Understanding Cell Injury in Medical Science

1. CELL INJURY for Medical (lecture 2) Sufia Husain Assistant Prof & Consultant KKUH, Riyadh.

2. APOPTOSIS • CELLULAR ACCUMULATION

3. APOPTOSIS • Apoptosis is programmed cell death. Apoptosis means “falling off”. It is sometimes referred to as cell suicide. • It is a pathway of cell death that is induced by a regulated intracellular program in which cells destined to die activate their own enzymes to degrade their own nuclear DNA, nuclear proteins and cytoplasmic proteins. • The cell's plasma membrane remains intact, but its structure is altered in such a way that the apoptotic cell sends signal to macrophages to phagocytose it. • The dead cell is rapidly phagocytosed and cleared, before its contents have leaked out, and therefore cell death by this pathway does not elicit an inflammatory reaction in the host. • Thus, apoptosis is fundamentally different from necrosis, which is characterized by loss of membrane integrity, enzymatic digestion of cells, and frequently a host reaction. Apoptosis and necrosis sometimes coexist.

4. CAUSES OF APOPTOSIS • It occurs normally/physiologically in many situations, and serves to eliminate unwanted or potentially harmful cells and cells that have outlived their usefulness. • It is also a pathologic event when cells are damaged beyond repair, especially when the damage affects the cell's DNA; in these situations, the irreparably damaged cell is eliminated. • Therefore apoptosis can be physiologic, adaptive, or pathologic.

5. Apoptosis in Physiologic Situations • The programmed destruction of cells during embryogenesis. • Hormone-dependent involution in the adult, such as endometrial cell breakdown during the menstrual cycle, the regression of the lactating breast after weaning, and prostatic atrophy after castration(adaptive atrophy). • Cell deletion in proliferating cell populations e.g. intestinal epithelia. • Death of host cells that have served their useful purpose, such as neutrophils in an acute inflammatory response, and lymphocytes at the end of an immune response. • Elimination of potentially harmful self-reactive lymphocytes.

6. Apoptosis in Pathologic Conditions • Cell death produced by a variety of injurious stimuli e.g. radiation and cytotoxic anticancer drugs damage DNA. • Cell injury in certain viral diseases, such as viral hepatitis. • Pathologic atrophy in parenchymal organs after duct obstruction, such as occurs in the pancreas, parotid gland, and kidney. • Cell death in tumors (usually accompanied by necrosis).

7. Mechanism of Apoptosis • Cell shrinkage. • Chromatin condensation: This is the most characteristic feature of apoptosis. The chromatin aggregates peripherally, under the nuclear membrane. The nucleus itself may break up into fragments. • Formation of cytoplasmic blebs and apoptotic bodies: The apoptotic cell first shows extensive surface blebbing, then undergoes fragmentation into membrane-bound apoptotic bodies composed of cytoplasm and tightly packed organelles, with or without nuclear fragments. • Phagocytosis of apoptotic cells or cell bodies, usually by macrophages: The plasma membrane of the apoptotic cell undergoes specific alterations which allow early recognition of these dead cells by macrophages. The macrophages phagocytose the apoptotic cell without the release of proinflammatory substance.and therefore there is no inflammation in the surrounding tissue.

9. Morphology of Apoptosis • On histologic examination, in tissues stained with hematoxylin and eosin, apoptosis involves single cells or small clusters of cells. The apoptotic cell appears as a round or oval mass of intensely eosinophilic cytoplasm with dense nuclear chromatin fragments. There is no inflammation. Top figure: apoptosis in liver cell Bottom figure: apoptosis in epidermis.

10. Important enzymes of apoptosis • Cysteine proteases named caspases: they cause protein cleavage or hydrolysis. • Ca2+- and mg2+-dependent endonucleases: they cause DNA breakdown in apoptotic cells. Regulation of apoptosis It is mediated by a number of genes and their products e.g: • bcl-2 gene inhibits apoptosis • baxgenes facilitates apoptosis • p53 facilitates apoptosis by inhibiting bcl2 and promoting baxgenes.

