Impaired Energy Production
High-energy phosphate bonds of adenosine triphosphate (ATP) represent the most efficient energy source for the cell. ATP is produced by phosphorylation of adenosine diphosphate (ADP), a reaction that is linked to the oxidation of reduced substances in the respiratory chain of enzymes. Oxygen is required (oxidative phosphorylation) (Figure 1-2).
Main biochemical pathways involved in cellular ATP (energy) production. Abnormalities that result in failure of energy production are noted by letters that correspond to the accompanying text description.
Causes of Defective Energy (ATP) Production
Glucose is the main substrate for energy production in most tissues and is the sole energy source in brain cells. Low glucose levels in blood (hypoglycemia) therefore result in deficient ATP production that is most profound in the brain.
Oxygen reaches the cells via arterial blood but is ultimately derived from the atmosphere. Most of the oxygen carried in blood is bound to hemoglobin. Lack of oxygen in the cells (hypoxia) may result from (1) respiratory obstruction or disease, preventing oxygenation of blood in the lungs; (2) ischemia, or failure of blood flow in the tissue, due either to generalized circulatory failure or to local vessel obstruction; (3) anemia (ie, decreased hemoglobin in the blood), resulting in decreased oxygen carriage by the blood; or (4) alteration of hemoglobin (as occurs in carbon monoxide poisoning), making it unavailable for oxygen transport and leading to the same result as anemia.
Cyanide poisoning is a good example of a chemical interfering with a vital enzyme. Cyanide inhibits cytochrome oxidase, the final enzyme in the respiratory chain, causing acute ATP deficiency in all cells of the body and rapid death.
Uncoupling of Oxidative Phosphorylation
Uncoupling of oxidation and phosphorylation occurs either through chemical reactions or through physical detachment of enzymes from the mitochondrial membrane. Mitochondrial swelling, which is a common change associated with many types of injury, causes uncoupling of oxidative phosphorylation.
Effects of Defective Energy Production
Generalized failure of energy production will first affect those cells with the highest demand for oxygen because of their high basal metabolic rate. Brain cells are maximally affected. The earliest clinical signs of hypoxia and hypoglycemia are disturbances of the normal level of consciousness.
Intracellular Accumulation of Water and Electrolysis
The earliest detectable biochemical evidence of diminished availability of ATP is dysfunction of the energy-dependent sodium pump in the plasma membrane. The resulting influx of sodium and water into the cell leads to cloudy swelling, or hydropic change, an early and reversible effect of cell injury. (The cloudy appearance is due to the cytoplasmic organelles dispersed in the swollen cell.) Changes also occur in the intracellular concentrations of other electrolytes (particularly K+, Ca2+, and Mg2+), that are maintained by energy-dependent activity of the plasma membrane. These electrolyte abnormalities may lead to disordered electrical activity and enzyme inhibition.
Swelling of cytoplasmic organelles follows influx of sodium and water. Distention of the endoplasmic reticulum detaches the ribosomes and interferes with protein synthesis. Mitochondrial swelling causes physical dissociation (uncoupling) of oxidative phosphorylation, which further impairs ATP synthesis.
Switch to Anaerobic Metabolism
In hypoxic conditions, cellular metabolism changes from aerobic to anaerobic glycolysis. The conversion leads to the production of lactic acid and causes a decrease in intracellular pH. Chromatin clumping in the nucleus and further disruption of organelle membranes then occur. Disruption of lysosomal membranes leads to release of lysosomal enzymes into the cytoplasm, which damages vital intracellular molecules.
The exact point at which cellular degeneration becomes irreversible, resulting in necrosis, is unknown.
Impaired Cell Membrane Function
Causes of Plasma Membrane Damage
Production of Free Radicals
(Figure 1-3.) Free radicals are highly unstable particles with an odd number of electrons (an unpaired electron) in their outer shell. The excess energy attributable to the unstable configuration is released through chemical reactions with adjacent molecules. One of the best known interactions is that between oxygen-based free radicals and cell membrane lipids (lipid peroxidation), which leads to membrane damage.
