1. Pathology of cellular injury and death

What is disease? A condition in which the presence of an abnormality causes a loss of normal health Manifests in signs and symptoms:  subjective: e.g., pain;  objective: confirmed by diagnostic tests Duration of disease:  short lasting – acute  long lasting – chronic Outcome: varies; can be lethal

What is pathology? The study (logos) of suffering (pathos) Devoted to the study of  the cause of the 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)

The morphologic change of an organ can be focal (localized abnormality) or diffuse

Teaching program of pathology for medical students General pathology  Basic reactions of cells and tissues to abnormal stimuli, i.e. common features of various disease processes in various cells and tissues Systematic pathology  The descriptions of specific diseases as they affect given organs or organ systems

The descriptions and terms are the basis of medical language

PATHOLOGY OF CELLULAR INJURY AND DEATH Cells react to adverse influences by 1) reversible cell injury: changes that can be reversed when the stimulus is removed 2) irreversible cell injury: changes that cause cell death 3) cellular adaptation: stimuli result in new but altered state that maintains the viability of the cell

Intracellular mechanisms particularly vulnerable to cellular injury  Maintenance of membrane integrity (critical for ionic and osmotic homeostasis of the cell)  Aerobic respiration (oxidative phosphorilation and ATP production in mitochondria)  Synthesis of enzymes and structural proteins

Common cellular injuries 1. Hypoxic/ischemic injury 2. O2-derived free radicals 3. Others: chemical injury (acid and alkali solutions), burns, frostbite, trauma, electric shock, etc.

Hypoxia Reduction in available oxygen Common causes  Upper airway obstruction (e.g., sudden swelling of laryngeal mucosa, aspiration of foreign body)  Inadequate oxygenation of blood in lung diseases  Inadequate O2 transport in blood because of decreased number of RBCs (anemia)  Inadequate perfusion of blood in the tissues in heart failure

Ischemia: inadequate blood supply to an organ or part of it due to impeded arterial flow or reduced venous drainage

REVERSIBLE ISCHEMIC INJURY  Leads to hydropic change of cells  Commonly observed in kidney biopsies Pathomechanism  A decrease of blood pressure for hours  Hypoxia of tubular epithelial cells  ATP depletion  malfunction of Na+/K+ ATPase  influx of sodium and water into tubular cells Morphologic features  Light microscopy (LM): the tubular epithelial cells are vacuolated, and the brush border of proximal tubules is lost  Electron microscopy (EM): accumulation of water in the cytoplasm, in the invaginations of the surface plasma membrane (hydropic vacuoles), in the cisterns of the RER, and in the mitochondria; loss of microvilli of proximal tubules Clinical consequence  Acute renal failure: decreased urinary output; hemodialysis is necessary  If systemic hypotension can be corrected, the renal function normalizes within days

IRREVERSIBLE ISCHEMIC INJURY AND CELL DEATH  The transition from reversible to irreversible state is gradual and occurs when adaptive mechanisms have been exhausted

1

1. Pathology of cellular injury and death

 Depletion of ATP, influx of Ca2+, activation of cellular enzymes, such as - phospholipases  degradation of membrane phospholipids - proteases  degradation of membrane and cytoskeletal protein - ATPases  enhance ATP depletion - endonucleases  chromatin fragmentation

EM features indicative of death of ischemic cells  Within 2 to 3 hours after the death of cells  Ruptured cell and plasma membranes  Lysis of cell and nuclear components (leakage of lysosomal enzymes result in digestion of organelles and other cytosolic components)

LM features indicative of death of ischemic cells  Evident approximately 24 hours after the death of ischemic cells  Loss of nuclear staining  Eosinophilia of the cytoplasm

Important  The cellular function is lost before cell death occurs, and the morphologic features of cell death lag far behind loss of function

INJURY INDUCED BY O2-DERIVED FREE RADICALS Inflammation, radiation, chemicals, reperfusion lead to the formation of: .  superoxide anion radical (O2 )  hydrogen peroxide (H2O2)  hydroxyl radical (OH.)  and nitric oxide (NO.)

These molecules cause oxidative stress of cells:  lipid peroxidation  membrane damage  cross-linkage of proteins  inactivation of enzymes  DNA breaks  blockade of DNA transcription

Important Insidiously ongoing oxidative stress of cells plays a role in the process of aging

Laboratory markers of cell death Cytoplasmic enzymes released through damaged cell membranes into the blood • Creatine kinase (CK) - cardiac or skeletal muscle injury • Aspartate aminotransferase (AST) and alanine aminotransferase (ALT) - liver cell injury • Lactate dehydrogenase (LDH) - released from ruptured RBCs

NECROSIS - morphology of irreversible injury  Necrosis (necros, dead): death of cells in a living organism characterized by loss of membrane integrity and enzymatic digestion of cells  Histological sign: loss of nuclear staining  Healing: substances released from dead cells induce a local inflammatory reaction, which serves to eliminate the debris and initiates the repair process

