2.7.1 General Pathology

Overview of General Pathology

General pathology studies the basic reactions of cells and tissues to injury and the mechanisms of disease that apply to many organs. Instead of focusing on a single system, it explains the common language of disease. Concepts like cell injury, inflammation, hemodynamic disorders, and neoplasia begin here, and later chapters apply them to specific organs.

For Step 1 preparation, the goal in general pathology is to recognize patterns. Once you see how different injuries lead to predictable cellular responses, clinical vignettes become easier to decode. This chapter introduces the central themes, but leaves organ specific details for systemic pathology.

Cellular Adaptations

When cells face stress that is not immediately lethal, they adjust their structure or function. These adjustments are cellular adaptations. They are usually reversible, and they can be physiologic, as in pregnancy, or pathologic, as in chronic hypertension.

The main forms of adaptation are hypertrophy, hyperplasia, atrophy, and metaplasia. In hypertrophy, individual cells increase in size, so the organ enlarges without increasing cell number. For example, the uterus in pregnancy shows physiologic hypertrophy of smooth muscle, while the left ventricle of a patient with long standing hypertension shows pathologic hypertrophy of cardiac myocytes.

Hyperplasia is an increase in the number of cells, leading to organ enlargement. It can be driven by hormones, as in glandular hyperplasia of the breast during puberty, or by growth factors, as in compensatory hyperplasia of the liver after partial hepatectomy. Pathologic examples, such as endometrial hyperplasia due to unopposed estrogen, increase the risk of malignancy.

Atrophy represents a decrease in cell size and sometimes number. It is a response to reduced workload, loss of innervation, diminished blood supply, inadequate nutrition, or aging. Atrophic cells show increased autophagic vacuoles and residual bodies, such as lipofuscin, reflecting ongoing self digestion.

Metaplasia is a reversible change in which one differentiated cell type is replaced by another that is better able to withstand the stress. Chronic smoking, for instance, can transform normal ciliated columnar epithelium of the bronchus into stratified squamous epithelium, a change that protects against irritation but compromises mucociliary clearance. Persistent metaplasia can set the stage for dysplasia and carcinoma, which are discussed separately under neoplasia.

Reversible and Irreversible Cell Injury

When a cell is exposed to more severe or prolonged stress, adaptation is not sufficient, and injury develops. General pathology distinguishes between reversible and irreversible injury.

Reversible injury is characterized by functional and morphologic changes that can return to normal if the damaging stimulus is removed. The classic pattern is cellular swelling due to failure of energy dependent ion pumps, which causes water influx. On light microscopy, this appears as hydropic change with pale, enlarged cells and small, clear cytoplasmic vacuoles. Fatty change, especially in liver and heart, is another reversible pattern and is due to disturbed lipid metabolism.

Irreversible injury culminates in cell death. The crucial point is loss of membrane integrity, profound mitochondrial dysfunction, and nuclear changes. Once membrane damage allows massive influx of calcium and leakage of cellular enzymes, the cell cannot recover. For exams, serum markers such as troponin and AST or ALT signal leakage of enzymes from irreversibly injured cells.

In general, the progression is from normal to reversible injury to irreversible injury and then to necrosis or apoptosis. Timing and severity of the insult determine where the cell lies on this spectrum.

Mechanisms of Cell Injury

Although many injurious stimuli exist, certain biochemical mechanisms are shared. These mechanisms are highly testable.

ATP depletion is central in many forms of hypoxic and ischemic injury. Reduced oxidative phosphorylation in mitochondria decreases ATP production. When ATP falls, the Na⁺/K⁺ ATPase fails, sodium and water accumulate inside the cell, and swelling appears. Glycolysis increases, lactic acid builds up, and intracellular pH falls, which can denature proteins and clump nuclear chromatin.

Mitochondrial damage is both a cause and a consequence of injury. Damaged mitochondria produce less ATP and more reactive oxygen species. If the mitochondrial permeability transition pore opens, the proton gradient collapses and ATP synthesis stops. Release of cytochrome c from the intermembrane space into the cytosol can activate apoptotic pathways.

Calcium influx is another critical event. The cytosolic free calcium concentration is normally very low compared with extracellular and stored calcium. Injury that disrupts membranes or ATP dependent pumps leads to calcium accumulation in the cytosol and mitochondria. Elevated calcium activates phospholipases, proteases, endonucleases, and ATPases that degrade cell membranes, cytoskeleton, nucleic acids, and further deplete ATP.