Free radicals and cell injury. The various agents that produce free radicals are shown in the left column, with mechanisms of action in the right column. Healthy cells possess a number of antioxidant mechanisms that limit the effects of toxic free radicals.
Activation of the Complement System
The final compounds of the activated complement pathway (Chapter 4: The Immune Response), probably a complex of C5b, C6, C7, C8, and C9, exert a phospholipase-like effect that can enzymatically damage the plasma membrane. This phenomenon (complement fixation and activation) is an important component of the immune response that causes the death of cells recognized as foreign.
Enzymes with lipase-like activity damage cell membranes. For example, pancreatic lipases—when they are liberated outside the pancreatic duct in acute pancreatic inflammation—damage nearby cells and cause extensive necrosis. Some microorganisms—eg, Clostridium perfringens, one of the causes of gas gangrene—produce enzymes that damage plasma membranes and cause extensive necrosis.
Cytopathic viruses cause lysis by direct insertion into the cell membrane. Other viruses cause lysis indirectly via an immune response to virally determined antigens on the surface of infected cells.
Lysis by Physical and Chemical Agents
Extremes of heat and cold and certain chemicals (solvents) may cause direct lysis of cells.
Effects of Plasma Membrane Damage
Loss of Structural Integrity
Severe injury to the plasma membrane leads to rupture and necrosis. Less severe injury produces localized damage, which may be repaired, although with some membrane loss. In erythrocytes, this process leads to the formation of microspherocytes (smaller and rounder red cells; see Chapter 25: Blood: II. Hemolytic Anemias; Polycythemia).
The plasma membrane maintains the internal chemical composition of the cell by means of selective permeability and active transport. Damage to the plasma membrane may result in abnormal entry of water, causing cloudy swelling and hydropic change identical to that resulting from injury due to defective energy production. Abnormal permeability occurs for Na+, K+, Ca2+, and other ions.
Deposition of Lipofuscin (Brown Atrophy)
Lipofuscin is a fine, granular, golden-brown pigment composed of phospholipids and proteins. It accumulates in the cytoplasm as a result of damage to the membranes of cytoplasmic organelles and is most commonly seen in myocardial cells (Figure 1-4), liver cells, and neurons. Lipofuscin causes no cellular functional abnormalities.
Myocardial fiber with lipofuscin pigment in the perinuclear region. On sections stained with hematoxylin and eosin, lipofuscin has a golden brown color.
Lipofuscin deposition occurs in elderly individuals, those suffering from severe malnutrition, and those with chronic diseases. It is due to a lack of cellular antioxidants that normally prevent lipid peroxidation of organelle membranes. Lipofuscin is also called “wear and tear” pigment.
Deoxyribonucleic acid (DNA) in the chromosomes represents the genetic basis of control of cellular function. DNA controls the synthesis of structural proteins (Figure 1-5), growth-regulating proteins, and enzymes.
Protein synthesis. Nucleic acids are represented as lines with multiple short projections representing the bases. Changes in the nucleotide sequence will lead to synthesis of an abnormal protein or failure of synthesis of the protein. Amino acids are represented as A1–A4.
Causes of DNA Abnormalities
Inherited genetic abnormalities are passed from generation to generation, frequently in predictable fashion according to mendelian laws (Chapter 15: Disorders of Development). Acquired genetic abnormalities are somatic mutations resulting from damage to genetic material by any of several agents, including ionizing radiation, viruses, and mutagenic drugs and chemicals.
Effects of DNA Abnormalities
The clinical and pathologic effects of genetic abnormalities depend on (1) the severity of damage, (2) the precise gene or genes damaged, and (3) when the damage was sustained. When genetic damage is inherited or occurs during gametogenesis or early fetal development, clinical effects may be present at birth (congenital genetic disease). Acquired genetic disease results when genetic damage occurs postnatally.