Types of necrosis  Grossly visible: coagulative necrosis, liquefactive necrosis, caseation, fat necrosis, and gangrene  Observed by LM: fibrinoid necrosis

Coagulative necrosis  Most common form of necrosis, predominated by protein denaturation with preservation of the cell and tissue framework  Arterial occlusion  distally: hypoxic (anoxic) death in tissues (exception: brain)  Types: anemic infarct, hemorrhagic infarct

Anemic infarct  Cause: occlusion of an end artery; in the heart, spleen, kidneys  Gross: circumscribed yellowish lesion, the margins are hyperemic  LM: dead cells become eosinophilic with loss of nuclear staining, the border of necrotic tissue is hyperemic and infiltrated by neutrophils

Hemorrhagic infarct  In the lungs, due to occlusion of a segmental pulmonary artery; sec. hemorrhage via bronchial arteries: wedge shaped, raised, dark-red area  In the small bowels, due to occlusion of the mesenteric superior artery; sec. hemorrhage via anastomosing arcades

Liquefactive necrosis The necrotic tissue undergoes softening due to action of hydrolytic enzymes Examples: brain infarct, abscess 2

1. Pathology of cellular injury and death

Brain infarct  Occlusion of cerebral artery leads to anemic infarct; then enzymes released from dead cells liquefy the necrotized area  Grossly, the necrotic area is softened and pale  LM: macrophages scavenge necrotic, lipid-rich debris

Abscess  Localized purulent inflammation  Hydrolytic enzymes derived from neutrophil granulocytes induce necrosis of infected area

Caseous necrosis  Distinctive form of coagulative necrosis in foci of tuberculous infection of the lung  Grossly, caseous necrosis is white and cheesy  LM features: the necrotic area is eosinophilic, amorphous, and is surrounded by activated macrophages (epitheloid cells) which mediate the focal necrosis of the lung and kill the bacteria

Enzymatic fat necrosis  Occurs in pancreatitis, induced by the action of lipases derived from injured pancreatic cells  Lipases catalyse decomposition of triglycerides to fatty acids, which complex with calcium to create calcium soaps  Gross: the swollen pancreas displays several yellowish foci of necrosis

Gangrene  This (mostly) clinical term refers to the severemost form of necrosis  Total destruction of all tissue components; often putrefactive bacteria invade the necrotic tissue  Three subtypes (detailed in Inflammation chapter)

One subtype: dry gangrene  In the leg of patients suffering from atherosclerosis-related occlusion of the tibial arteries  The affected tissues appear black because of the deposition of iron sulphide from degraded hemoglobin

Fibrinoid necrosis  Occurs in arteries, arterioles, and capillaries; seen in autoimmune disoders (SLE, arteritis) for example  The wall of these vessels undergo necrosis and is impregnated with fibrinogen and other plasma proteins

Apoptosis: programmed cell death • A form of energy-dependent process for selective deletion of unwanted individual cells • An internal suicide program becomes activated • The dead cell’s membrane remain intact; the dead cell is rapidly cleared by macrophages before its content have leaked out; therefore, apoptosis does not induce an inflammatory reaction (remember! features of necrosis: loss of membrane integrity, enzymatic digestion of cells, and an inflammatory reaction) Apo is prevented or induced by a variety of stimuli. Reduced Apo contributes to cell accumulation, e.g. neoplasia; increased Apo results in excessive cell loss, e.g. atrophy Apo is initiated via two pathways, the extrinsic and intrinsic pathways, which lead to the activation of execution caspases  cell death • Extrinsic (death receptor) pathway: if death receptors on the cell surface (TNF-R, FAS-R) cross-link with the ligand, activation of execution caspases occurs. • Intrinsic (mitochondrial) pathway: when cells are deprived of survival signals or subjected to stress, anti-apoptotic Bcl- 2 protein is replaced by pro-apoptotic Bax protein in the mitochondrial membrane, and in turn, the execution caspases become activated • Execution caspases: cascade of proteolytic enzymes Morphologic features of Apo: breakdown of cytoskeleton, cell shrinkage, chromatin condensation and fragmentation, formation of apoptotic bodies

ADAPTATIONS Changes that occur in cells and tissues in response to prolonged stimulation or chronic injury • Atrophy • Hypertrophy • Hyperplasia • Metaplasia • Dysplasia (to be lectured later) • Intracellular accumulation of various substances

ATROPHY Decreased cell mass: reduction in size of cells (nucleus and cytoplasm), tissue, or organs. Atrophied organs are smaller than normal. Normal weight (g) of parenchymal organs: spleen 150, kidneys 150-150, heart 300 to 350, lungs 400-400, brain 1300, liver 1500

Physiologic atrophy  Involution of the thymus in adolescence  Senile atrophy in aging  Atrophy of female genitalia in menopause Pathologic atrophy 1. In skeletal muscles due to

