Fibrinoid Change: Fibrinoid change, also known as fibrinoid necrosis and fibrinoid degeneration, is a term applied to a pattern of lesions most often observed in the vascular system. The terms fibrinoid degeneration and fibrinoid necrosis are inappropriate because the process is not a true regressive alteration of cells. Rather, fibrinoid change is the result of the deposition of immunoglobulin, complement, and/or plasma proteins, including fibrin in the wall of a vessel. This lesion is caused by injury to the intima and media such as occurs in the immune-mediated vasculitides.

Grossly, fibrinoid change cannot be observed; however, it is often accompanied by thrombosis and hemorrhage, and when these two lesions are present in a vascular pattern of distribution, fibrinoid change of the vasculature should be considered. Microscopically, direct injury to endothelial cells, basement membrane, or myocytes, such as caused by viruses or toxins, or indirect injury such as caused by activation of complement proteins, can lead to activation of the acute inflammatory cascade and the deposition of plasma proteins in the vessel walls. These proteins, especially fibrin, stain intensely red (eosinophilic) with H&E stains and involve the vessel wall circumferentially to varying depths of the tunica intima and tunica media (Fig. 1-52). This lesion is also often accompanied by cellular and nuclear debris from injured vascular cells and inflammatory cells. These proteins contribute to the vascular “eosinophilia,” which has been described somewhat differently by different pathologists. There is general agreement that the material is eosinophilic, which is sometimes described as “smudgy” or “deeply eosinophilic.” Some pathologists add “homogeneous” and others “amorphous” to the descriptive terminology of fibrinoid change.

Gout: Gout is the deposition of sodium urate crystals or urates in tissue. It occurs in humans, birds, and reptiles but has not been reported in domestic mammals.

In the most common form in humans, urate crystals are deposited in the articular and periarticular tissues and elicit an acute inflammatory response characterized by the presence of neutrophils and macrophages and large aggregations of urate crystals called tophi. These tophi may be visible grossly and are pathognomonic of gout. Later in the course of the disease, the inflammation becomes chronic and a foreign body reaction to the tophi develops. Microscopically, urate crystals are acicular and birefringent and they, or the spaces left after they have been dissolved during the preparation of paraffin-embedded histologic sections, are visibly surrounded by numerous neutrophils, macrophages, and giant cells.

In birds and reptiles, there are two forms, namely (1) the articular type, which is rare, and (2) a visceral type. The latter characteristically affects the visceral serosae, particularly the parietal pericardium, and the kidneys. The serosa is covered with a thin layer of gray granules, and the gross appearance is diagnostic. In the renal form, urate deposits are visible in renal tubules and ureters. Uric acid and urates are the end-products of purine metabolism, and in birds and reptiles, these products are eliminated as semisolid urates. Visceral gout is usually diagnosed only at necropsy and is seen sporadically as the result of vitamin A deficiency, high-protein diets, and renal injury.

Pseudogout: Pseudogout is characterized by deposits of calcium pyrophosphate crystals. It is well recognized in humans but has been reported in the dog, in which it is rare. The pathogenesis of the canine disease is unknown, but in humans, one form is inherited as an autosomal dominant trait. Grossly, there are chalky white deposits in the joints, which histologically show a chronic reaction with aggregates of crystalline material, macrophages, and fibrosis. The disease may be differentiated from gout by the chemical analysis of the crystalline deposits.

Cholesterol: Cholesterol crystals are the by-products of hemorrhage and necrosis. They are dissolved out of the tissue specimen during the preparation of paraffin-embedded sections, leaving characteristic clefts that, in tissue sections, resemble shards of glass. Actually, the crystals are thin rhomboidal plates with one corner notched out, their outline resembling that of the state of Utah. Cholesterol crystals in tissue have no significance except that they indicate the site of an old hemorrhage or tissue necrosis, and they may be present in atheromas (i.e., mass of degenerated, thickened arterial intima occurring in atherosclerosis). However, in the choroid plexus of the lateral ventricles of old horses, cholesterol crystals can induce a granulomatous response, and the resultant cholesterol granuloma or cholesteatoma can become so large as to obstruct the outflow of cerebrospinal fluid through the interventricular foramen (foramen of Munro), resulting in obstructive hydrocephalus. It is thought that these granulomas are secondary to cholesterol crystals from hemorrhages into the choroid plexus. Grossly, the cholesterol appears as firm, crumbly gray nodules in the cholesteatomas.