DNA abnormalities are manifested at a cellular level in several ways.
Failure of Synthesis of Structural Proteins
Severe damage to DNA in the nucleus—as occurs after high doses of radiation and some viral infections—causes necrosis due to inhibition of synthesis of vital intracellular structural proteins. Less severe damage may result in a variety of effects, depending on the extent of inhibition and the type of protein synthesis that is inhibited.
Interference with mitosis in actively dividing cells (eg, bone marrow cells) may result in depletion of erythrocytes (anemia) and neutrophils (neutropenia). Similar depletion of cells may occur in intestinal mucosa, resulting in abnormal structure and function. Failure of mitosis in the testis may result in decreased spermatogenesis, manifested as infertility.
Failure of Growth-Regulating Proteins
Changes in growth regulation that result from DNA damage may result in cancer (see Chapter 18: Neoplasia: II. Mechanisms & Causes of Neoplasia).
Failure of Enzyme Synthesis
Enzyme deficiency in the embryo may result in congenital diseases (inborn errors of metabolism). Acquired enzyme defects result in necrosis if a vital biochemical system is affected. Enzyme defects involving less vital biochemical reactions result in a variety of sublethal degenerative changes (Chapter 15: Disorders of Development).
Many exogenous injurious agents, including alcohol, drugs, heavy metals, and infectious agents, cause cellular degeneration and necrosis by interfering directly with various specific biochemical reactions. Individual injurious agents and their effects on cellular metabolism are discussed in Section III (Chapter 8: Immunologic Injury, Chapter 9: Abnormalities of Blood Supply, Chapter 10: Nutritional Diseases, Chapter 11: Disorders Due to Physical Agents, Chapter 12: Disorders Due to Chemical Agents, Chapter 13: Infectious Diseases: I. Mechanisms of Tissue Changes in Infection, and Chapter 14: Infectious Diseases: II. Diagnosis of Infectious Diseases). Depending upon their severity, they may produce cellular degeneration or necrosis.
Accumulation of Endogenous Substances
Endogenous Substances Accumulating in Tissues As a Result of Deranged Metabolism.
Fatty Change (Fatty Degeneration)
Fatty change is the accumulation of triglyceride in the cytoplasm of parenchymal cells. It is common in the liver and rare in the kidney and myocardium and occurs as a nonspecific response to many types of injury.
Normal Triglyceride Metabolism in the Liver
The liver plays a central role in triglyceride metabolism (Figure 1-6). Free fatty acids are carried in the blood to the liver, where they are converted to triglycerides, phospholipids, and cholesteryl esters. After these lipids form complexes with specific lipid acceptor proteins (apoproteins), which are also synthesized in the liver cell, they are secreted into the plasma as lipoproteins. When triglycerides are metabolized normally, there is so little triglyceride in the liver cell that it cannot be seen in routine microscopic sections.
Fat metabolism in the liver cell. Numbers shown correspond with circled numbers in the section on causes of fatty liver as described in the text.
Accumulation of triglycerides in the cytoplasm of liver cells (fatty liver) represents an abnormality of the metabolic pathway shown in Figure 1-6 and occurs in the following conditions:* ① When there is increased mobilization of adipose tissue, resulting in an increase in the amount of fatty acids reaching the liver, eg, in starvation and diabetes mellitus. ② When the rate of conversion of fatty acids to triglycerides in the liver cell is increased because of overactivity of the involved enzyme systems. This is the main mechanism by which alcohol, a powerful enzyme inducer, causes fatty liver. ③ When oxidation of triglycerides to acetyl-CoA and ketone bodies is decreased, eg, in anemia and hypoxia. ④ When synthesis of lipid acceptor proteins is deficient. Protein malnutrition and several hepatotoxins, eg, carbon tetrachloride and phosphorus, cause fatty liver in this way.
*Circled numbers in the following text correspond to heavy numbered arrows in Figure 1-6.
