Irreversible Cell Injury L4 PDF

Prepared by Ass. Proff. Dr. Rasha M. El-Sawi IRREVERSIBLE CELL INJURY Pathology department  Learning objectives: Explain the mechanism of irreversible cell injury, Free radical induced cell injury Define and correctly use "necrosis...

Prepared by Ass. Proff. Dr. Rasha M. El-Sawi IRREVERSIBLE CELL INJURY Pathology department  Learning objectives: Explain the mechanism of irreversible cell injury, Free radical induced cell injury Define and correctly use "necrosis“. List & Distinguish the different types of necrosis with examples Define what is apoptosis Differentiate between necrosis and apoptosis Identify the different types of apoptosis (physiologic and pathologic). Explain the mechanism of apoptosis. Describe the key morphological changes in irreversible cell injury Differentiate between reversible & irreversible forms of cell Pathology injury department Pathology department  Mechanism of reversible cell injury ATP Depletion Caused by hypoxia or toxins. Leads to dysfunction of Na⁺/K⁺ ATPase, resulting in cellular swelling due to ion imbalance and water influx. Mitochondrial Dysfunction Reduced oxidative phosphorylation decreases ATP production. Cell Membrane Alterations Impaired ion pumps cause ionic imbalance and swelling. Detachment of ribosomes from the rough endoplasmic reticulum disrupts protein synthesis. Accumulation of Intracellular Substances Fatty change (steatosis) due to lipid metabolism disturbances. Water accumulation (hydropic change). Cytoskeletal Damage Disruption of microfilaments affects membrane integrity. Oxidative Stress Mild reactive oxygen species (ROS) production can occur but is reversible. Reversible Nuclear Changes Chromatin clumping due to lowered intracellular pH. Key Morphological Changes in Reversible Injury: Cellular swelling (hydropic change). Plasma membrane blebbing. Mitochondrial swelling (no permanent damage). Dilated ER with ribosomal detachment. Pathology department  Mechanism Irreversible Cell Injury Persistence of ischaemia and hypoxia leads to irreversible cell damage → CELL DEATH Stage at which Point of No return is reached is Inability to reverse mitochondrial dysfunction on reperfusion or reoxygenation Disturbance in cell membrane function in general and plasma membrane in particular Pathology department  Mechanism Irreversible Cell Injury 1- Severe Mitochondrial Damage: Loss of mitochondrial function is central to irreversible injury. Depletion of ATP and the generation of reactive oxygen species (ROS) lead to oxidative damage. Persistent mitochondrial membrane damage results in the release of CYTOCHROME C, triggering apoptosis. 2- Disruption of Plasma Membrane Integrity: Irreversible damage to the plasma membrane causes the leakage of intracellular enzymes (e.g., troponin, transaminases). Influx of calcium, sodium, and water leads to cellular swelling and further mitochondrial injury. 3- Massive Calcium Influx: Sustained elevation of cytosolic calcium activates destructive enzymes: Phospholipases → Degrade cell membranes. accelerated membrane damage Proteases → Disrupt cytoskeletal proteins. irreversible membrane damage Endonucleases → Fragment DNA and chromatin. Irreversible damage to the nucleus ATPases → Deplete ATP. Pathology department  Mechanism Irreversible Cell Injury 4- DNA and Nuclear Damage: Persistent DNA damage leads to activation of repair mechanisms that fail in irreversible injury. Nuclear changes like pyknosis (condensation), karyorrhexis (fragmentation), and karyolysis (dissolution) are hallmarks of irreversible injury. 5- Lysosomal Rupture: Leakage of lysosomal enzymes (RNAase, DNAase, proteases, phospholipases etc.) into the cytoplasm leads to autolysis, further accelerating cell death DUE TO → Digestion of cellular components 6- Loss of Functional Homeostasis: Inability to reverse mitochondrial dysfunction, repair membrane damage, or regulate ionic balance ensures the transition to irreversible injury. Pathology department Pathology department  Irreversible injury occurs when cellular damage surpasses the point of repair, with dysfunction in mitochondria, membranes, calcium homeostasis, and DNA integrity being critical determinants. Pathology department Pathology department  Ischaemic – Reperfusion Injury While restoring blood flow is usually helpful, it can actually cause additional damage to the tissue Pathology department  Ischaemic – Reperfusion Injury Depending on duration of ischaemia/hypoxia, restoration of blood flow may result into three consequences: 1. From ischemia to reversible injury When the period of ischemia is of short duration, reperfusion with resupply of oxygen restores the structural and functional state of the injured cell i.e. reversible cell injury. 