1. Renal failure. Renal failure results in retention of phosphates, which induce a secondary renal hyperparathyroidism and hypercalcemia. Calcium is deposited in the gastric mucosa, kidney, and alveolar septa.

2. Vitamin D toxicosis. The ingestion of calcinogenic plants, such as Cestrum diurnum by herbivores, results in severe soft tissue mineralization, chiefly involving the aorta, heart, and lungs. In the heart, the endocardium of the right and left atria and left ventricle is often strikingly mineralized. Acute vitamin D toxicosis in dogs and cats is commonly caused by ingestion of rodenticides containing cholecalciferol. Intestinal mucosa, vessel walls, lung, and kidneys are mineralized.

3. Parathormone (PTH) and PTH-related protein. Primary hyperparathyroidism is rare. Hypercalcemia and elevated concentrations of PTH-related protein can be associated with canine malignant lymphomas and canine adenocarcinoma of the apocrine glands of the anal sac. Intestinal mucosa, vessel walls, lung, and kidneys are mineralized.

4. Destruction of bone from primary or metastatic neoplasms.

Carbon: Carbon is the most common exogenous pigment. The usual route of entry into the body is via inhalation, and its accumulation in the lung results in a condition called anthracosis (also known as black lung).

Carbon is ubiquitous in the air and all animals are exposed, but those most likely to show gross lesions live in an environment with substantial air pollution, such as adjacent to busy highways (e.g., animals in a zoo near a highway or animals living in a house with a smoker). In the alveoli, the carbon is phagocytosed by macrophages, which transport it via the lymphatics to the regional tracheobronchial lymph nodes. Because elemental carbon is inert and not metabolized by the body, it remains in the tissue for the life of the animal.

Grossly, the lungs are usually speckled with fine 1- to 2-mm-diameter subpleural black foci, which are most visible if the lungs have been exsanguinated (Fig. 1-56, A). In severely affected cases, the medulla of the tracheobronchial lymph nodes may be black. The heavy deposits are in this location because of the concentration of sinus histiocytes (macrophages) in the medulla.

Microscopically, carbon presents as fine black granules and may be extracellular or intracellular (within macrophages). Carbon pigment may be within the alveolar walls or be frequently present as black peribronchiolar or peribronchial foci (Fig. 1-56, B). Because of the nonreactiveness of carbon, there are no histochemical tests for it. Unlike many other pigments, it is resistant to solvents and bleaching agents.

Tattoos: Animals are frequently tattooed as a method of identification. These pigments, which include carbon, are introduced into the dermis. Some of the pigments are phagocytosed by macrophages, whereas the remainder remains free in the dermis where it can remain indefinitely and does not evoke any inflammatory reaction.

Dusts: Pneumoconiosis is the general term used for any dust inhaled into and retained in the lung. Anthracosis, from the inhalation of carbon, is a subtype of pneumoconiosis. Inhalation of silicon (e.g., from quarries) is called silicosis. These minute particles enter the lungs by escaping the mucociliary defense mechanisms of the nasal and upper respiratory systems (see Chapter 9) and are deposited in pulmonary alveoli where they may be phagocytosed and carried to the peribronchial regions. Some types of silica evoke a fibrous reaction, which may ultimately form nodules. Microscopically the mineral is visible as birefringent crystals under polarized light.

Carotenoid Pigments: Carotenoid pigments are also called lipochrome pigments, although this term is sometimes confused with lipofuscin (see later discussion). They are fat-soluble pigments of plant origin and include the precursor of vitamin A, namely β-carotene.

Grossly, these pigments normally occur in a wide variety of tissue, such as adrenal cortical cells, corpus luteum-lutein cells, Kupffer cells, and testicular cells, and in the plasma/serum and fat of horses and Jersey and Guernsey cattle and sometimes dogs (Fig. 1-57). Carotenoids discolor fat yellow to orange-yellow. The concentration of carotenoids retained in tissue depends on the species of animal. Some animals store little or no carotenoids and have white fat and clear serum. These animals include Holstein cattle, sheep, goats, and cats. As fat stores are depleted (e.g., in starvation or cachexia), carotenoids become concentrated in the adipocytes, giving them a dark yellowish-brown color.