3

1. Pathology of cellular injury and death

- disuse as in prolonged bed rest or immobilization of limb for healing of bone fracture - loss of innervation 2. Loss of endocrine stimulation - lack of trophic hormones in pituitary disease 3. Diminished blood supply. Slow but progressive reduction of blood supply leads to renal atrophy or atrophy of the brain 4. Malnutrition. Atrophy of skeletal muscles, parenchymal organs, and general wasting (marasmus) 5. Increased pressure, e.g., hydrocephalus or hydronephrosis

HYPERTROPHY An increased cell mass leading to an increased size of organs Physiologic  Hypertrophy of uterus in pregnancy  Compensatory hypertrophy of the remnant kidney after unilateral nephrectomy  Exercise Pathologic  In the muscles: cells of muscles are not able to divide, therefore an increased demand for action can be met only by enlarging the size of cells.  Hypertrophy of the myocardium: triggered by action of mechanical stimuli ( workload) and vasoactive substances (e.g., angiotensin II). Left ventricular free wall thickness: above 15 mm  Hypertrophy of the detrusor muscles of urinary bladder due to urethral obstruction

HYPERPLASIA Hormonal stimulation results in an increase in the size of a tissue or organ due to an increased number of constituent cells. The cells may have an increased volume. Physiologic  Proliferation of the glandular epithelium of the breast during lactation Pathologic hyperplasias  Endometrial hyperplasia, induced by estrogens; clinical feature: bleeding from the uterus between menstrual periods (metrorrhagia)  Hyperplasia of prostate, induced by dihydrotestosterone, estrogens and peptide growth factors; clinical consequence: urinary tract obstruction  Bilateral adrenal cortex hyperplasia, induced by increased ACTH secretion; clinical consequence: increased production of corticosteroids leading to the Cushing’s sy  etc.

METAPLASIA Replacement of one adult cell type by another adult cell type; reversible. Examples  Squamous metaplasia of the bronchus: chronic irritation-induced replacement of bronchial stratified columnar epithelium by squamous epithelium in smokers  Gastric metaplasia of the esophagus: chronic irritation induced by gastric juices in gastroesophageal reflux leads to the replacement of squamous epithelium by gastric epithelium If the adverse circumstances persist, metaplasia may progress to dysplasia (precancerous)

INTRACELLULAR ACCUMULATIONS • Lipids - triglycerides, cholesterol • Proteins • Pigments

Accumulation of triglycerides Most common in the liver, but also occurs in the heart; reversible

Fatty change/steatosis of liver: due to  alcohol abuse  morbid obesity  diabetes  protein-energy malnutrition  hypoxia  hepatotoxins Biochemical pathways of uptake and metabolism of fatty acids by the liver, formation of triglycerides, and secretions of lipoproteins: not detailed here

Gross: the liver is enlarged, yellow and greasy, resembles to goose liver LM: the lipid molecules accumulate in large vacuoles

In atherosclerosis, cholesterols and cholesterol esters accumulate extracellularly and intracellularly in the intima of aorta and large arteries and form atheromatous plaques LM  the lipids are dissolved during normal histologic processing; the dissolved cholesterol crystals appear as cleftlike cavities  Accumulation of lipids in macrophages In cerebral infarction, macrophages phagocytose membrane lipids derived from dead oligodendrocytes and transform into foamy macrophages

Accumulation of proteins

4

1. Pathology of cellular injury and death

Hyaline change: any alteration within cells that imparts a homogeneous, glassy pink appearance in H&E-stained histologic sections. Examples  Hyaline droplets in proximal tubular cells in heavy proteinuria  Mallory-hyaline in hepatocytes in alcoholic liver injury

Accumulation of pigments Exogeneous  Inhaled coal dust (black) - leading to anthracosis of lungs; stored in pulmonary macrophages  Pigments of tattooing, taken up by macrophages

Endogeneous  Jaundice (icterus): systemic bilirubin retention; yellow skin and sclera discoloration  Hemosiderin (brown), hemoglobin-derived intracellular pigment composed of aggregated ferritin, indicates previous hemorrhage. Systemic accumulation: termed hemosiderosis  Melanin (brown): product of nevus cells

PATHOLOGIC CALCIFICATION Abnormal deposition of Ca-salts in soft tissues

Dystrophic  In nonviable or dying tissues; the serum Ca2+ level is normal.  Precipitation of a crystalline Ca-phosphate starts with nucleation (initiation) on membrane fragments, followed by propagation of crystal formation.  Very common, with serious clinical consequences

Examples • Arteries in atherosclerosis • Damaged heart valves • Areas of various necrosis

Metastatic calcification  Results from hypercalcemia  Causes: destruction of bones by myeloma, metastases, secretion of parathormone in hyperparathyroidism, etc.  Deposits in the arteries, and at sites of acidification: kidneys, lungs, and stomach

5