Acute fatty liver is a rare but serious condition associated with acute liver failure (Chapter 42: The Liver: I. Structure & Function; Infections). In acute fatty liver, triglyceride accumulates as small, membrane-bound droplets in the cytoplasm (microvacuolar fatty change, Figure 1-7).
Acute microvacuolar fatty change of the liver in Reye's syndrome. The cytoplasm of the liver cells is filled with numerous small vacuoles representing the lipid that has been dissolved out of the tissue during processing. The nuclei are centrally located.
Chronic fatty liver is much more common. It is associated with chronic alcoholism, malnutrition, and several hepatotoxins. Fat droplets in the cytoplasm fuse to form progressively larger globules (macrovacuolar fatty change, Figure 1-8). The distribution of fatty change in the liver lobule varies with different causes (Figure 1-9). Grossly, the fatty liver is enlarged and yellow, with a greasy appearance when cut. Even when severe, chronic fatty liver is rarely associated with clinically detectable liver dysfunction.
Macrovacuolar fatty change of the liver in chronic alcoholism. The large fat globules in the cytoplasm appear as empty spaces that have displaced the nucleus to the side. The degree of fatty change varies from slight in the bottom left to marked at the top right of this photograph.
Distribution of fatty change (tinted circles) in the liver in hypoxic and toxic liver injuries. In hypoxic injury, fatty change is centrizonal; in toxic injury, fatty change occurs around the portal areas. The rules relating to this distribution, which are dependent on the mode of entry of oxygen and toxins into the liver lobule, are not without exception. Carbon tetrachloride, for example, causes centrizonal fatty change.
Fatty Change of the Myocardium
Triglyceride deposition in myocardial fibers occurs in chronic hypoxic states, notably severe anemia. In chronic fatty change, bands of yellow streaks alternate with red-brown muscle (“thrush breast” or “tiger skin” appearance); this usually causes no clinical symptoms. Toxic diseases such as diphtheritic myocarditis and Reye's syndrome produce acute fatty change. The heart is flabby and shows diffuse yellow discoloration; myocardial failure commonly follows.
Microscopic Features of Fatty Change
Any fat present in tissues dissolves in the solvents that are used to process tissue samples for microscopic sections. In routine tissue sections, therefore, cells in the earliest stages of fatty change have pale and foamy cytoplasm. As fat accumulation increases, cytoplasmic vacuoles appear. Positive demonstration of fat requires the use of frozen sections made from fresh tissue. Fat remains in the cytoplasm in frozen sections, where it can be demonstrated by fat stains such as oil red O and Sudan black B.
Deposition of Iron (Hemosiderosis and Hemochromatosis)
(Figure 1-10.) Iron metabolism is normally regulated so that the total amount of iron in the body is maintained within a narrow range. The body has no effective mechanism for eliminating excess iron, although women lose 20–30 mg of iron each month in menstrual blood. Iron overload is therefore rare in premenopausal women, whereas iron deficiency is common.
Iron metabolism. Normally, iron loss is balanced by intestinal absorption. Negative balance due to a loss that cannot be compensated for by increased absorption leads to depletion of iron stores and development of anemia. Positive iron balance due to increased absorption or administration of excessive iron (usually in blood transfusions) leads to excessive iron storage.
Hemosiderosis and Hemochromatosis
An increase in the total amount of iron in the body is termed hemosiderosis or hemochromatosis. The excess iron accumulates in macrophages and parenchymal cells as ferritin and hemosiderin and may cause parenchymal cell necrosis (Figure 1-11).
Hemochromatosis of the liver, showing hemosiderin pigment deposited in hepatocytes and Kupffer cells. Hemosiderin stains golden brown with hematoxylin and eosin and deep blue with Prussian blue stain.
Causes and Effects of Deposition of Iron
Localized hemosiderosis is common in any tissue that is the site of hemorrhage. Hemoglobin is broken down and its iron is deposited locally, either in macrophages or in the connective tissue, in the form of hemosiderin (as in a bruise). Localized hemosiderosis has no clinical significance.