Microscopically, carotenoids are not seen in routine formalin-fixed paraffin-embedded sections because the alcohols and clearing agents remove the fat-soluble pigments.

The significance of carotenoids is that they may obscure or confuse the detection of icterus. In those animals whose fat and serum are devoid of carotenoids, a yellow discoloration is easily detected and is most likely to be caused by bilirubin (i.e., icterus).

Tetracycline: Tetracycline-based antibiotics administered during the development of teeth are deposited in mineralizing dentin, enamel, and cementum, staining the teeth or portions of them yellow or brown (Fig. 1-58). Thus tetracycline administered to a pregnant animal stains the deciduous teeth of the offspring. Tetracycline also stains bone that is being laid down and has been used experimentally as a marker for that bone.

Melanin: Melanin is the pigment normally present in the epidermis and is responsible for the color of the skin and hair. It is also normally present in the retina, iris, and in small amounts in the pia-arachnoid of black animals (e.g., Suffolk sheep [Fig. 1-59]) and in the oral mucous membrane of some breeds (e.g., Jersey cows and Chow dogs).

Cells called melanocytes secrete melanin. In the skin of animals, these cells are in the basal layer and transfer their pigment by means of dendritic processes to adjacent keratinocytes, where the melanin is often arranged as a cap over the nucleus to provide some protection from ultraviolet radiation. Melanin is formed by the oxidation of tyrosine, which requires the copper-containing enzyme tyrosinase. Thus, in copper deficiency, particularly in cattle and sheep, there is a fading of the coat color, and this is most obvious in black wool. A general lack of melanin can be the result of a metabolic defect: a lack of tyrosinase. This condition is called albinism, and the affected animal is called an albino. Histologically, the melanocytes appear normal.

Pathologically, melanin is present in hyperpigmentation of the skin associated with many types of chronic injury and endocrinopathies, such as hyperadrenalism, and in primary neoplasms of melanocytes (melanocytomas and malignant melanomas), although highly malignant tumors may have little or no pigment.

Microscopically, melanin is stored in melanosomes in the cytoplasm of melanocytes. However, if there is irreversible injury to the cells containing melanin (e.g., necrosis of melanocytes and basal cells of the skin), melanin is released from the dead cells and is phagocytosed by macrophages, which are termed melanophages.

Extensive deposits of congenital melanin in tissues are termed congenital melanosis. It occurs in the lungs and aorta (intima) of cattle, sheep, and pigs as brown-to-black spots up to a couple of centimeters in diameter (Fig. 1-60). Melanosis of the lung is visible both subpleurally and in cross-sections of the parenchyma. These deposits of melanin have no adverse effects, but organs with extensive melanosis may be aesthetically unacceptable as food and thus will be condemned at the packing plant.

Lipofuscin-Ceroid: Lipofuscin, known as “wear-and-tear” pigment, has in the past been described as accumulating with age and in certain pathologic conditions. However, in recent years, lipofuscin, now referred to as “age” pigment, has been differentiated from a pathologically accumulating similar pigment called ceroid, which is described later.

Lipofuscin accumulates in a time-dependent manner in postmitotic cells (neurons, cardiac myocytes [Fig. 1-61], and skeletal muscle myocytes) and in slowly dividing cells, such as hepatocytes and glial cells, and this process is present at a few months of age. Lipofuscin is also found in other cells, but as these replicate, the lipofuscin is divided between the daughter cells and thus does not accumulate to the same extent as it does in postmitotic cells. Lipofuscin is the end result of autophagocytosis of cell constituents, such as organelles, and is the final undegradable remnant of that process. As the pigment cannot be removed by further lysosomal degradation or exocytosis, it accumulates in lysosomes, a form of biologic garbage.

Ceroid has many of the same histochemical features as lipofuscin (see later discussion) but is found in response to severe malnutrition, including vitamin E deficiency, cachexia from cancer, irradiation, and in the inherited disease neuronal ceroid-lipofuscinosis. It accumulates in Kupffer cells and to a lesser extent in hepatocytes, skeletal and smooth muscle myocytes, and in inherited neuronal ceroid-lipofuscinosis, where it accumulates in neurons. It can be either intracellular or extracellular. Unlike lipofuscin, it is considered to have a deleterious effect on the cell.