2. From ischemia to irreversible injury Another extreme is when much longer period of ischemia has resulted in irreversible cell injury during ischemia itself i.e. when so much time has elapsed that neither blood flow restoration is helpful nor reperfusion injury can develop. 3. From ischemia to reperfusion injury When ischemia is for somewhat longer duration, then restoration of blood supply to injured but viable cells (i.e. reperfusion), rather than restoring structure and function of the cell, paradoxically deteriorates the already injured cell and leads it to cell death. This is termed ischemia-reperfusion injury. occurs when blood supply to an ischemic tissue is restored, paradoxically causing additional damage to the tissue. Examples of Ischemia-Reperfusion Injury: myocardial infarction, stroke, and organ transplantation. Pathology department  Mechanism of Ischemia- Reperfusion Injury: 1-Oxidative Stress: Excessive production of ROS overwhelms the antioxidant defenses of the cells. Lipid peroxidation damages cellular membranes. 2-Inflammatory Response: Reperfusion triggers the release of inflammatory mediators and recruitment of neutrophils. Neutrophils release proteases and ROS, amplifying tissue injury. 3- Mitochondrial Dysfunction: Damaged mitochondria during ischemia cannot handle the sudden influx of oxygen, leading to more ROS production and apoptosis. 4-Calcium Overload: Reperfusion exacerbates intracellular calcium accumulation, activating destructive enzymes (phospholipases, proteases, endonucleases) that damage membranes, cytoskeleton, and DNA. 5- Endothelial Dysfunction: Reperfusion damages endothelial cells, impairing vasodilation due to reduced nitric oxide availability and promoting further inflammation and thrombosis. Pathology department Pathology department  ISCHAEMIA-REPERFUSION INJURY AND FREE RADICAL-MEDIATED CELL INJURY THUS Ischemia-reperfusion injury occurs due to EXCESSIVE ACCUMULATION OF FREE RADICALS OR REACTIVE OXYGEN SPECIES. Clinical Significance: IRI highlights the need for careful management during reperfusion therapies. Therapeutic strategies focus on minimizing ROS production, modulating calcium levels, and controlling inflammation to reduce injury. THUS IN SOME CASES HIGH OXYGEN THERAPY TO IMPROVE HYPOXIA IS NOT GIVEN BECAUSE IT GENERATES OXYGEN DERIVED FREE RADICALS ( REACTIVE OXYGEN SPECIES ROS) Pathology department  SO WHAT IS Reactive Oxygen Species (ROS) Pathology department  Reactive Oxygen Species (ROS) Definition: ROS are molecules with an unpaired electron, making them highly unstable and reactive. Produced during normal metabolism or due to external stimuli (e.g., radiation, toxins). Sources: Endogenous: Mitochondria, NADPH oxidase, peroxisomes, endoplasmic reticulum. Exogenous: Radiation, toxins, pollutants. Types: Free Radicals: Superoxide (O₂⁻), Hydroxyl radical ( OH). Non-Radicals: Hydrogen peroxide (H₂O₂), Singlet oxygen (¹O₂). Roles: Physiological: Cell signaling, microbial killing. Pathological: Oxidative stress damages lipids, proteins, and DNA, contributing to diseases like cancer, neurodegeneration, and cardiovascular disorders. Antioxidant Defenses: Enzymatic: Superoxide dismutase (SOD), catalase, glutathione peroxidase. Non-Enzymatic: Vitamins C and E, glutathione, flavonoids. Takeaway: ROS are essential at controlled levels but harmful in excess, emphasizing the need for a balance between ROS production and antioxidants. Pathology department  Pathological Effects of ROS: Free radicals initiate autocatalytic reactions whereby molecules they react with become converted into free radicals – propagate chain of damage Oxidative Stress: Excessive ROS overwhelm antioxidant defenses, leading to cell damage. Damage to Cellular Components: 1-Lipid peroxidation of membranes : Lipid peroxidation damages membranes, increasing permeability. 2-Protein destruction: Oxidation alters enzymatic functions and structural integrity. 