11. The sequential ultrastructural changes seen in necrosis (left) and apoptosis (right). In apoptosis, the initial changes consist of nuclear chromatin condensation and fragmentation, followed by cytoplasmic budding and phagocytosis of the extruded apoptotic bodies. Signs of cytoplasmic blebs, and digestion and leakage of cellular components.

13. Intracellular Accumulations Intracellular accumulation of abnormal amounts of various substances. (1) a normal cellular constituent accumulated in excess, such as water, lipids, proteins, and carbohydrates (2) an abnormal substance, either exogenous, such as a mineral or products of infectious agents, or endogenous, such as a product of abnormal synthesis or metabolism (3) a pigment. The substance may accumulate in either the cytoplasm or the nucleus.

14. 1. Accumulation of LIPIDS All major classes of lipids can accumulate in cells: • triglycerides, • cholesterol/cholesterol esters, • and phospholipids • In addition, abnormal complexes of lipids and carbohydrates accumulate in the lysosomal storage diseases.

15. Steatosis (Fatty Change) • Fatty change is the abnormal accumulation of triglycerides inside parenchymal cells. It is caused by an imbalance between the uptake, utilization, & secretion of fat. Fatty change is usually seen in the liver, but it is also seen in heart, muscle, and kidney. The causes of steatosis include: • Toxins/poisoning, • protein malnutrition, • diabetes mellitus, • obesity, • anoxia • alcohol abuse • Starvation • pregnancy

16. Steatosis (Fatty Change) • Free fatty acids from ingested food or adipocytes are normally transported into the liver, where they are converted to triglycerides or cholesterol or phospholipids, or ketone bodies by the hepatocytes. Release of triglycerides from the hepatocytes requires apoproteins to form lipoproteins. • Excess accumulation of triglycerides within the hepatocytes occurs when there is defect in the any one of the events in the sequence from fatty acid entry to lipoprotein exit by any of the following mechanisms: a. Increased uptake of triglycerides into parenchymal cells. b. Decreased use of fat by cells. c. Overproduction of fat in cells. d. Decreased secretion of fat from the cells. • Fatty change in the heart is seen in severe anemia and in starvation.

17. Morphology of Steatosis • Clear vacuoles within parenchymal cells. • Stains like Sudan IV or Oil Red-O, impart an orange-red color to the contained lipids. • Liver. In mild fatty change gross appearance is normal. With progressive accumulation, the organ enlarges and becomes increasingly yellow and greasy. • Light microscopy: vacuoles in the cytoplasm displacing the nucleus to the periphery of the cell Occasionally, contiguous cells rupture, and the enclosed fat globules coalesce, producing so-called fatty cysts

18. Cholesterol and Cholesterol Esters • Cells normally use cholesterol for the synthesis of cell membranes. Accumulation of cholesterol in the form of intracellular vacuoles, are seen in several pathologic processes e.g.atherosclerosis. • In atherosclerotic plaques there is intracellular accumulation of cholesterol and cholesterol esters in the smooth muscle cells and macrophages in the wall of arteries and also extracellular accumulation. The extracellular cholesterol esters may crystallize in the shape of long needles, producing clefts in atherosclerotic plaques.

19. 2. ACCUMULATION OF GLYCOGEN • Glycogen is a readily available energy store that is present in the cytoplasm. Excessive intracellular deposits of glycogen are seen in patients with an abnormality in either glucose or glycogen metabolism. • They appear as clear vacuoles within the cytoplasm. Stains like mucicarmine or the periodic acid schiff (PAS) stain give a pinkish/violet color to the glycogen. • Diabetes mellitus is the prime example of a disorder of glucose metabolism. In this disease, glycogen is found in the epithelial cells of the proximal convoluted tubules of kidney, liver, the β cells of the islets of Langerhans, heart muscle cells etc. • Glycogen also accumulates within the cells in a group of genetic diseases, collectively referred to as the glycogen storage diseases. In these diseases, enzymatic defects in the synthesis or breakdown of glycogen result in massive accumulation, and cell death.

