Cellular Adaptation, Injury, and Death

Comprehensive vocabulary flashcards covering the concepts of cellular adaptation, injury mechanisms, necrosis patterns, apoptosis, and hybrid pathways of cell death.

Cellular Adaptation

A reversible adjustment in cell number, size, phenotype, metabolic activity, or function in response to physiologic needs or pathologic stress that allows cells to modulate their structure and escape injury.

Labile Cells

Continuously dividing cells that cycle throughout life under normal physiologic conditions, passing directly from M to $G_1$ phase and easily regenerating from stem cell pools (e.g., epidermis, gastrointestinal tract, hematopoietic bone marrow).

Stable Cells

Quiescent cells in the $G_0$ phase that have decreased proliferative activity after adolescence but retain the capacity to multiply in response to appropriate stimuli (e.g., parenchymal cells of the liver, kidneys, pancreas, and fibroblasts).

Permanent Cells

Non-dividing cells that have permanently left the cell cycle and lost the ability to proliferate around the time of birth; injury to these tissues inevitably results in scar formation (e.g., neurons, cardiac myocytes, and skeletal muscle).

Hyperplasia

An increase in the number of cells in an organ or tissue in response to a stimulus, occurring exclusively in labile and stable cell populations capable of synthesizing DNA.

Compensatory Hyperplasia

A physiologic form of hyperplasia wherein residual tissue proliferates to restore organ mass and function after damage or partial resection, such as liver regeneration following partial hepatectomy.

Benign Prostatic Hyperplasia (BPH)

A form of pathologic hyperplasia seen in elderly men driven by excessive androgen stimulation, producing irregular hyperplastic nodules in the prostate that can obstruct urinary outflow.

Hypertrophy

An increase in the size of cells resulting in an increased organ size, driven by the synthesis and assembly of additional intracellular structural components and contractile proteins.

Myocardial Hypertrophy

Enlargement of cardiac myocytes triggered by chronic mechanical stretch and neurohumoral factors (such as $\alpha$-adrenergic agonists and angiotensin II), activating transcription factors GATA4, NFAT, and MEF2 to express fetal and contractile proteins.

Atrophy

A reduction in the size of an organ or tissue resulting from a decrease in both cell size and cell number, characterized by diminished metabolic activity and reduced protein content.

Ubiquitin-Proteasome Pathway

The primary cellular mechanism for accelerated protein degradation during atrophy, in which ubiquitin ligases tag target proteins with ubiquitin peptides for breakdown within proteasomes.

Lipofuscin Granules

Membrane-bound residual bodies remaining in the cytoplasm during cellular atrophy that resist autophagic digestion, serving as a yellow-brown wear-and-tear pigment of aging or atrophy.

Denervation Atrophy

A form of pathologic atrophy that occurs when skeletal muscle fibers lose their nerve supply, resulting in rapid shrinkage of the affected muscle fibers (as seen in poliomyelitis or motor neuron disease).

Metaplasia

A reversible adaptive change in which one differentiated cell type (epithelial or mesenchymal) is replaced by another cell type that is better suited to withstand a hostile environmental stress.

Squamous Metaplasia

The replacement of a glandular or ciliated columnar epithelium by durable stratified squamous epithelium, classically observed in the bronchial respiratory tract of chronic cigarette smokers.

Barrett's Esophagus

A pathologic adaptation where the normal stratified squamous epithelium of the lower esophagus undergoes columnar (intestinal) metaplasia with goblet cells due to chronic exposure to refluxed gastric acid.

Hypoxia

A deficiency of oxygen delivery to tissues that impairs mitochondrial oxidative phosphorylation, reducing cellular ATP production; recognized as the most common underlying cause of cellular injury.

Ischemia

A mechanical reduction or loss of blood supply to a tissue that halts both oxygen and nutrient delivery while preventing the clearance of toxic metabolic waste products.

Hydropic Degeneration

Acute cellular swelling resulting from ATP depletion and the failure of plasma membrane $\text{Na}^+/\text{K}^+$ ATPase pumps, causing intracellular accumulation of sodium and water accompanied by vacuolation of the endoplasmic reticulum.

Reversible Ischemic Injury (Tubular)

Early, recoverable cellular damage characterized ultrastructurally and histologically by microvillar loss, surface bleb formation, cytoplasmic eosinophilia, and mitochondrial/ER swelling without loss of nuclei.

Steatosis

An abnormal intracellular accumulation of triglycerides (fat droplets) within the cytoplasm of parenchymal cells, such as hepatocytes in fatty liver disease.

Fenton Reaction

A biochemical reaction wherein hydrogen peroxide ($\text{H}_2\text{O}_2$) interacts with ferrous iron ($\text{Fe}^{2+}$) to produce the highly destructive hydroxyl free radical ($\text{OH}^\bullet$).

Haber-Weiss Reaction

A biochemical reaction occurring in the presence of oxygen ($\text{O}_2$) where superoxide and hydrogen peroxide react to generate the highly reactive hydroxyl free radical ($\text{OH}^\bullet$).

Superoxide Dismutase (SOD)

An antioxidant defense enzyme that converts cytotoxic superoxide ions ($\text{O}_2^{\bullet-}$) into hydrogen peroxide ($\text{H}_2\text{O}_2$).

