Apoptotic pathways are a series of cellular events that result in programmed cell death, or apoptosis. There are two main pathways: the extrinsic pathway, which is triggered by signals from outside the cell, and the intrinsic pathway, which is activated by signals from within the cell.

I. Extrinsic Pathway of Apoptosis/ Extrinsic apoptotic pathway

• The extrinsic pathway is triggered by signals from outside the cell, such as death receptors on the cell surface.
• These receptors, such as tumor necrosis factor (TNF) and Fas (CD95) receptors, bind to their respective ligands and undergo a conformational change that allows them to recruit adaptor proteins.
• These adaptor proteins, such as FADD and TRADD, recruit and activate the caspase cascade, which is a series of proteases that ultimately lead to the destruction of the cell.
• The initiator caspase in this pathway is caspase 8, which is activated by the adaptor proteins.
• Caspase 8 in turn activates the executioner caspases, caspase 3 and caspase 7, which cleave and degrade a variety of cellular proteins, leading to the characteristic morphological changes of apoptosis.

II. Intrinsic Pathway of Apoptosis/ Intrinsic apoptotic Pathway

• The intrinsic pathway is activated by signals from within the cell, such as DNA damage or mitochondrial dysfunction.
• This pathway involves the release of pro-apoptotic molecules, such as cytochrome c and SMAC/DIABLO, from the mitochondria into the cytosol.
• These molecules bind to and activate the initiator caspase, caspase 9, which is a part of the Apaf-1 complex.
• Caspase 9 in turn activates the executioner caspases, caspase 3 and caspase 7, which cleave and degrade a variety of cellular proteins, leading to the characteristic morphological changes of apoptosis.
• The intrinsic pathway is also regulated by the Bcl-2 family of proteins, which can either promote or inhibit the release of pro-apoptotic molecules from the mitochondria.
• It is important to note that the intrinsic and extrinsic pathway converge at the activation of executioner caspases, caspase 3 and caspase 7, which leads to the apoptotic process.