Generalized hemosiderosis is less common, occurring with relatively minor iron excess following multiple transfusions, excessive dietary iron, or excess absorption of iron in some hemolytic anemias. The excess iron is deposited as hemosiderin in macrophages throughout the body, notably in bone marrow, liver, and spleen. Generalized hemosiderosis can be diagnosed in bone marrow and liver biopsies and, apart from indicating the presence of iron overload of minor degree, has no clinical significance.
Hemochromatosis is uncommon, occurring both as an idiopathic (inherited) disease and as a secondary phenomenon following major iron overload. The distinction between hemosiderosis and hemochromatosis is somewhat arbitrary, the major differences being the degree of iron overload and the presence of parenchymal cell damage or necrosis in hemochromatosis.
It is postulated that once intracellular storage mechanisms are exhausted, free ferric iron accumulates and undergoes reduction to produce toxic oxygen-based free radicals. The liver, heart, and pancreas are the most severely affected tissues in hemochromatosis (Chapter 43: The Liver: II. Toxic & Metabolic Diseases; Neoplasms).
Deposition of Copper (Wilson's Disease)
Copper is normally transported in the plasma as ceruloplasmin, composed of copper complexed with an α2-globulin, and “free” copper, which is loosely bound to albumin. Normally, copper absorption is balanced by excretion, mainly in bile.
In Wilson's disease, excretion of copper into bile is defective and leads to an increase in total body copper, with accumulation of copper in cells. The liver, basal ganglia of the brain, and the cornea (Kayser-Fleischer ring) (Chapter 43: The Liver: II. Toxic & Metabolic Diseases; Neoplasms) are the most severely affected tissues.
Accumulation of Bilirubin (Jaundice or Icterus)
(Figure 1-12.) Bilirubin is the catabolic end product of the porphyrin ring of the hemoglobin molecule; it contains neither iron nor protein. It is formed in the reticuloendothelial system, where senescent erythrocytes are destroyed. Bilirubin is then transported in the plasma to the liver in an unconjugated form, bound to albumin. Unconjugated bilirubin is lipid-soluble. In the liver, bilirubin is conjugated enzymatically with glucuronide to form water-soluble conjugated bilirubin, which is excreted by liver cells into the bile and thence to the intestine. In the intestine, bacterial activity converts bilirubin to urobilinogen, which is disposed of in one of three ways: (1) directly excreted in feces (as stercobilin); (2) absorbed in the portal vein and reexcreted into bile by the liver in the enterohepatic circulation; or (3) excreted in urine, normally in small amounts (Figure 1-12).
Bilirubin metabolism and causes of jaundice. In hemolytic jaundice ①, there is increased bilirubin formation due to increased hemoglobin breakdown. In hepatocellular jaundice ②, conjugation and excretion of bilirubin by the liver are defective. In obstructive jaundice ③, conjugated bilirubin refluxes into the blood.
(See also Chapter 42: The Liver: I. Structure & Function; Infections.) An increase in serum bilirubin is called jaundice, or icterus. Jaundice may result from three distinct mechanisms (Table 1-2): increased production, decreased excretion by the liver, or bile duct obstruction.
Differential Features of the Different Types of Jaundice.
Differential Features of the Different Types of Jaundice.
Excessive production of bilirubin
Defective uptake, conjugation or excretion of bilirubin by liver cells
Obstruction of bile ducts
Elevation of serum bilirubin
Type of bilirubin in plasma
Conjugated and unconjugated
Bilirubin in urine
Urobilinogen in urine
Stercobilin in feces
Red cell survival
Liver function tests
May contain pigment stones
Obstructed, with proximal dilatation
Hemolytic Jaundice (Increased Production)
Increased destruction of erythrocytes, if sufficiently severe, overwhelms the capacity of the liver to conjugate bilirubin and results in accumulation of unconjugated bilirubin in serum. Because unconjugated bilirubin is lipid-soluble and bound to albumin in the blood, it is not excreted in the urine (acholuric jaundice) (Figure 1-12).