Reactive oxygen species and other free radicals are highly reactive molecules with unpaired electrons. Significant sources include the mitochondrial electron transport chain, inflammatory cells, and some drugs or toxins. Free radicals damage lipids through peroxidation, alter proteins by oxidation, and cause DNA breaks. The cell counters them through antioxidants, such as vitamin E, vitamin C, and glutathione, and through enzymes like superoxide dismutase and catalase.

Membrane damage can result from lipid peroxidation, cytoskeletal disruption, or direct toxins. Once the plasma membrane is compromised, ion homeostasis is lost. Injury to lysosomal membranes allows release of hydrolytic enzymes into the cytoplasm, worsening damage.

Finally, DNA and protein damage trigger repair mechanisms. If the damage is too severe, these pathways can activate cell death programs. Tumor suppressor proteins like p53 can halt the cell cycle to allow DNA repair. If repair fails, p53 can initiate apoptosis, which is protective against malignant transformation.

Necrosis

Necrosis is a form of cell death that results from severe injury, typically involving large groups of cells in a tissue, and is almost always pathologic. It is often associated with inflammation, because the cell contents leak into the extracellular space and attract immune cells.

On microscopy, necrotic cells show increased eosinophilia, loss of nuclear detail, and disruption of cellular outlines. Nuclear changes include pyknosis, in which the nucleus shrinks and becomes intensely basophilic, karyorrhexis, in which the nucleus fragments, and karyolysis, in which nuclear material fades due to DNAse activity.

Several patterns of necrosis are classic in general pathology. Coagulative necrosis preserves the basic outline of the dead cells for some days, even though cellular detail is lost. It is typically seen in infarcts in most solid organs supplied by end arteries, such as the heart, kidney, and spleen. Liquefactive necrosis, by contrast, results in complete digestion of dead cells, leaving a liquid, viscous mass. It characteristically occurs in brain infarcts and in abscesses, where enzymes from neutrophils and microbes digest the tissue.

Caseous necrosis combines features of both and is often associated with granulomatous inflammation, such as in tuberculosis. On gross examination, the tissue has a soft, friable, cheese like appearance. Fat necrosis refers to focal areas of fat destruction, often due to release of pancreatic lipases in acute pancreatitis or trauma to fatty tissue. The liberated fatty acids combine with calcium to form chalky, white deposits.

Fibrinoid necrosis is a special pattern seen in immune mediated vascular injury. Immune complexes and plasma proteins deposit in the vessel wall and create a bright pink, fibrin like appearance on microscopy using H and E stain. This pattern is important in some vasculitides and severe hypertension.

Clinically, knowledge of necrosis types allows you to connect a vignette describing gross and microscopic findings with the underlying disease process and often with the organ involved.

Apoptosis

Apoptosis is programmed cell death that is energy dependent and tightly regulated. Unlike necrosis, apoptosis usually involves single cells or small groups of cells, and it does not elicit significant inflammation because the plasma membrane remains intact until the cell is removed by phagocytes.

Morphologically, apoptotic cells shrink, their chromatin condenses, and the cell fragments into membrane bound apoptotic bodies. These are quickly phagocytosed by neighboring cells or macrophages. Biochemically, apoptosis is executed by caspases, a family of proteases that cleave specific cellular substrates to dismantle the cell.

Two main pathways activate apoptosis. The intrinsic, or mitochondrial, pathway is controlled by the balance between pro apoptotic and anti apoptotic members of the BCL2 family. Stress signals, such as DNA damage, misfolded proteins, or withdrawal of growth factors, tilt this balance toward pro apoptotic proteins, which form channels that allow cytochrome c to escape from mitochondria into the cytosol. Cytochrome c then helps activate initiator caspases.

The extrinsic, or death receptor, pathway involves cell surface receptors such as Fas and TNF receptor. When engaged by their ligands, these receptors recruit adaptor proteins and form a death inducing signaling complex that activates caspases. This pathway is important in immune regulation, including deletion of self reactive lymphocytes.

Apoptosis has physiologic roles, including embryogenesis, hormone dependent involution of tissues like the endometrium during the menstrual cycle, and elimination of self reactive or no longer needed immune cells. It also has pathologic roles, such as in neurodegenerative diseases, viral infections, and after some types of DNA damage. On exams, apoptosis is often contrasted with necrosis through features like cell size, inflammation, and caspase activation.