3- DNA alteration: Causes mutations, strand breaks, and base modifications. Diseases Associated with ROS: Neurodegenerative diseases: Alzheimer’s, Parkinson’s. Cardiovascular diseases: Atherosclerosis, myocardial infarction. Cancer: ROS-induced mutations contribute to carcinogenesis. Pathology department Pathology department Pathology department  Irreversible cell injury is characterized by a) Dispersion of ribosomes b) Cell swelling c) Nuclear chromatin clumping d) Lysosomal rupture e) Cell membrane defects – characterized by mitochondrial dysfunction and profound disturbances in membrane function Pathology department  Irreversible cell injury is characterized by a) Dispersion of ribosomes b) Cell swelling c) Nuclear chromatin clumping d) Lysosomal rupture e) Cell membrane defects – characterized by mitochondrial dysfunction and profound disturbances in membrane function Pathology department The most important factor in irreversible cell injury is a) ATP depletion b) Decreased protein synthesis c) Decreased pH d) Membrane damage – profound membrane dysfunction is a hallmark e) Loss of intracellular K+ Pathology department The most important factor in irreversible cell injury is a) ATP depletion b) Decreased protein synthesis c) Decreased pH d) Membrane damage – profound membrane dysfunction is a hallmark e) Loss of intracellular K+ Pathology department  Types of cell death Pathology department  NECROSIS Pathology department  Necrosis Definition: Morphological changes that follow death of group of cells within the living body which occur either directly or follow reversible injury. Accompanied by inflammatory reaction Affect both nucleus and cytoplasm Due to pathologic process –NEVER PHYSIOLOGIC NB: Cell placed immediately in fixative are dead but not necrotic). Morphologic change in necrosis: The changes don’t appear in the affected cells by light microscopy before 2-6 hours according to the type of the affected tissue. Pathology department  Morphology of Cell Injury and Necrosis Cytoplasmic changes : Cytoplasmic eosinophilia due to loss of normal basophilia & increased binding of eosin to denaturated proteins Necrotic cell have more glassy homogenous appearance Nuclear changes: Pyknosis: shrinkage-increased staining with haematoxylin Karyorrhexis: fragmentation of nuclear Karyolysis: total disappearance (fading of basophila of chromatin) Pathology department Pathology department Pathology department Pathology department Pathology department  Types of necrosis 1-Coagulative Necrosis 2- liquifactive Necrosis 3- Caseous Necrosis 4-Fat Necrosis 5-Fibrinoid Necrosis Pathology department  Coagulative necrosis This is the most common type of necrosis caused by irreversible focal injury, mostly from sudden cessation of blood flow (ischemia), and less often bacterial and chemical agents. Gross appearance: Foci of Coagulative necrosis in the early stage are pale, firm, and slightly swollen. With progression, they become more yellowish, softer, and shrunken. Microscopic appearance: the hallmark of Coagulative necrosis is the conversion of normal cells into their ghost cells. outlines of the cells are retained and the cell type can still be recognized but their cytoplasmic and nuclear details are lost. Pathology department  Homogenous, Structureless, Pink Pathology department  Coagulative necrosis. Cell’s basic outline is preserved but details are lost. Protein dénaturation prédominâtes enzymatic digestion. Pathology department  LIQUEFACTION NECROSIS Liquefactive necrosis is characterized by complete enzymatic digestion of necrotic cells, often resulting in a semi-liquid mass. The common examples are infarct brain and abscess cavity. Gross appearance: The affected area is soft with liquefied centre containing necrotic debris. Later, a cyst wall is formed. Microscopic examination: Cystic Spaces: Cavitation with liquefied contents (common in the brain). Prominent Neutrophilic Infiltrate: Seen in abscesses and infections Foamy Macrophages: Pathology departmentLipid-laden macrophages in brain necrosis.  LIQUEFACTION NECROSIS Infarction of CNS Pyogenic abscess Pathology department  CASEOUS NECROSIS This is found in the centre of foci of tuberculous infections. It combines features of both Coagulative and liquefactive necrosis. Gross appearance: foci of caseous necrosis, resemble dry cheese and are soft, granular and yellowish. Microscopic appearance: The necrosed foci are structureless, eosinophilic, and contain granular debris. The surrounding tissue shows characteristic granulomatous inflammatory reaction consisting of epithelioid cells with interspersed giant cells of Langerhan's’ or foreign body type and peripheral Pathology mantle department of lymphocytes.  