Both lipofuscin and ceroid have many common histologic and histochemical features, such as autofluorescence (golden yellow) and staining with stains for fat such as Sudan black (sudanophilia), although oil-red-O is more sensitive, PAS positive, and acid-fast (long Ziehl-Neelsen technique). All of these characteristics increase in intensity with age for lipofuscin but not for ceroid. Lipofuscin consists chiefly of proteins and lipids with very little carbohydrate, but lectin-binding histochemistry in humans and rats has revealed differences in the saccharides of lipofuscin and ceroid.

Grossly, large amounts of lipofuscin in the heart and skeletal muscles impart a brown tinge. It is commonly seen in aged dairy cows sent to slaughter. Ceroid is grossly evident in the small intestine of dogs with so-called intestinal lipofuscinosis (Fig. 1-62) (also see Chapter 7) and in nutritional panniculitis in cats, mink, foals, and pigs. Both of these conditions are associated with a vitamin E deficiency and the ingestion of unsaturated fatty acids. In the dog, the tunica muscularis, usually of the caudal small intestine, is discolored brown because of accumulations of ceroid in myocytes. In the cat with nutritional panniculitis, the subcutaneous fat is discolored lemon yellow to orange. This disease is considered to be the result of the ingestion of fish products with a high concentration of unsaturated fatty acids and a vitamin E deficiency, frequently brought about by the fats becoming rancid and destroying the vitamin E.

Microscopically, in routine H&E-stained sections or in unstained sections, lipofuscin varies from a light golden brown to dark brown with advancing age. Because it is intralysosomal, it is perinuclear in neurons and in cardiac, skeletal, and smooth muscle myocytes. In feline nutritional panniculitis, globules of ceroid are extracellular in the interstitial tissue or have been ingested by macrophages and giant cells.

The significance of these two pigments is that lipofuscin is a clear indicator of the age of the cell and ceroid is a pathologic pigment, often associated with vitamin E deficiency. Lectin-binding histochemistry, which has shown differences between lipofuscin and ceroid from rats and humans, may be applicable to differentiating these pigments in domestic animals, but it is a very laborious research tool and only provides semiquantitative data. Isolation and physicochemical analysis is more precise but even more laborious. Thus, until some other specific test becomes available, differentiation between the two pigments for diagnostic purposes will be based on the features listed in Table 1-1.

Hemoglobin: The normal pigment of erythrocytes, hemoglobin, can be responsible for gross changes in the color of the body. Oxygenated hemoglobin is red and imparts the pink appearance to unpigmented skin and tissues. Normally, arterial blood (oxygenated hemoglobin) is red, and venous blood with more unoxygenated blood is bluish. However, if the blood is not sufficiently oxygenated (unoxygenated hemoglobin), the tissues appear blue, exhibiting so-called cyanosis (Fig. 1-63).

In acute cyanide poisoning, cyanide binds to cytochrome oxidase, the enzyme in the cell responsible for oxidative phosphorylation, and this results in paralysis of cellular respiration. Tissues cannot use the oxygen delivered by the blood. Consequently, in acute cyanide toxicity the oxygen content and color of venous blood may be similar to those of arterial blood, and the venous blood will be bright red.

In carbon monoxide (CO) poisoning, as from exhaust gases from automobiles, the blood is a bright cherry red from the formation of carboxyhemoglobin (Fig. 1-64). Methemoglobin is an oxide of hemoglobin, in which the ferrous ion of hemoglobin is converted to the ferric ion, resulting in a reddish-brown (chocolate brown) color to the blood and tissue (Fig. 1-65). Methemoglobin is seen most often in poisoning by nitrites, especially after ingestion of nitrate-accumulating plants, but has been reported as a result of ingestion of acetaminophen, naphthalene, and chlorates and of treatment with local anesthetic agents (e.g., lidocaine, benzocaine, and tetracaine).

In intravascular hemolysis, hemoglobin is released from the lysed erythrocytes and stains the plasma pink. This hemoglobin may be excreted by the kidney, staining it dark red to reddish-black and the urine red (Fig. 1-66). Similar changes can result from myoglobinuria after the destruction of large numbers of myofibers (see Chapter 15).

Hematins: The hematin category of pigments includes “formalin pigment” and the excreta of parasites such as Fascioloides magna (liver fluke) and Pneumonyssus simicola (lung mite).