Intracellular Accumulations

General pathology also studies abnormal accumulations of substances in cells. These can be normal cellular components present in excess, abnormal proteins or lipids, or exogenous materials.

Lipids may accumulate in cells, particularly triglycerides in hepatocytes during steatosis. Causes include alcohol abuse, obesity, and metabolic syndromes. Cholesterol esters can deposit in macrophages in atherosclerotic plaques or in xanthomas, creating foamy cells.

Proteins can accumulate as eosinophilic inclusions in the cytoplasm. One clinically important form is amyloid, an abnormal protein that deposits in the extracellular space in characteristic diseases, but its detailed types belong to systemic pathology. Within hepatocytes, misfolded proteins can appear as hyaline inclusions, while in neurodegenerative diseases, misfolded proteins aggregate within neurons.

Glycogen accumulation occurs in some metabolic disorders involving enzymes of glycogen metabolism. On light microscopy, glycogen appears as clear vacuoles in the cytoplasm, which can be highlighted by special stains.

Pigments also accumulate. Endogenous pigments include lipofuscin, a wear and tear pigment composed of lipid peroxidation products that accumulates in aging cells, especially in the heart and liver. Melanin is a normal brown black pigment produced by melanocytes. Hemosiderin is a golden yellow to brown pigment containing iron that appears in conditions of excess iron, such as local hemorrhage with hemosiderosis.

Pathologic calcification refers to abnormal deposition of calcium salts in tissues. In dystrophic calcification, calcium deposits in dead or dying tissues, even if serum calcium levels are normal, such as in areas of caseous necrosis or on damaged heart valves. In metastatic calcification, calcium deposits in otherwise normal tissues due to hypercalcemia, often in settings like hyperparathyroidism or vitamin D disorders.

These accumulations can be clues to underlying systemic disease in biopsy specimens and vignettes, and they illustrate how metabolic disturbances manifest at the cellular level.

Cellular Aging

Cellular aging in general pathology concerns the gradual decline in cellular function and replicative capacity with time. It involves both extrinsic factors, like environmental stress, and intrinsic factors, such as genetic programs.

One central concept is replicative senescence, which refers to the limited number of divisions that many somatic cells can undergo. With each cell division, telomeres, the repetitive nucleotide sequences at chromosome ends, become shorter. When telomeres reach a critical length, cells enter a nondividing state. Telomerase can extend telomeres and is active in germ cells and stem cells, but is usually low in most somatic cells. Many cancers reactivate telomerase, which contributes to limitless replicative potential.

Accumulation of DNA damage over time also promotes aging. Reactive oxygen species and other insults produce lesions that, if not repaired, trigger cell cycle arrest or apoptosis. Similarly, proteins accumulate damage or misfolding over many years, and failure of quality control systems such as the ubiquitin proteasome pathway can further impair cellular function.

Aging is not purely detrimental. Some of the same pathways that limit proliferation or trigger apoptosis in aging cells protect against cancer. For Step 1, you mainly need to recognize how general mechanisms like telomere shortening, DNA damage, and reactive oxygen species fit into the big picture of aging and disease predisposition.

Hemodynamic Disorders and Thrombosis

Hemodynamic disorders arise when normal blood flow or fluid balance is disturbed. In general pathology, this includes edema, hyperemia and congestion, hemorrhage, thrombosis, embolism, and infarction. Their detailed organ specific effects are left for systemic pathology, but the shared mechanisms begin here.

Edema is the accumulation of fluid in the interstitial space or body cavities. It results from imbalances in Starling forces, which govern movement of fluid across capillary walls. Increased hydrostatic pressure, as in heart failure or venous obstruction, and decreased plasma oncotic pressure, as in hypoalbuminemia from liver disease or nephrotic syndrome, both promote movement of fluid out of capillaries into tissues.

Hyperemia and congestion both refer to increased blood volume in a tissue, but their causes differ. Hyperemia is an active process due to arteriolar dilation, as in inflammation or exercise, and produces tissues that are red and warm. Congestion is a passive process due to impaired venous outflow. Chronically congested tissues are often cyanotic and may develop parenchymal cell death and fibrosis.