Hepatocellular Jaundice (Decreased Uptake, Conjugation, or Excretion)
Failure of the liver to take up, conjugate, or excrete bilirubin results in an increase in serum bilirubin. Usually, both conjugated and unconjugated bilirubin levels are elevated, the proportions depending on which metabolic failure predominates. Conjugated, water-soluble bilirubin is commonly present in urine. Urinary urobilinogen levels are usually elevated because liver dysfunction prevents normal uptake and reexcretion of urobilinogen absorbed from the intestine.
Obstructive Jaundice (Decreased Excretion)
Biliary tract obstruction results in an accumulation of conjugated bilirubin proximal to the obstruction in the biliary tract and liver (cholestasis). In a manner not clearly understood, reflux of conjugated bilirubin into the plasma occurs, causing jaundice; some conjugated bilirubin is then excreted in the urine. Failure of bilirubin to reach the intestine causes a decrease in fecal and urinary urobilinogen levels. In complete biliary obstruction, absence of bilirubin alters the normal color of the feces (producing clay-colored stools).
Effects of Deposition of Bilirubin
Deposition in Connective Tissue
The increase in serum bilirubin leads to deposition of bilirubin in the connective tissue of the skin, scleras, and internal organs. The resulting yellow-green discoloration is characteristic of jaundice. No functional abnormality results from bilirubin accumulation in connective tissue.
Deposition in Parenchymal Cells
Basal ganglia–Kernicterus is an uncommon condition in which unconjugated bilirubin is deposited in the basal ganglia (nuclei) of the brain (Figure 1-13). It occurs only with an increase in unconjugated bilirubin, which is lipid-soluble and can cross the blood-brain barrier.
The most common cause of kernicterus is severe neonatal hemolysis, usually as a result of Rh blood group incompatibility between mother and baby (Figure 1-13). (See also Chapter 25: Blood: II. Hemolytic Anemias; Polycythemia.)
Intracellular accumulation of bilirubin in brain cells causes neuronal dysfunction and necrosis, which may cause death in the acute phase. Infants who survive the acute phase show the effects of neuronal loss.
Liver–Accumulation of bilirubin in liver cells in obstructive jaundice results in toxic injury associated with cellular swelling and, if severe, necrosis. Fibrosis follows and may lead to biliary cirrhosis and chronic liver failure (Chapter 42: The Liver: I. Structure & Function; Infections and Chapter 43: The Liver: II. Toxic & Metabolic Diseases; Neoplasms).
Factors involved in the pathogenesis of kernicterus. Increased hemolysis ① leads to increased production of unconjugated bilirubin ②, which, in the neonate, is not cleared efficiently owing to immaturity of liver enzyme systems ③. Unconjugated bilirubin is normally complexed with plasma albumin, levels of which may also be low in neonates ④. Unconjugated bilirubin that is not complexed to albumin (Free ucb) can cross the blood-brain barrier in the neonatal period ⑤, causing toxic neuronal injury ⑥ and kernicterus ⑦.
The synthesis of ubiquitin and the family of heat shock proteins is increased soon after injury due to any cause. Heat shock proteins are believed to protect other cell proteins from denaturation. Ubiquitin serves a housekeeping function by linking with damaged proteins. In severe injury, ubiquitin-protein complexes may form cytoplasmic inclusions (eg, Mallory bodies in hepatocytes, ubiquitin/keratin; Lewy bodies in neurons of Parkinson's disease, ubiquitin/neurofilaments).
Accumulation of Other Substances
Other endogenous products that may accumulate in cells or in interstitial tissues are discussed in Chapter 2: Abnormalities of Interstitial Tissues (see also Table 1-1). Toxic substances that accumulate in hepatic and renal disease are discussed in Chapter 33: The Eye and Chapter 48: The Kidney: II. Glomerular Diseases, respectively.