICAM-1 CD11/CD18 (b2) Adhesion, arrest, (immunoglobulin integrins transmigration family) (LFA-1, (neutrophils, monocytes, Mac-1) lymphocytes) VCAM-1 VLA-4 (b1) Adhesion (eosinophils, (immunoglobulin integrin monocytes, lymphocytes) family) CD31 (PECAM) CD31 Leukocyte migration through endothelium *L-selectin is expressed weakly on neutrophils. It is involved in the binding of circulating T-lymphocytes to the high endothelial venules in lymph nodes and mucosal lymphoid tissues, and subsequent “homing” of lymphocytes to these tissues. avidity of the adhesion molecules. These modulating molecules induce leukocyte adhesion in inflammation by three general mechanisms: Redistribution of preformed adhesion molecules to the cell surface. After histamine exposure, P-selectin is rapidly translocated from the endothelial Weibel-Palade body membranes to the cell sur- face, where it can bind leukocytes. Induction of adhesion molecules on endothelium. Interleukin-1 (IL-1) and tumor necrosis factor (TNF) increase endothelial expression of E-selectin, ICAM-1, and VCAM-1; such activated endothelial cells have increased leukocyte adherence. Increased avidity of binding. This is most important for integrin binding. Integrins are normally present on leukocytes in a low- affinity form; they are converted to high-affinity forms by a variety of chemokines. Such activation causes firm adhesion of the leukocytes to the endothelium and is required for subsequent transmigration. Leukocyte Migration through Endothelium (p. 50) Transmigration (also called diapedesis) is mediated by homotypic (like-like) interactions between platelet-endothelial cell adhesion molecule-1 ¼ CD31 (PECAM-1) on leukocytes and endothelial cells. Once across the endothelium and into the underlying connective tissue, leukocytes adhere to the extracellular matrix via integrin binding to CD44. Chemotaxis of Leukocytes (p. 50) After emigrating through interendothelial junctions and traversing the basement membrane, leukocytes move toward sites of injury along gradients of chemotactic agents (chemotaxis). For neutrophils,  Acute and Chronic Inflammation 29 these agents include exogenous bacterial products and endogenous mediators (detailed later), such as complement fragments, arachi- donic acid metabolites, and chemokines. Chemotaxis involves binding of chemotactic agents to specific leukocyte surface G protein–coupled receptors; these trigger the production of phosphoinositol second messengers, in turn causing increased cytosolic calcium and guanosine triphosphatase (GTPase) activities that polymerize actin and facilitate cell movement. Leukocytes move by extending pseudopods that bind the extracellu- lar matrix and then pull the cell forward (front-wheel drive). Recognition of Microbes and Dead Tissues (p. 51) Having arrived at the appropriate site, leukocytes distinguish offending agents and then destroy them. To accomplish this, inflammatory cells express a variety of receptors that recognize pathogenic stimuli, and deliver activating signals (Fig. 2-2). Receptors for microbial products: These include toll-like receptors (TLRs), one of 10 different mammalian proteins that recognize distinct components in different classes of microbial pathogens. Thus, some TLRs participate in cellular responses to bacterial lipopolysaccharide (LPS) or unmethylated CpG nucleotide fragments, whereas others respond to double-stranded RNA made by some viral infections. TLRs can be on the cell surface or within endosomal vesicles depending on the likely location of the pathogen (extracellular versus ingested). They function through receptor-associated kinases that in turn induce produc- tion of cytokines and microbicidal substances. G protein–coupled receptors: These receptors typically recognize bacterial peptides containing N-formyl methionine residues, or they are stimulated by the binding of various chemokines (see preceding discussion), complement fragments, or arachidonic acid metabolites (e.g., prostaglandins and leukotrienes). Ligand binding triggers migration and production of microbicidal substances. Receptors for opsonins: Molecules that bind to microbes and ren- der them more “attractive” for ingestion are called opsonins; these include antibodies, complement fragments, and certain lectins (sugar-binding proteins). Binding of opsonized (coated) particles to their leukocyte receptor leads to cell activation and phagocytosis (see later). Cytokine receptors: Inflammatory mediators (cytokines) bind to cell surface receptors and induce cellular activation. One of the most important is interferon-g, produced by activated T cells and natural killer cells, and the major macrophage-activating cytokine. Removal of the Offending Agents (p. 52) Recognition through any of the preceding receptors induces leukocyte activation (see Fig. 2-2). The most essential functional consequences of activation are enhanced phagocytosis and intracellular killing, although the release of cytokines, growth factors, and inflammatory mediators (e.g., prostaglandins) is also important. Phagocytosis (p. 52) Phagocytosis begins with leukocyte binding to the microbe; this is facilitated by opsonins, the most important being the immunoglob- ulin Fc fragment and the complement fragment C3b. Macrophage integrins, and the macrophage mannose and scavenger receptors (mannose is expressed as a terminal sugar on many microbes), are also important recognition proteins for phagocytosis.  Microbe 30 General Pathology Chemokines Cytokines N-formyl- Lipid (e.g., IFN-γ) methionyl mediators LPS peptides G protein– CD14 Toll-like coupled Cytokine Recognition receptor receptors receptor Phagocytic of microbes, receptor mediators Cellular response Cytoskeletal changes, signal transduction Production of Production of reactive Phagocytosis of mediators oxygen species (ROS); microbe into (e.g., arachidonic lysosomal enzymes phagosome Increased Chemotaxis acid metabolites, integrin avidity cytokines) Functional outcomes Adhesion to Migration Amplification of the Killing of microbes endothelium into tissues inflammatory reaction FIGURE 2-2 Leukocyte receptors and responses. Different classes of receptors recognize different stimuli, initiating responses that mediate leukocyte function. IFN-g, Interferon-g; LPS, lipopolysaccharide.  Acute and Chronic Inflammation 31 Engulfment (p. 53) After binding to receptors, cytoplasmic pseudopods enclose the particle and eventually pinch off to make a phagosome vesicle. Subsequent fusion of phagosomes and lysosomes (forming a phagolysosome) discharges lysosomal contents into the space around the microbe but can also occasionally dump lysosomal granules into the extracellular space. Killing and Degradation (p. 53) Killing of phagocytosed particles is most efficient in activated leukocytes, and is accomplished largely by reactive oxygen species (ROS). Phagocytosis stimulates an oxidative burst—a surge of oxy- gen consumption with production of reactive oxygen metabolites through activation of nicotinamide-adenine dinucleotide phos- phate (NADPH) oxidase. The enzyme converts oxygen to super- oxide anion (O2 ), eventually resulting in hydrogen peroxide (H2O2). Lysosomal myeloperoxidase (MPO) then converts H2O2 and C