Hemosiderin: Iron is stored in the body in two forms, ferritin and hemosiderin, both of which are protein-iron complexes. Ferritin is in all tissues, but the heaviest concentrations are found in the liver, spleen, bone marrow, and skeletal muscle. Hemosiderin is formed from intracellular aggregates of ferritin (Fig. 1-69). It appears as golden-yellow to golden-brown globules and is the most visible form of storage iron. Normally, most storage iron is found in the spleen.

Excess iron from the breakdown of senescent erythrocytes, or the result of a hemolytic crisis (e.g., because of autoimmune diseases or hemotropic parasites) or reduced erythropoiesis (e.g., from malnutrition), is stored mainly in the spleen. Rarely in veterinary medicine, excess iron can be present in the body because of excessive absorption from the gut, multiple injections of iron, or from multiple blood transfusions.

Besides splenic storage, there may be local iron storage at sites of erythrocyte breakdown, such as in hemorrhages, and in areas of poor blood flow, as in chronic passive congestion of the lungs. In the latter case, because of the poor blood flow through the lungs, erythrocytes may come to the end of their natural life and be lysed or enter the alveoli by diapedesis, where they are phagocytosed by alveolar macrophages. These cells are termed heart failure cells (Fig. 1-70). Localized deposits of iron may also be present from the intramuscular injection of iron dextran, and this iron may drain to the regional lymph node.

Grossly, no change is seen in an organ or tissue if there are only small amounts of hemosiderin, but very large amounts cause a yellow-to-brown discoloration (Fig. 1-71). This color change can also be seen at sites of old bruises and other hemorrhages or hematomas. The spleen and the liver in hemolytic disease and the lungs in chronic passive congestion also appear brown. Microscopically, hemosiderin deposits are golden-yellow to golden-brown globules, which may be intracellular or extracellular (see Fig. 1-69, A). It can be confirmed by the Prussian blue reaction (see Fig. 1-69, B), which is sometimes incorrectly called a stain but is a chemical reaction, of which the end-product is Prussian blue. In the acid solution that liberates ferric iron from the hemosiderin, the ferric iron is reacted with potassium ferrocyanide (colorless) to form ferric ferrocyanide, which is Prussian blue.

The significance of hemosiderin deposits depends on their location and the amount. Normally, the spleen contains some hemosiderin, but excess hemosiderin is seen in the spleen and liver (Kupffer cells and hepatocytes) from hemolytic diseases, such as in autoimmune hemolytic anemia, and hemotropic diseases, such as babesiosis, anaplasmosis, or equine infectious anemia. Local tissue aggregations of hemosiderin are usually the result of the breakdown of erythrocytes in an old hemorrhage.

Excess hemosiderin is called hemosiderosis and must be differentiated from hemochromatosis, in which there are extreme accumulations of hemosiderin.

Hematoidin: Grossly, hematoidin is yellow-brown to orange-red pigment derived from hemoglobin but free of iron. Hematoidin closely resembles bilirubin (see next section) but is formed by cells of the macrophage-monocyte system when they phagocytose and digest red blood cells and hemoglobin in areas of hemorrhage. Microscopically, hematoidin is crystalline and polarizes light.

Bilirubin: Low concentrations of bilirubin are normally present in the plasma from the breakdown of senescent erythrocytes (see Chapter 13). Briefly, when erythrocytes have come to the end of their natural lifespan (average 70 days for a cat; average 150 days for cattle and horses), they are phagocytosed by the macrophage-monocyte system, chiefly by macrophages of the spleen and to a lesser extent by those of the bone marrow and liver (Kupffer cells). Within these cells, the iron is removed and stored, and the remainder of the porphyrin ring is broken down to bilirubin, which is released into the blood where it attaches to albumin. This bilirubin-albumin complex is too large to be excreted by the kidney. It is carried to the liver, where it enters the space of Disse, is taken up by the microvilli of the hepatocytes, is conjugated to form bilirubin glucuronide or diglucuronide, and is then excreted into the bile canaliculus.

Icterus (jaundice), the yellow staining of the tissue by bilirubin, is the result of an imbalance between production and clearance of bilirubin because there is either excess production or reduced clearance of bilirubin such that it accumulates in the plasma. The mutant Corriedale sheep model is an animal model for Dubin-Johnson syndrome in humans (Fig. 1-72).