Caseation Necrosis Pathology department  Caseation Necrosis Pathology department  FAT NECROSIS This is a special form of cell death occurring mainly in fat-rich anatomic locations in the body. The examples are: Traumatic fat necrosis of the breast, especially in heavy and pendulous breasts, Enzymatic mesenteric fat necrosis due to acute pancreatitis. Gross appearance: Fat necrosis appears as yellowish-white and firm deposits. Formation of calcium soaps imparts the necrosed foci firmer and chalky white appearance. Microscopic examination: The necrosed fat cells have cloudy appearance and are surrounded by an inflammatory reaction. Formation of calcium soaps is identified in the tissue sections as amorphous, granular and basophilic material. Pathology department  Traumatic Fat necrosis In fat necrosis, there is hydrolysis and rupture of adipocytes, causing release of neutral fat which changes into glycerol and free fatty acids. The leaked out free fatty acids complex with calcium to form calcium soaps (saponification) Pathology department  Traumatic Fat necrosis Pathology department  Enzymatic fat necrosis In pancreatitis. The enzyme lipase escapes from the ruptured pancreatic ducts and digests the surrounding. Fatty acids deposit with calcium as small dull opaque white patches Pathology department  FIBRINOID NECROSIS This is characterised by deposition of fibrin-like material which has staining properties of fibrin. It is encountered in various examples of immunologic tissue injury and arterioles in hypertension. Microscopic examination: Fibrinoid necrosis is identified by brightly eosinophilic, hyaline-like deposition in the vessel wall. Necrotic focus is surrounded by nuclear debris of neutrophils (leucocytoclasis). Pathology department  Fibrinoid Necrosis Nuclear debris Fibrinoid necrosis Viable PNLs Pathology department  Feature Coagulative Caseous Liquefactive Fat Fibrinoid Amorphous, Immune- Preservation of Destruction of fat "cheese-like" Complete tissue mediated injury tissue architecture tissue by lipase- Definition necrosis in digestion into a with deposition o with protein mediated granulomatous liquid mass. fibrin-like denaturation. damage. conditions. material. Tuberculosis, Acute Immune complex most common Brain infarction, fungal infections pancreatitis, diseases (e.g., Causes ischemia (except abscess, (e.g., trauma to fat-rich polyarteritis brain), infarction. infections. histoplasmosis). tissues (breast). nodosa). Firm and opaque Bright pink Yellow-white, Soft, liquefied Chalky white tissue with amorphous Morphology granular "cheesy" tissue; cystic deposits preserved deposits in vessel appearance. cavities. (saponification). architecture. walls. Necrotic Preserved cell Granular, Loss of Eosinophilic adipocytes with outlines with eosinophilic architecture, "fibrin-like" Microscopic basophilic eosinophilic center with necrotic debris, deposits in vessel Features cytoplasm. (ghost calcium deposits granulomatous and neutrophilic walls, surrounded and inflammatory cells) inflammation. infiltrate. by inflammation. cells. Lungs, lymph Breast, Heart, kidney, spleen Arterioles, small SitesPathology department nodes (TB), fungal Brain, abscesses, subcutaneous fat, (ischemia). vessels. infections. pancreas.  Fate of Necrosis: Superadded Small area Large area putrefaction Healing by Surrounded Gangrene regeneration by fibrous or fibrosis capsule & Ca deposition (calcification) Pathology department  A 50-year-old chronic alcoholic presents to the emergency room with 12 hours of severe abdominal pain. The pain radiates to the back and is associated with an urge to vomit. Physical examination discloses abdominal tenderness. Which of the following morphologic changes would be expected in the peripancreatic tissue of this patient? (A) Coagulative necrosis (B) Caseous necrosis (C) Fat necrosis (D) Fibrinoid necrosis (E) Liquefactive necrosis Pathology department  A 50-year-old chronic alcoholic presents to the emergency room with 12 hours of severe abdominal pain. The pain radiates to the back and is associated with an urge to vomit. Physical examination discloses abdominal tenderness. Which ofthe following morphologic changes would be expected in the peripancreatic tissue of this patient? (A) Coagulative necrosis (B) Caseous necrosis (C) Fat necrosis (D) Fibrinoid necrosis (E) Liquefactive necrosis Pathology department  A 70-year old woman suddenly lost consciousness and on awakening