Cell Necrosis and Injury

• In irreversible cell injury, plasma, mitochondrial, and lysosomal membrane damage lead to cell death.
• Cell injury is initially reversible; however, with ongoing damage, it reaches a point of irreversibility
• Irreversibly injured cells undergo morphological changes recognized as cell death.
• Morphological hallmark of cell death is the loss of the nucleus.
• There are two mechanisms of cell death: necrosis and apoptosis.
• Necrosis and apoptosis differ in their morphology, roles in disease, and physiology.

Necrosis

• Necrosis is the uncontrolled death of a large group of cells.
• It occurs within a living organism, and is followed by acute inflammation and structural changes.
• It is due to pathologic processes and never physiologic.

Cytoplasmic changes in necrosis:

• Cellular changes do not become visible immediately(1-3 hr by EM).
• Swelling of cells occurs
• Increased Eosinophilia is observed due to increased binding of eosin stain to denatured cytoplasmic proteins.
• Cells appear Homogenous due to the loss of cytoplasmic glycogen (granular).
• Whorled phospholipid masses, called myelin figures, are derived from damaged cell membranes.
• Myelin figures digested to Fatty acids combine with calcium, and the cells eventually calcify.

Nuclear changes In Necrosis:

• Nucleus may show:
  • Pyknosis: Nuclear condensation and shrinkage with increased basophilia.
  • Karyorrhexis: Nuclear fragmentation.
  • Karyolysis: Nuclear fading with decreased basophilia.

Morphological appearance of necrotic area involves:

• Denaturation of intracellular proteins
• Unregulated enzymatic digestion of cell components by lysosomes of damaged cell itself
• Loss of cell membrane integrity leads to the release of cellular products.
• Inflammatory cells produce enzymes to digest the dead tissue (Heterolysis).
• Distinctive morphologic patterns depending on whether enzyme lysis or protein denaturation predominates

Types of Necrosis

• Necrosis is divided into several types, depending on morphological patterns:
  • Coagulative necrosis (most common)
  • Liquefactive necrosis
  • Caseous necrosis
  • Gangrenous necrosis
  • Fat necrosis
  • Fibrinoid necrosis

Coagulative Necrosis

• The characteristic of ischemic death of cells occur in all tissues except for the brain.
• Protein denaturation is more prominent than enzymatic breakdown (cell lysis)
• A localized area of coagulative necrosis is called an infarct.
• Necrotic cells are eventually removed by proteolysis and phagocytosis.

Gross picture of Coagulative Necrosis:

• Affected tissues exhibit a pale or yellowish color, with a firm texture.
• The architecture of dead tissues is preserved by coagulation of cellular protein for some days.
• A local vascular/inflammatory reaction to necrotic tissue may be observed.

Microscopic picture of Coagulative Necrosis:

• The basic outlines are preserved, but cellular details are lost.
• An-nucleated cells are cells without a nucleus.
• There’s a Homogenous, glassy eosinophilic appearance is apparent due to loss of cytoplasmic RNA and glycogen.

Liquefactive Necrosis

• Enzymatic breakdown is more prominent than protein denaturation.
• Necrotic cells are digested via leukocytes and affected cells (lysosomal hydrolytic enzymes)
• These processes transform the tissue into a liquid viscous mass.
• Typically found in the central nervous system (CNS) and abscesses

In liquefactive necrosis:

• Autolysis predominates in the brain, resulting in a liquefied mass in hypoxia.
• Pyogenic abscesses releases of hydrolytic enzymes from neutrophils and bacteria.

Gross appearance Liquefactive Necrosis:

• Affected tissue is liquefied into a soft, viscous or fluid mass, surrounded by oedema and hyperemia.

Microscopic appearance Liquefactive Necrosis:

• Structureless eosinophilic area of necrotic tissue is observed
• The region contains debris and lysed cells.
• Architecture is completely obliterated.
• Surrounded by inflammatory cells.

Caseous Necrosis

• Appearance of "cheese-like," in the area of necrosis: soft, friable, yellow-white.
• Common in foci of tuberculous infections (TB).
• Surrounded by inflammatory border, which indicates the inflammation known as a granuloma.

Morphology of Caseous Necrosis:

• Gross: soft, cheesy material, granular and pale yellow.
• Microscopic: No visible cell outlines, obliterated tissue architecture
• Amorphous, structureless material that is pink in color and a rim inflammatory cells.
• Granulomas containing epithelioid macrophages, a rim of lymphocytes, and Langhans cells.

Gangrenous Necrosis

• Commonly used clinical term.
• Coagulative necrosis characterized by black discoloration.
• Caused by a loss of blood supply (chronic ischemia), especially in the lower limb
• If a bacterial infection is superimposed liquefactive action of bacteria and leukocytes causes wet gangrene.
• Loss of blood also dries the tissue causing dry gangrene.