Mechanisms leading to icterus can involve one or more of the following:

Icterus is classified several different ways. A convenient approach uses the classification of prehepatic, hepatic, and posthepatic. The most common cause of prehepatic icterus is a hemolytic crisis, which produces high plasma concentrations of unconjugated bilirubin that exceed the uptake capacity of the hepatocytes. Hepatic icterus is caused by hepatocellular damage, which results in release of bilirubin, both conjugated and unconjugated into the blood and can be the result of one or more of factors 2 to 4. Posthepatic icterus is secondary to obstruction of the biliary system, either intrahepatic or extrahepatic (hepatic bile ducts and the common bile duct), with reflux of the conjugated bilirubin into the blood. In contrast to unconjugated bilirubin, which is carried in the blood attached to albumin and cannot be excreted by the kidney, conjugated bilirubin is not bound to a plasma protein such as albumin and can be excreted.

Grossly, icteric tissues are discolored yellow, and the color change is distributed systemically. Clinically, icterus is most easily recognized in lightly pigmented animals. In living animals, icterus is detected in mucous membranes of the oral cavity, urogenital systems, and alimentary system and in normally white areas such as the sclera of the eyes. At necropsy, in addition to the sites listed previously, icterus can be identified in the omentum, mesentery, and adipose tissue (Fig. 1-73), except in Jersey and Guernsey cattle, horses, and nonhuman primates, whose sera and fat are normally discolored yellow by carotenoids. The intima of the large vessels is also a good site to detect icterus, and unless the plasma concentration is extremely high, the brain is usually unaffected.

Microscopically, icterus is not detected, but excessive quantities of bilirubin can be seen in the bile ducts and bile canaliculi in obstructive jaundice (Fig. 1-74).

Icterus is a very important clinical sign and may be detected by examination of the sclera, and in cases of anemia, in which the mucous membranes are pale; it may be visible there. Laboratory tests to determine the exact plasma or serum concentrations of bilirubin, and preferably whether it is or is not conjugated, are essential. It is critical to realize that hyperbilirubinemia is not the same as icterus. Most domestic animals normally have very low serum concentrations of bilirubin, usually less than 1 mg/dL, except for the horse, in which it may range from 1 to 3 mg/dL. Icterus is not detected until the serum concentration exceeds 1.5 to 2.0 mg/100 mL. Thus hyperbilirubinemia can be present without causing icterus.

Porphyria: Congenital erythropoietic porphyria of calves, cats, and pigs is an inherited metabolic defect in heme synthesis caused by a deficiency of uroporphyrinogen III cosynthetase. The disease is sometimes incorrectly called osteohemachromatosis. It is also known colloquially as pink tooth because of the discoloration by the porphyrins accumulating in dentin and bone (Fig. 1-75, also see Chapter 7). The teeth and bones of young animals are reddish (pink tooth) and those of adults are dark brown. In these cases, both bones and teeth fluoresce red under ultraviolet radiation.

Replicative Senescence

The concept that many cells have a limited capacity for replication was developed from a simple experimental model for aging. Normal human fibroblasts, when placed in tissue culture, have limited division potential. Cells from children undergo more rounds of replication than cells from older people. After a fixed number of divisions, all cells become arrested in a terminally nondividing state, known as cellular senescence. Many changes in gene expression occur during cellular aging, but a key question is which of these changes are causes and which are effects of cellular senescence.

How dividing cells can count their divisions is under intensive investigation. One likely mechanism is that with each cell division, there is incomplete replication of chromosome ends (telomere shortening), which ultimately results in cell cycle arrest (Web Fig. 1-2). Telomeres are short repeated sequences of DNA (TTAGGG) present at the linear ends of chromosomes and are important for ensuring the complete replication of chromosome ends and for protecting them from fusion and degradation.

When somatic cells replicate, a small section of the telomere is not duplicated, and telomeres become progressively shortened. As the telomeres become shorter, the ends of chromosomes cannot be protected and are seen as broken DNA, which signals cell cycle arrest. The lengths of the telomeres are normally maintained by nucleotide addition, mediated by an enzyme called telomerase. Telomerase is a specialized RNA-protein complex that uses its own RNA as a template for adding nucleotides to the ends of chromosomes. The activity of telomerase is repressed by regulatory proteins, which restrict telomere elongation, thus providing a length-sensing mechanism. Telomerase activity is present in germ cells and is present at low levels in stem cells, and it is usually absent in most somatic tissue. Therefore, as cells age, their telomeres become shorter and ultimately the cell ceases to cycle in the cell cycle, resulting in an inability to generate new cells to replace damaged ones. Conversely, in immortal cancer cells, telomerase is reactivated and telomeres are not shortened, suggesting that telomere elongation might be an important—possibly essential—step in tumor formation. However, the relationship of telomerase activity and telomeric length to aging and cancer still needs to be fully established.

