The Pathogenesis and molecular changes in cell injury and necrosis:
There are several biochemical and molecular changes evident in cell injury & necrosis. To properly understand these changes let’s discuss the patterns of cell injury and necrosis:
Reduced ATP production from hypoxia decreases oxidative phosphorylation of ADP. Inefficient Na+/K+ pump drives Na+, water into the cell, & K+ out of the cell. Swelling causes rough endoplasmic reticulum to distend. Ribosomes detach from ER resulting in reduced protein synthesis.
Anaerobic Glycolysis causes build up of lactic acid which reduces PH level. The reduction of enzyme activity takes place , with clumping of nuclear chromatin. This is part of the molecular changes.
Mitochondrial damage from cytosolic calcium, hypoxia, and toxins increase permeability of mitochondria. This reduces oxidative phosphorylation and ATP that leads to cell injury & eventually cell death.
Increased cytosolic calcium due to ischemia activates intracellular enzymes and increases mitochondria membrane permeability. Enzymes like endonucleases, proteases, phospholipases destroy cellular components. Mitochondria membrane damage decreases ATP synthesis as part of the molecular changes.
Generation of free radicals cause oxidation of free metallic ions & molecular compounds like carbohydrates & proteins. This oxidation damages protein synthesis, DNA, and plasma membrane. Common free radicals include superoxide anion; hydrogen peroxide & hydroxyl ion, Nitrogen dioxide, trichloromethyl. Free radicals are produced in mitochondria, peroxisomes, lysosomes and by radioactive agents. Antioxidants like vitamin A, E, C, and superoxidase dismutases and catalase prevent oxidative stress from occurring.
Reactive oxygen species (ROS) are a type of oxygen-derived free radicals. Excess ROS production or ineffective scavenging systems results in oxidative stress.
Increased membrane permeability in mitochondria, lysosomes, & cell from injurious stimuli causes extensive damages. Mitochondria ATP depletion and cell death; digestion of RNA, DNA, glycogen, protein, & phosphoric esters by lysosomal enzymes; plasma cell swelling and death.
In necrosis, nuclear membrane is damaged in this forward pattern: Pyknosis- Karyorrhexis – Karyolysis.
Clinically important causes of cell injury:
In medical practice, we may encounter cases of hypoxic-ischemic cell injury, chemical intoxication, & ischemia-reperfusion injury. Common causes as these, disrupt cell membrane intergrity by producing free radicals.
In ischemia-reperfusion injury, parenchymal and endothelial cells release reactive oxygen and nitrogen molecules.
Cytochrome P-450 enzyme converts drugs to highly reactive metabolites in chemical intoxication.
Apoptosis and its molecular changes
Apoptosis is the irreversible death of cells which may be physiologic or pathologic.
Physiologic causes include: programmed cell death during embryogenesis; hormone withdrawal; failure of immature lymphocytes in the bone marrow & thymus to express useful antigenic receptors; potentially harmful self-reactive lymphocytes; lack of growth factors for neutrophils & lymphocytes.
Pathologic causes are: DNA damage from cytotoxic anticancer drugs, radiation, & hypoxia; accumulation of misfolded proteins; viral infections; cytotoxic T-cell immune response to viral hepatitis, tumors, & allografts; duct obstruction resulting in atrophy of pancreas, kidney, & parotid glands.
The hallmark of apoptosis: Intact plasma cell & nuclear membrane; Cell shrinkage; chromatin condensation & digestion; plasma membrane blebs and apoptotic bodies formation; phagocytosis.
Pathogenesis of apoptosis:
The most common pathway is the mitochondrial intrinsic pathway. Growth factors withdrawal, DNA damage, & misfolded proteins activate Bcl-family sensors that activate pro-apoptotic Bcl-family and inhibit anti-apoptotic BCL-family. The prop-apoptotic Bcl-family attaches to mitochondria membrane to create pores. Cytochrome c is released into the cytosol which then binds to Apaf-1 protein (apoptosis-activating factor-1) to form apoptosome complex.
Apoptosome complex binds to initiator caspase-9. Activation of initiator caspase-9 occcurs following autoproteolysis. Active initiator caspase-9 then rapidly & sequentially activates executioner caspases including caspase-3 & -6. These executioner caspases promote nuclei fragmentation.
Intracellular accumulation of molecular substances:
Molecular compounds like, lipids, carbohydrates, proteins, iron remain in the cell vacuoles. Slow metabolism, defective enzymes to transport them or excess amount of them are found to be the cause.
Fatty change (steatosis), causes excessive triglyceride to build up in the vacuoles of liver or heart. This may be due to excessive entry or defective metabolism & export of lipids. Causes include: alcohol abuse, diabetes mellitus, obesity, anorexia, & protein malnutrition. Fatty change appear as orange-red color on Sudan-IV stain.
In atherosclerotic plaques, cholesterol & cholesterol esters accumulate in vacuoles of macrophages and smooth muscles of large arteries. They appear as yellow-cholesterol laden atheromas in the intima of large arteries.
The intracellular accumulation of proteins commonly appear as rounded, eosinophilic droplets, vacuoles or aggregates. Reabsorption droplets in proximal renal tubule occur following renal disorders with heavy protein leakage across the glomerular filter. Increased reabsorption of protein into vesicles appear as pink hyaline droplets within the cytoplasm of proximal tubular cell.
Lipofuscin is an insoluble pigment, derived from undigested debris within the cytoplasm. It gives a yellow-brown granular color on H&E stain. Seen in the liver & heart of aging patients, patients with severe malnutrition, & cancer cachexia.
The carbon pigment is an exogenous pigment, formed from the inhalation of coal dust. It produces a black pigment in lung tissue as in the case of anthracosis.
Hemosiderin pigment is derived from hemoglobin & it is one of the major storge forms of iron. It produces a golden yellow-brown granular pigment on H&E stain. On a Prussian blue histochemical reaction, it creates a blue-black color.
Dystrophic Calcification is the calcification of tissues after necrosis. Macroscopically, it appears as fine, white granules. Microscopically, creates a basophilic amorphous granule on H&E stain.