one hour later, she could not speak nor move her right arm and leg. Two months later, a head MRI showed a large cystic area in the left parietal lobe. Which of the following pathologic processes has most likely occurred in the brain? A. karyolysis B. Fat necrosis C. Apoptosis D. Liquefactive necrosis Pathology department  A 70-year old woman suddenly lost consciousness and on awakening one hour later, she could not speak nor move her right arm and leg. Two months later, a head MRI showed a large cystic area in the left parietal lobe. Which of the following pathologic processes has most likely occurred in the brain? A. karyolysis B. Fat necrosis C. Apoptosis D. Liquefactive necrosis Pathology department  APOPTOSIS Pathology department  Apoptosis (programmed cell death) Distinctive morphologic pattern of cell death affecting a single cell or small group of cells. Definition: death of individual cells surrounded by viable cells It is an active process—energy dependent Does NOT ELICIT INFLAMMATORY response May be physiologic or pathologic Pathology department  PHYSIOLOGICal causes of Apoptosis to eliminate cells that are no longer needed………maintain constant no cells During E.g. removal of interdigital webs during embryogenesis embryonic development of toes and fingers Hormone- e.g. endometrial cell loss in menstruation, regression of dependent lactating breast after withdrawal of breast-feeding Involution of the thymus in early age In aging Pathology department  PATHOLOGICal Causes of Apoptosis To eliminate irreparably (unable to be fixed) damaged cell (DNA & Protein damage) 1.Cell death in tumours exposed to chemotherapeutic agents. 2. Cell death by cytotoxic T cells in immune mechanisms such as in graft-versus-host disease and rejection reactions. 3.Progressive depletion of CD4+T cells in the pathogenesis of AIDS. 4.Cell death in viral infections e.g. formation of Councilman bodies in viral hepatitis. Pathology department  PATHOLOGICal Causes of Apoptosis 5.Pathologic atrophy of organs and tissues on withdrawal of stimuli e.g. prostatic atrophy after orchiectomy, atrophy of kidney or salivary gland on obstruction of ureter or ducts, respectively. 6.Cell death in response to low dose of injurious agents involved in causation of necrosis e.g. radiation, hypoxia and mild thermal injury. 7.In degenerative diseases of CNS e.g. in Alzheimer’s disease, Parkinson’s disease, and chronic infective dementias. 8.Heart diseases e.g. in acute myocardial infarction (20% necrosis and 80% apoptosis). Pathology department Pathology department  Microscopic Features of Apoptosis Involvement of single cells or small clusters of cells in the background of viable cells. Cell Shrinkage: The cell becomes smaller with dense cytoplasm and tightly packed organelles. Nuclear Changes: Chromatin Condensation: Dark, basophilic nuclear material clumps along the nuclear membrane. Fragmentation: Nucleus breaks into smaller pieces (karyorrhexis). Formation of Apoptotic Bodies: Small membrane-bound fragments containing cytoplasm, organelles, and nuclear debris. Intact Plasma Membrane: Maintains integrity but shows altered phospholipid distribution (e.g., phosphatidylserine flips outward). Absence of Inflammation: Surrounding tissue lacks significant neutrophilic or inflammatory infiltrates. Clearance by Phagocytes: Apoptotic bodies are quickly engulfed by macrophages or neighboring cells without triggering inflammation. Pathology department  Morphological Changes Pathology department Pathology department Pathology department  Mechanism of Apoptosis 1. Triggering Pathways Intrinsic Pathway (Mitochondrial): Happens when the cell is stressed (e.g., DNA damage, lack of nutrients). The mitochondria release cytochrome c, which activates a set of proteins to start the process. Extrinsic Pathway (Death Receptor): Happens when signals from outside the cell (like Fas ligand or TNF) bind to special receptors on the cell surface. This sends a signal inside the cell to begin apoptosis. 2. Caspase Activation Both pathways lead to the activation of caspases (special enzymes). Caspases act like scissors, cutting up proteins and DNA inside the cell. 3. Execution and Clearance The cell breaks into small pieces called apoptotic bodies. These pieces are quickly eaten by nearby cells or macrophages without causing inflammation. Pathology department  Mechanism of Apoptosis Two main pathways EXTRINSIC PATHWAY INTRINSIC PATHWAY (MITOCHONDRIAL): (DEATH RECEPTOR): Happens when the cell is stressed Happens when signals from outside (e.g., DNA damage, lack of nutrients). the cell (like Fas ligand or TNF) bind to The mitochondria release CYTOCHROME C, special receptors on the cell surface. which activates a set of proteins to start the This sends a signal inside the cell to process. begin apoptosis. 