Hemorrhage is the escape of blood from vessels, either externally or into tissues to form a hematoma. Small pinpoint hemorrhages in skin or mucosa are petechiae, slightly larger ones are purpura, and larger, rounded collections are ecchymoses. Hemorrhage can result from trauma, vascular disease, or disorders of platelets or coagulation.

Thrombosis is the formation of a blood clot within intact vessels. A key concept in general pathology is Virchow’s triad, which describes three major factors that predispose to thrombosis: endothelial injury, abnormal blood flow, and hypercoagulability.

Endothelial injury, as in atherosclerotic plaques or after myocardial infarction, exposes subendothelial factors that promote platelet adhesion and coagulation. Abnormal flow, such as stasis in dilated veins or turbulence at vessel bifurcations, disrupts laminar flow and brings platelets into contact with endothelium. Hypercoagulability includes both inherited conditions, such as factor V Leiden mutation, and acquired states like prolonged immobilization, malignancy, or pregnancy.

An embolus is a detached intravascular mass that travels in the bloodstream to a distant site. Most emboli are fragments of thrombi, termed thromboemboli, but they can also be composed of fat, air, or amniotic fluid. Emboli lodge in vessels that are too small to permit further passage and can cause ischemic necrosis of downstream tissue, known as infarction.

Infarction is a localized area of ischemic necrosis produced by occlusion of the arterial supply or venous drainage. Many infarcts are thrombotic or embolic in origin, and their appearance as pale, wedge shaped lesions in solid organs or as hemorrhagic lesions in tissues with dual blood supply becomes important in systemic pathology.

Acute Inflammation

Acute inflammation is an immediate and early response to tissue injury or infection. It aims to eliminate the initial cause of cell injury, remove necrotic cells, and initiate repair. The hallmarks of acute inflammation are fluid exudation and neutrophil dominated cellular infiltration.

Two major components define acute inflammation. The vascular component involves changes in blood vessels that increase blood flow and permeability. The cellular component involves recruitment and activation of leukocytes. Together, they produce the classic signs of inflammation: redness, heat, swelling, pain, and loss of function.

Vascular changes begin with transient vasoconstriction followed by vasodilation of arterioles. This increases blood flow, which causes redness and warmth. Increased vascular permeability allows plasma proteins and leukocytes to leave the circulation and enter the site of injury. Fluid rich in proteins, particularly fibrinogen and immunoglobulins, leaks into tissues and forms an exudate, which contributes to swelling.

The cellular component begins with margination of leukocytes along the vascular endothelium as blood flow slows. Leukocytes then roll along the endothelium through weak interactions mediated by selectins, adhere firmly via integrins binding to immunoglobulin superfamily molecules, and finally transmigrate between endothelial cells into the extravascular space. Chemokines and other chemotactic agents guide leukocytes toward the site of injury along chemical gradients.

Neutrophils are usually the first leukocytes to respond in acute inflammation. They phagocytose microbes and debris and kill ingested pathogens through reactive oxygen species, granule enzymes, and antimicrobial peptides. However, their powerful weapons can also damage host tissues if not tightly controlled. As the response continues, monocytes may replace neutrophils and become macrophages in the tissue.

Chemical mediators orchestrate acute inflammation. Some are derived from plasma proteins, such as complement and kinins, while others are produced by cells, including histamine, prostaglandins, leukotrienes, cytokines, and chemokines. These mediators increase vascular permeability, cause vasodilation, recruit cells, and amplify the response. Importantly, they are produced in response to stimuli and are quickly degraded, which helps limit the reaction.

Outcomes of acute inflammation include complete resolution with restoration of normal structure and function, healing by connective tissue replacement when tissue destruction is substantial or regenerative capacity is limited, and progression to chronic inflammation if the offending agent persists or if there are repeated episodes of injury. Recognition of these outcomes prepares you for more detailed discussions in the chapters on inflammation and tissue repair.

Chronic Inflammation and Granulomas

Chronic inflammation is a prolonged inflammatory response in which active inflammation, tissue destruction, and attempts at repair occur simultaneously. It can follow unresolved acute inflammation or begin insidiously, as in many autoimmune diseases.

