General Veterinary Pathology

APOPTOSIS

Programmed cell death in which there is death of individual cells without inciting inflmmatory processes. In embryogenesis and normal growth, physiologic cell death occurs which may be referred as programmed cell death or apoptosis. Apoptosis can also occur in pathologic diseases.

Mechanism of Apoptosis

There are two processes:

• Initiation phase mediated by caspases
• Execution phase in which enzymatic degradation leads to cell death

Initiation phase

There are two pathways of initiation of apoptosis:

• Extrinsic receptor initiated pathway
• Intrinsic mitochondrial pathway

These two pathways are interconnected and converge to activate caspases

• Extrinsic pathway: On cross linkage of Fas (Death domain) by its ligand three or more molecules come together and bind to cytoplasmic Fas-associated death domain (FADD) which in turn binds to inactive forms of caspase-8 via death domain. These activated caspases trigger a cascade of caspase activation and mediate execution phase of apoptosis. FLIP protein inhibits apoptosis by binding to procaspase-8. This mechanism is used to protect infected normal cells from Fas mediated apoptosis.
• Intrinsic mitochondrial pathway: There are more than 20 antiapoptotic proteins. Of which Bcl-2 and Bcl-x are located on the mitochondrial membrane of cytoplasm. Bcl-2 and Bcl-x are replaced when cells are deprived of survival signals or stress by proapoptotic members like Bak, Bax and Bim. This leads to increased mitochondrial membrane permeability and release of several proteins which activate caspase cascade e.g. cytochrome C from mitochondria which binds to Apaf-1(Apoptosis activating factor-1protein). The complex activates caspase-9. Apoptosis activating factor from mitochondria also neutralizes various apoptotic inhibitors which block caspase activation.

Execution phase

• The final proteolytic cascade is mediated by the proteases (Caspase: ‘c’- cystine protease that cleaves aspartic acid residues). There are more than 10 members in caspase family which are grouped into initiator and executioner groups depending on their order in which they are activated during apoptosis e.g. caspase-8 and 9 are initiator caspases and caspase-3 and 6 are executioner caspases.
• These caspases are hydrolysed autocatalytically following cleavage of initiator caspase to generate the active form. The enzymatic death programme sets in motion by rapid and sequential activation of other caspases. These caspases can act on many cellular components like cytoskeleton and nuclear matrix proteins. Cytoskeleton destruction and nuclear break down occurs. Caspase target proteins of transcription, DNA replication and DNA repair in the nucleus e.g. caspase-3 activates cytoplasmic DNAs.
• Not only gross changes, but microscopical changes are also not obvious since single cell death occurs.

Histopathologically,

• Shrinkage of individual cells: cells-size smaller, cytoplasm is dense and organelles are tightly packed.
• Condensation of chromatins: Most characteristic in apoptosis. Aggregation of chromatins under nuclear membrane with variable shape and size (Semilunar shape)
• Cytoplasmic fragmentation
• Cytoplasmic buds containing fragments of nucleus: Cytoplasm shows excessive surface budding and formation of membrane bound fragments (Apoptotic bodies) containing cytoplasm and tightly packed organelles with or without nuclear fragments. Nucleus itself may break up into two or more fragments
• Presence of apoptotic bodies in the adjacent cells and phagocytes
• Inflammation is absent.