2. CASPASE ACTIVATION Both pathways lead to 3. EXECUTION AND the activation of CLEARANCE caspases (special The cell breaks into enzymes). small pieces called apoptotic bodies. Caspases act like scissors, These pieces are quickly cutting up proteins and eaten by nearby cells or DNA inside the cell. macrophages without causing inflammation.. Pathology department Pathology department Pathology department  Feature Apoptosis Necrosis Programmed cell death, energy-dependent, Unregulated cell death due to injury or Definition tightly regulated. damage. Physiological (e.g., embryogenesis, immune Always pathological (e.g., ischemia, Cause regulation) or pathological (e.g., DNA damage). toxins, infections). Cell shrinkage, chromatin condensation, Cell swelling, membrane rupture, and Cellular Morphology apoptotic bodies. loss of cellular architecture. Pyknosis, karyorrhexis, karyolysis with Nuclear Changes Pyknosis, karyorrhexis without inflammation. inflammation. Intact but altered (phosphatidylserine flips to Disrupted, leading to leakage of Plasma Membrane outer membrane). intracellular contents. Inflammatory Prominent inflammation due to leakage None (contents contained in apoptotic bodies). Response of cell contents. Energy Requirement Requires ATP for execution. Passive process; no energy required. Release of cytochrome c triggers caspase Severe damage leads to loss of Mitochondrial Role activation. membrane potential and dysfunction. - Embryogenesis. - Myocardial infarction. - Hormone-dependent involution (e.g., Examples - Stroke. endometrium). Pathology department - Infections (e.g., gangrene). - Cytotoxic T-cell mediated killing.  Irreversible cell injury Pathology department  Comparison Between Reversible and Irreversible Cell Injury Feature Reversible Cell Injury Irreversible Cell Injury Injury that can be reversed if the damaging Definition Injury that leads to permanent cell death. stimulus is removed. Cell Size Cell swelling (hydropic change). Cell swelling followed by rupture. Intact but altered (blebbing and loss of Disrupted, with leakage of cellular Plasma Membrane microvilli). contents. Severe damage, including rupture and Mitochondria Swelling with preservation of function. release of cytochrome c. Irreversible nuclear changes: pyknosis, Nucleus Chromatin clumping. karyorrhexis, karyolysis. Swelling of endoplasmic reticulum and Lysosomal rupture and autodigestion of Organelles detachment of ribosomes. organelles. Complete ATP depletion, leading to loss Energy Status Decreased ATP production but can recover. of function. Intensely eosinophilic due to denatured Cytoplasm Cloudy, with granular appearance. proteins. Inflammatory Prominent inflammation due to leaked None or minimal. Response cellular contents. Necrosis or apoptosis, depending on the Outcome Cell returns Pathology department to normal if stimulus is removed. context.  Apoptosis has the following features except: A. There is cell shrinkage in apoptosis B. There are no acute inflammatory cells surrounding apoptosis C. There may be single cell loss or affect clusters of cells D. Apoptosis is seen in pathologic processes only Pathology department  Apoptosis has the following features except: A. There is cell shrinkage in apoptosis B. There are no acute inflammatory cells surrounding apoptosis C. There may be single cell loss or affect clusters of cells D. Apoptosis is seen in pathologic processes only Pathology department Pathology department Pathology department  "Answering questions isn't just a test—it's a powerful way to learn, reinforce, and master what you know. Every answer brings you closer to expertise!" https://forms.gle/BKwHiqqTWn2WvcK86 Pathology department  Short essay QUESTIONS List the main mechanisms of irreversible cell injury. What is the role of reactive oxygen species (ROS) in cell injury? What are the nuclear changes seen in necrosis? Name the types of necrosis and provide one example of each. Name the two main pathways of apoptosis. Compare between apoptosis & necrosis? What are the main differences between reversible and irreversible cell injury? Pathology department