Accumulation of Metabolic and Genetic Damage

In addition to the importance of age and a genetic clock, cellular lifespan may also be determined by the balance between cellular damage resulting from metabolic activities occurring within the cell and the response of the counteracting molecular mechanisms responsible for repair. Smaller animals have generally shorter lifespans and faster metabolic rates, suggesting that the lifespan of a species is limited by fixed total metabolic consumption over a lifetime.

One group of products of normal metabolism is reactive oxygen species. These by-products of oxidative phosphorylation cause covalent modifications of proteins, lipids, and nucleic acids. The amount of oxidative damage, which increases as an organism ages, may be an important component of senescence, and the accumulation of lipofuscin in aging cells is seen as the indicator of this damage. The role of oxidative damage in aging is supported by the following observations: (1) variation in longevity among different species is inversely correlated with the rates of mitochondrial generation of a superoxide anion radical and (2) overexpression of the antioxidative enzymes SOD and catalase extends the lifespan in transgenic forms of Drosophila. Thus part of the mechanism that monitors the duration of aging may be the cumulative damage caused by toxic by-products of metabolism such as oxygen radicals. Increased oxidative damage could result from repeated environmental exposure to such influences as ionizing radiation, progressive reduction of antioxidant defense mechanisms (e.g., vitamin E and glutathione peroxidase), or both.

A number of protective responses counterbalance progressive damage in cells, and an important one is the cell’s recognition and repair of damaged DNA. Although most DNA damage is repaired by endogenous DNA repair enzymes, some persists and therefore damaged DNA progressively accumulates as the cell ages. Thus the balance between cumulative metabolic damage and the effectiveness of the response to that damage could determine the rate at which animals age. It is possible that aging may be delayed by decreasing the accumulation of the effects of that damage or by increasing the response to that damage.

Not only damaged DNA but also damaged cellular organelles accumulate as cells age. In part, this may be the result of declining function of the proteasome, the proteolytic machine that serves to eliminate abnormal and unwanted intracellular proteins.

Nuclear Chromosomes

Each animal species has a unique chromosomal complement called a karyotype (i.e., the number and morphology of the chromosomes that make up its genome). With the exception of cells that develop into ova and spermatozoa (i.e., germline cells), all cells in the body are called somatic cells (e.g., soma: body). The genome contained in the nucleus of somatic cells consists of chromosomes arranged in pairs. All except one pair are similar in both males and females and are called autosomes, and the remaining pair is the sex chromosomes: two X chromosomes in females and an X and a Y chromosome in males. Although each chromosome has different genes, members of a pair of chromosomes, also called homologous chromosomes or homologues, carry matching genetic information, that is, they have the same genes in the same sequence. Any specific locus (i.e., the specific location of a gene) on a chromosome may have either identical or slightly different forms of the same gene, called alleles. One member of each pair of chromosomes is inherited from the sire, the other from the dam.

Mitochondrial Chromosomes

Mitochondria are the site of aerobic energy production in all cells of the body. In highly active cells, such as type I myofibers of equine athletes, up to 10,000 mitochondria may be present in the cytoplasm of a myofiber. Each mitochondrion contains a single chromosome formed by circular double-stranded DNA, called mitochondrial DNA (mtDNA). The genome of a mitochondrial chromosome encodes for 37 genes, including those for messenger RNAs (mRNAs), ribosomal RNAs (rRNAs), transfer RNAs (tRNAs), and 13 protein subunits for enzymes, such as cytochrome b and cytochrome oxidase, which are involved in the production of energy by oxidative phosphorylation. The mitochondrial genome also has distinct transcription and protein-synthesis (i.e., translation) systems. A specialized RNA polymerase, encoded in the nuclear genome, is used to transcribe the mitochondrial genome and then the mitochondrial transcripts are processed to generate the various individual mitochondrial mRNAs, tRNAs, and rRNAs.