20. 3. ACCUMULATION OF PIGMENTS • Pigments are colored substances, some of which are normal constituents of cells (e.g., melanin), whereas others are abnormal and collect in cells only under special circumstances. • Pigments can be exogenous, coming from outside the body, or endogenous, synthesized within the body itself.

21. Endogenous Pigments • A) Lipofuscin is an insoluble pigment, also known as wear-and-tear or aging pigment. Lipofuscin is not injurious to the cell or its functions. • Its importance lies in its being the telltale sign of free radical injury and lipid peroxidation. • In tissue sections, it appears as a yellow-brown, finely granular intracytoplasmic, often perinuclear pigment • It is prominent in the liver and heart of aging patients, atrophic tissue, patients with severe malnutrition and cancer cachexia Top 2 figure are from heart and bottom from liver

22. Endogenous Pigments cont. • B) Melanin, is an endogenous, non-hemoglobin-derived, brown-black pigment normally present in the cytoplasm of melanocytes found in the basal layer of the epidermis. • It is responsible for the color of our skin. • It is derived from tyrosine and stored in melanosomes of the melanocytes. • The function of melanin is to prevent the harmful effects of UV light. • It accumulates in large amounts in benign and malignant melanocytic tumors and its presence is helpful in making a diagnosis on biopsy. • In inflammatory conditions of the skin it spills out into the underlying dermis where it is phagocytosed by the dermal macrophages and remains like that in the dermis. This leads to post inflammatory hyperpigmentation (PIH) of the skin. • Masson-Fontana stain is used to identify melanin. • Top 2 figures: normal skin with basal melanin. • Bottom figure: PIH

23. Endogenous Pigments cont. • C) Hemosiderinis a hemoglobin-derived, golden yellow-to-brown, iron containing pigment in cells. Iron is normally stored in the form of ferritin micelles. When ther there is excess iron ee.g. whenthere is break down of rbcs or hemorrhage, hemosiderin aacumulates. Hemosiderin exists normally in small amounts within tissue macrophages of the bone marrow, liver, & spleen as physiologic iron stores. • It accumulates in tissues in excess amounts in certain diseases. This excess accumulation is divided into 2 types: • Hemosiderosis: accumulation of hemosiderin is primarily within tissue macrophages & is not associated with tissue damage. It is either • a localized hemosiderosis (e.g. common bruise: there is lysis of the rbcs , release of hemoglobin which eventually undergoes transformation to hemosiderin) • or as a systemic hemosiderosis. • Hemochromatosis; there is more extensive accumulation of hemosiderin, often within parenchymal cells, which leads to tissue damage, scarring & organ dysfunction resulting in liver fibrosis, heart failure, and diabetes mellitus • Systemic overload of iron is seen with: (1) increased absorption of dietary iron, (2) impaired use of iron, (3) hemolytic anemias, and (4) transfusions because the transfused red cells constitute an exogenous load of iron. In most cases it causes systemic hemosiderosis, where the pigment does not damage the parenchymal cells or impair organ function. But is severe cases it leads to hemochromatosis.

24. Endogenous Pigments cont. • Morphology. Iron pigment appears as a coarse, golden, granular pigment lying within the cell's cytoplasm usually in the macrophages. the pigment may accumulate in the parenchymal cells like the liver, pancreas, heart, and endocrine organs. Iron can be visualized in tissues by the Pearl Prussian blue histochemical reaction, in which it appears blue-black.

25. Exogenous Pigments • The most common exogenous pigment is carbon or coal dust, which is an air pollutant. When inhaled, it is picked up by macrophages within the alveoli and is then transported through lymphatic channels to the regional lymph nodes. • Accumulations of this pigment blacken the tissues of the lungs (anthracosis) and the involved lymph nodes. In coal miners, the aggregates of carbon dust or silica may induce a fibroblastic reaction or even emphysema and thus cause a serious lung disease known as coal worker's pneumoconiosis.

26. Anthracosis lung

27. Exogenous Pigments • Tattooing is a form of localized, exogenous pigmentation of the skin. The pigments inoculated are phagocytosed by dermal macrophages. • Plumbism is lead poisoning and argyria is silver poisoning. In both cases there may be permanent grey discoloration of skin and conjunctivae.