Catalase

A major antioxidant enzyme that detoxifies hydrogen peroxide ($\text{H}_2\text{O}_2$) by converting it directly into safe water ($\text{H}_2\text{O}$) and oxygen.

Dystrophic Calcification

Pathologic deposition of calcium salts into necrotic, damaged, or aging tissues in the presence of normal systemic serum calcium ($\text{Ca}^{2+}$) levels.

Metastatic Calcification

Deposition of calcium mineral salts into normal, undamaged tissues (such as the kidneys, lungs, and gastric mucosa) secondary to systemic hypercalcemia.

Necrosis

A spectrum of morphological changes that follow irreversible cell death in living tissue, characterized by cellular swelling, denaturation of proteins, plasma membrane rupture, enzymatic digestion, and prominent inflammation.

Pyknosis

A nuclear morphologic pattern in necrotic cells characterized by nuclear shrinkage and condensation into a dense, solid, deeply basophilic mass.

Karyorrhexis

The nuclear fragmentation stage of necrosis where the shrunken pyknotic nucleus breaks up into multiple dispersed chromatin granules.

Karyolysis

The progressive fading and enzymatic dissolution of the basophilic nucleus due to DNA breakdown by endonucleases, leading to complete nuclear disappearance within 1 to 2 days.

Damage-Associated Molecular Patterns (DAMPs)

Endogenous intracellular molecules (such as ATP and uric acid) released into the extracellular space following plasma membrane rupture that bind to macrophage receptors and elicit a vigorous inflammatory response.

Myelin Figures

Large, whorled phospholipid precipitates derived from damaged cell membranes that accumulate in injured or necrotic cells and eventually calcify.

Coagulative Necrosis

A pattern of necrosis typical of ischemic infarcts in solid organs (except the brain) where structural protein and enzymatic denaturation delays proteolysis, preserving basic tissue outlines for several days.

Liquefactive Necrosis

A form of necrosis characterized by rapid enzymatic digestion of dead cells into a viscous liquid mass (pus), typical of bacterial or fungal abscesses and hypoxic infarcts of the central nervous system.

Caseous Necrosis

A distinctive pattern of necrosis classically seen in tuberculosis infections, grossly resembling friable, cheesy white material and microscopically presenting as amorphous, eosinophilic granular debris enclosed within a granuloma.

Fat Necrosis

Focal areas of fat destruction resulting either from trauma or from the release of activated pancreatic lipases in acute pancreatitis, leading to enzymatic digestion of adipocytes and saponification.

Saponification

The process whereby fatty acids released by pancreatic lipases combine with ionized calcium ($\text{Ca}^{2+}$) to form gross, chalky-white, macroscopic calcium soaps.

Fibrinoid Necrosis

A specialized pattern of vascular damage caused by immune-mediated vasculitis or severe hypertension, where antigen-antibody complexes and leaking fibrin deposit into arterial walls, staining bright pink on H&E.

Apoptosis

A pathway of energy-dependent, programmed cell suicide in which a cell degrades its own DNA and nuclear proteins, breaking down into intact membrane-bound bodies that are cleared without eliciting inflammation.

Efferocytosis

The silent engulfment and clearance of apoptotic fragments by phagocytes, driven by the outward flipping of phosphatidylserine to the external membrane leaflet to signal 'eat me' without generating pro-inflammatory cytokines.

Apoptosome

A cytosolic multiprotein complex formed by cytochrome c and APAF-1 in the intrinsic apoptotic pathway that binds and activates initiator Caspase-9.

BH3-Only Proteins

Pro-apoptotic stress-sensing proteins (including BAD, BIM, and BID) that detect cellular injury or DNA damage, neutralize anti-apoptotic guardians (BCL-2, BCL-XL, MCL-1), and activate BAX and BAK.

Executioner Caspases

Downstream cysteine proteases (primarily Caspase-3 and Caspase-6) cleaved and activated by initiator caspases to execute cell death via widespread structural protein cleavage and DNA fragmentation.

Necroptosis

A caspase-independent form of programmed cell death triggered by death receptors (such as TNFR1) involving RIPK1 and RIPK3 kinase activation and MLKL phosphorylation, culminating in plasma membrane rupture and inflammation.

Pyroptosis

A form of programmed inflammatory cell death initiated by microbial infection that activates the multiprotein inflammasome and Caspase-1, causing swelling, lysis, and the massive release of pyrogenic Interleukin-1 (IL-1).

Ferroptosis

An iron-dependent form of cell death characterized by overwhelming accumulation of reactive oxygen species and unchecked lipid peroxidation of cell membranes when antioxidant defenses fail.

Autophagy

A survival pathway in starvation where a cell sequesters damaged or obsolete organelles within double-membrane autophagosomes (marked by lipidated LC3) that fuse with lysosomes for degradation and nutrient recycling.

Ischemia-Reperfusion Injury

Paradoxical exacerbation of cellular injury and necrosis upon the restoration of blood supply, caused by a sudden burst of reactive oxygen species, intracellular calcium overload, incoming inflammatory neutrophils, and complement activation.