"Dry" gangrene:

• Usually due to arterial supply interference, but with preserved venous return.
• Results from chronic tissue ischemia.
• Shriveled/mummified appearance
• Dark or black discoloration is observed
• The spread is slow.
• Clear Line of inflammatory reaction occurs between dead tissue and health tissue causes line of demarcation

“Wet” gangrene:

• Bacterial superinfection is present.
• The tissue appears wet due to extensive liquefaction and black discoloration.
• There is no clear line of demarcation.
• Results from interference with venous return
• Occurs in areas that anaerobic bacteria could access such as in extremities, and the intestine.

Fat Necrosis

• Refers to focal areas of necrotic adipose tissue with a chalky-white appearance.
• Results from deposition of calcium (saponification).

There are two classifications of Fat Necrosis:

• Enzymatic fat necrosis: occurs following acute pancreatitis.
• Traumatic fat necrosis: from breast traumas

Enzymatic fat necrosis:

• Release of activated pancreatic lipases occurs into the substance of the pancreas and the peritoneal cavity.
• Pancreatic enzymes liquefy the membranes of fat cells to release fatty acids.
• Fatty acids released by trauma or lipase combine with calcium, forming calcium soaps (saponification).
• Saponification is an example of dystrophic calcification.

Traumatic fat necrosis :

• Usually happens in the breast trauma.
• May form a hard mass that mimics cancer.
• Typically surrounded by fibrosis & calcification
• Dystophic calcification is example of Saponification

Fat Necrosis Histology:

• Foci of necrosis contain shadowy outlines of necrotic fat cells showing eosinophilic cytoplasm.
• Surrounded by basophilic calcium deposits.
• Inflammatory reaction that may include a high variable number of lipid-laden macrophages and neutrophils.

Fibrinoid Necrosis

• It usually occurs in the wall of blood vessels.
• Observed in immunologic diseases due to antigen-antibody deposition.
• It is observed malignant hypertension and vasculitis.
• Necrosis of smooth muscle cells of the tunica media and endothelial damage, Plasma proteins deposit
• These plasma proteins including fibrin
• This result to bright pink appearance on H&E.

Clinical Correlation of Necrosis

• Injured membranes tend to be leaky
• Enzymes and other proteins that leak into the blood stream, and assessed by clinicians to assess damage.
• These processes occur in clinical conditions like Myocardiac infarction where troponins T and I are measured from a sample of blood.

Apoptosis

• Programmed, energy-dependent mechanism resulting in cell death.
• It works by the elimination of unwanted cells
• No host reaction is observed to apoptosis.
• Involves single cells or small groups of cells.
• Can occur in either physiological and pathological conditions.

Apoptosis:

• A normal physiologic process in embryonic development (e.g., separates webbed fingers and toes)
• Also responsible for Loss of endometrial cells during menstruation, and for Regression of breast after weaning
• Functions in the removal of neutrophils during Inflammation, elimination of lymphocytes
• Can be involved in deletion in the proliferating cell population of Epithelial cells in the GI tract

Apoptosis in pathological conditions:

• Important mechanism for the removal of cells with irreversible DNA damage (from viruses, free radicals, chemical....etc)
• Protects against neoplastic transformation
• Seen in viral diseases and tumors
• Causes atrophy in parenchymal organs, from duct obstruction (pancreas, parotid gland)
• Atherosclerosis of renal artery can cause kidney atrophy

Mechanism of Apoptosis

• Mediated by caspases.
• Caspases activate Nucleases break down DNA into nucleosomal chromatin fragments.
• Proteases break down cytoskeleton.
• Fragmentation of cells is a result.
• Extrinsic and Intrinsic pathways lead to caspases activation.

The extrinsic pathway

• Also known as the death receptor pathway.
• Death signals come from outside the cell.
• Mediated by cell surface death receptors (TNF family receptor)
• Its protein called FAS activate enzymes called CASPASES.

Intrinsic Pathway:

• Cell injury, DNA damage, or a decrease of hormonal stimulation that inactivates anti-apoptotic molecules Bcl-2 and Bcl-xL.
• Mitochondrial permeability increases and causes cytochrome c release
• Stimulation of pro-apoptotic proteins such as Bax releases CASPASES within.

Morphologic Features of Apoptosis

• Cell shrinkage and loss of junction (becomes eosinophilic)
• Maintains the plasma membrane
• Chromatin condensation and DNA fragmentation
• Formation of cytoplasmic blebs and apoptotic bodies
• Fragments are quickly phagocytized without causing an inflammatory response.

Explore cell necrosis, irreversible cell injury, and associated morphological patterns. Understand the sequence of nuclear events, enzymatic digestion, and the role of inflammation. Learn to identify key characteristics such as eosinophilia and myelin figures.