1. Single-gene disorders caused by mutations in DNA of a single gene such as point, frameshift, and trinucleotide-repeat mutations

2. Chromosomal disorders caused by alterations in the number and/or structure of chromosomes (i.e., its karyotype)

3. Complex multigenic disorders

Single-Gene Disorders

Most germline cells are meiotic cells, and disorders involving them can be inherited. Somatic cells are mitotic cells, and disorders involving mitotic cells are not heritable but are important in the genesis of cancers and some congenital malformations. Single-gene disorders can affect either germline cells or somatic cells and usually result from mutations in DNA from (1) environmental causes such as exposure to excessive ultraviolet light, excessive radiation, or certain chemicals (i.e., mutagens) or (2) errors in cell division when somatic or germline cells copy their DNA in preparation for mitosis or meiosis, respectively. The actual occurrence of mutations is very low because cells have DNA repair proteins that correct mistakes in the DNA caused by mutagens (Fig. 1-79). These repair proteins determine which nucleotide bases are paired incorrectly, and then replace the incorrect base with the correct one. Mutations of the genes for these repair proteins often have serious outcomes, especially in neoplastic transformation of somatic cells. Single-gene disorders must be differentiated from single nucleotide polymorphisms (SNPs). SNPs represent differences in a single DNA nucleotide between animals of the same species (i.e., different breeds) and are the most common type of genetic variation among animals. They are usually found in regions between genes whose functions are known, thus they have no effect on health or development. However, if they occur within a gene or in a regulatory region near a gene, they may play a more direct role in disease. SNPs most commonly serve as biologic markers to locate genes that are associated with disease or hereditable traits such as muscling, weight gain, and milk production.

Because single-gene disorders arise from a mutation in a single gene, they result in a permanent change of the cell’s nuclear DNA (Figs. 1-80 and 1-81). Such mutations can affect the synthesis of proteins by disrupting one or more steps in the normal transcriptional and translational processes and lead to the following:

Virtually any type of protein may be affected in single-gene disorders, and the mechanisms involved can be classified into the following four categories:

In enzyme defects, mutations may result in the synthesis of a defective enzyme with reduced activity or in a reduced amount of a normal enzyme. An enzyme defect may lead to three major consequences as follows:

Mutations resulting in the accumulation of substrates or the blockage of normal metabolic pathways are best illustrated by a group of diseases called storage diseases, in which defective processing of a metabolic substrate leads to the accumulation of the substrate in cell cytoplasm or within lysosomes in the cell. Such diseases are discussed in the organ system chapters of this book. Storage diseases are commonly grouped as lysosomal storage diseases and glycogen storage diseases. Lysosomal storage diseases are characterized by a deficiency of lysosomal acid hydrolases, in which breakdown of their substrates remain incomplete, leading to the accumulation of the partially degraded insoluble metabolite within the lysosomes (Fig. 1-82). Lysosomal acid hydrolases break down a variety of complex macromolecules derived from the metabolic turnover of intracellular organelles or acquired from outside the cells by phagocytosis. Stuffed with incompletely digested macromolecules, these organelles become large and numerous enough to interfere with normal cell functions. Lysosomal storage diseases are exemplified by globoid cell leukodystrophy in which a deficiency in the function of a lysosomal enzyme called lysosomal galactocerebroside β-galactosidase (galactosylceramidase) results in the accumulation of a substrate, galactocerebroside, in lysosomes of macrophages recruited as monocytes from the vascular system (see Fig. 14-64).

Glycogen storage diseases (glycogenoses) are caused by a deficiency of one of the enzymes involved in the synthesis or sequential degradation of glycogen (Figs. 1-83 and 14-63). Glycogen storage diseases are exemplified by glycogenosis type III (Cori’s disease), in which a deficiency in the function of an amylo-1, 6-glucosidase (debranching enzyme) results in the accumulation of a structurally abnormal glycogen within hepatocytes.

A failure to inactivate tissue-damaging substrates is illustrated by α₁-antitrypsin deficiency. Animals that have an inherited deficiency of serum α₁-antitrypsin are unable to inactivate neutrophil elastase in their lungs. Unchecked activity of this protease leads to destruction of elastin in the walls of lung alveoli, eventually leading to pulmonary emphysema. The consequences of defects in membrane receptors and transport systems; alterations in the structure, function, or quantity of nonenzyme proteins such as collagen, spectrin, and dystrophin in osteogenesis imperfecta; or mutations resulting in unusual reactions to drugs have not been adequately documented in animals.

Chromosomal Disorders

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