Unlike acute inflammation, which is dominated by neutrophils and edema, chronic inflammation features infiltration by mononuclear cells, namely macrophages, lymphocytes, and plasma cells. Macrophages are central players. They derive from circulating monocytes and from tissue resident precursors and are activated by cytokines from T cells, microbial products, and other signals. Activated macrophages secrete cytokines, growth factors, and reactive oxygen and nitrogen species, which sustain inflammation and participate in tissue destruction and repair.

Lymphocytes interact with macrophages in a bidirectional loop. Macrophages present antigens and secrete cytokines that activate T cells, and activated T cells, in turn, produce cytokines that further activate macrophages. This collaboration is particularly important in immune mediated chronic inflammatory diseases.

One distinctive pattern of chronic inflammation is granulomatous inflammation, in which activated macrophages aggregate into tight clusters and may transform into epithelioid cells, so named because they resemble epithelial cells. These clusters are often surrounded by a collar of lymphocytes, and sometimes multinucleated giant cells form from fused macrophages.

Granulomas arise in specific settings. Persistent, poorly degradable microbes, such as Mycobacterium tuberculosis, can provoke granulomas known as immune granulomas. Foreign materials like suture fragments can trigger foreign body granulomas. Some systemic diseases, such as sarcoidosis, also feature granulomatous inflammation, but their detailed features are best learned with systemic pathology and organ specific chapters.

Recognition of granulomas has high diagnostic value. In exam questions, descriptions of epithelioid macrophages, giant cells, and caseous necrosis, especially in the lung, almost always point to granulomatous inflammation and narrow the differential diagnosis.

Tissue Repair and Wound Healing

After injury and inflammation, tissues attempt to restore integrity through repair. General pathology divides repair into regeneration and scar formation. Regeneration restores normal structure and function by proliferating surviving cells and replacing those that were lost. Scar formation, or fibrosis, involves deposition of collagen rich connective tissue that provides structural stability but may reduce function.

The capacity for regeneration depends on the type of tissue. Labile tissues, such as bone marrow and most epithelia, continually divide and can regenerate well if their stem cells and supporting framework remain intact. Stable tissues, like liver and kidney, have limited ongoing proliferation but can reenter the cell cycle and regenerate to some degree after injury. Permanent tissues, such as cardiac muscle and neurons, have very limited capacity to regenerate, so injury often leads to scar formation.

Wound healing by primary intention describes repair of a clean, closely approximated incision, such as a surgical cut. In this setting, there is minimal tissue loss, and reepithelialization and collagen deposition occur quickly. Healing by secondary intention occurs when there is substantial tissue loss or when wound edges cannot be approximated. It involves more extensive granulation tissue, wound contraction by myofibroblasts, and a larger scar.

Growth factors produced by macrophages and other cells regulate cellular proliferation, migration, and extracellular matrix synthesis during repair. Angiogenesis, the formation of new blood vessels from preexisting ones, is crucial for delivering nutrients and oxygen to healing tissue. Fibroblasts migrate into the site, proliferate, and deposit collagen and other matrix components. Over time, granulation tissue, which is rich in new vessels and fibroblasts, matures into a scar as vessels regress and collagen remodels.

Factors that influence wound healing include infection, which is the most important cause of delayed healing, nutritional status, particularly vitamin C and protein, diabetes, poor blood supply, mechanical stress, and glucocorticoid therapy, which can impair collagen synthesis.

Complications of repair include inadequate formation of granulation tissue or matrix, which can lead to wound dehiscence or ulceration, excessive scar formation with hypertrophic scars, excessive collagen leading to keloids, and excessive contraction causing contractures, particularly in burns. Recognizing these patterns and their risk factors is central when answering clinical scenario questions that link general pathology principles with patient outcomes.

Clinical Correlation and Exam Focus

General pathology provides the foundational concepts that recur throughout pathology and clinical medicine. On Step 1, these topics are often integrated into multi step questions that ask you to connect an initial injury with cellular responses, chemical mediators, and eventual clinical outcomes.

To use general pathology effectively, focus on patterns. When you see cell swelling and fatty change, think reversible injury and energy failure. When you see cytochrome c release and caspases, think apoptosis. When you read about endothelial injury, stasis, and hypercoagulability, recall Virchow’s triad and the risk of thrombosis. When a vignette describes epithelioid cells and caseating necrosis, identify granulomatous inflammation.

By mastering the shared mechanisms in this chapter, you build a mental framework that supports the system specific details you will learn in subsequent chapters.