Cell Injury and Necrosis

Cell Response to Injury: Causes

• Hypoxia, caused by ischaemia, inadequate oxygenation, and anaemia, can lead to cell injury
• Living irritants like bacteria, viruses, fungi, and parasites are causes of cell injury
• Physical and chemical irritants can cause cell injury
• Immunological reactions can be a cause of cell injury
• Nutritional deficiencies may lead to cell injury
• Genetic and enzymatic abnormalities can result in cell injury

Necrosis: Definition and Appearance

• Necrosis involves the local death of cells or tissue in the living body
• The causes and pathogenesis are the same as reversible cell injury
• Necrotic tissue appears opaque, white, or yellow, surrounded by a red zone of acute inflammation

Microscopic Changes in Necrosis

• Immediately after necrosis, dead cells appear relatively normal under a microscope
• Autolytic changes occur due to lysosomal enzymes

Nuclear Changes in Necrosis

• Pyknosis involves the nucleus becoming small, irregular, and deeply stained
• Karyorrhexis is the fragmentation of the nucleus into small pieces
• Karyolysis is the dissolution of nuclear fragments

Cytoplasmic Changes in Necrosis

• Cells swell, resulting in cytomegaly, and the cytoplasm becomes homogenous
• Cells lose their cell membrane and the necrotic cells appear as a homogenous structureless area

Types of Necrosis: Coagulative Necrosis

• Necrotic tissues appear firm and dry
• Coagulative necrosis is caused by cut of blood supply
• Early on, cellular details are lost, but the tissue architecture is preserved for some time, both are eventually lost

Types of Necrosis: Liquefactive Necrosis

• Necrotic tissue appears as liquid
• In CNS infarction, liquefaction is due to the high lipid and fluid content
• In pyogenic abscesses, liquefaction results from lysosomal enzymes released by dead polymorphs (pus cells)
• In amoebic abscesses, liquefaction is caused by liquefactive enzymes produced by the amoeba

Types of Necrosis: Caseous Necrosis

• Begins as coagulative necrosis, followed by slow partial liquefaction
• The necrotic area appears semi-solid, semifluid, like casein of milk
• Commonly seen in tuberculosis and syphilis
• An antigen-antibody reaction causes caseous necrosis
• There is a loss of cellular details followed by loss of architecture

Types of Necrosis: Fat Necrosis

• There are two types of fat necrosis: enzymatic and traumatic

Types of Necrosis: Enzymatic Fat Necrosis

• Occurs in acute haemorrhagic pancreatitis
• Lipase enzyme escapes from ruptured pancreatic ducts, acting on fat in the abdominal cavity and splitting it into fatty acid and glycerol
• Glycerol is absorbed, and fatty acids combine with calcium, depositing as opaque white patches
• Affected fat cells swell and are surrounded by foreign body giant cell reaction and fibrosis

Types of Necrosis: Traumatic Fat Necrosis

• Occurs as a result of trauma to fatty areas, such as the female breast
• Self-digestion of the fat splits it into fatty acid and glycerol
• Glycerol is absorbed, and fatty acids combine with calcium, depositing and inducing a fibrosed hard mass
• Affected fat cells are swollen and surrounded by foreign body giant cell reaction and fibrosis

Apoptosis: Definition and Characteristics

• Apoptosis is a special type of cell death affecting a single cell or a small group of cells, referred to as cell suicide
• Morphological changes include shrinkage of the nucleus
• Condensation and fragmentation of chromatin occur
• Chromatin fragments are enclosed in parts of the cell membrane to form apoptotic bodies, which are then phagocytosed by macrophages or adjacent phagocytic cells
• There is no inflammation in the surrounding tissues

Apoptosis: Occurrence

• Apoptosis occurs during normal cell turnover
• It occurs as programmed cell death during embryonic development
• It occurs as endocrine-dependent tissue involution, exemplified by the menstrual cycle
• It can occur in pathological conditions, such as radiation cell injury, cell death by cytotoxic lymphocytes, liver cells in viral hepatitis, and pathological atrophy

Dystrophic Calcification

• Deposition of calcium slats in dead and degenerating tissues
• More common
• Occurs with normal blood calcium level
• Caused by local alkalinity of dead tissue and increased phosphatase activity
• Examples include areas of necrosis, fibrosis, hyalinosis, thrombosis and tuberculosis

Metastatic Calcification

• Deposition of calcium salts in living tissues
• Less common
• Occurs with high blood calcium level, due to Increased Ca intake or hyper-vitaminosis D.
• Increased Ca mobilization from bone as in hyperparathyroidism & bone tumours.
• Caused by relative alkalinity of affected tissue
• Sites are stomach, kidney and lung because these organs secrete acidic secretions and their wall is always relatively alkaline.