Which of the following features best distinguishes the intrinsic (mitochondrial) pathway of apoptosis from the extrinsic (death receptor) pathway?

Explanation

## Distinguishing Feature: Mitochondrial Involvement ### Intrinsic vs Extrinsic Pathways — Core Difference While both pathways converge at executioner caspases (caspase-3/7), the **intrinsic pathway is uniquely defined by mitochondrial outer membrane permeabilization (MOMP)** and the release of cytochrome c into the cytoplasm. This is the cardinal feature that distinguishes it from the extrinsic pathway. ### Intrinsic Pathway Mechanism **Key Point:** The intrinsic pathway is triggered by **intracellular stress** (DNA damage, oxidative stress, growth factor withdrawal) and depends on: 1. p53 activation and upregulation of pro-apoptotic Bcl-2 family members (Bax, Bak) 2. Mitochondrial outer membrane permeabilization (MOMP) 3. Cytochrome c release into the cytoplasm 4. Formation of the apoptosome (cytochrome c + Apaf-1 + pro-caspase-9) 5. Caspase-9 activation → caspase-3/7 activation ### Extrinsic Pathway Mechanism **Key Point:** The extrinsic pathway is triggered by **extracellular ligands** (Fas, TNF, TRAIL) and depends on: 1. Death receptor engagement (Fas, TNF-R1, TRAIL-R) 2. Formation of the death-inducing signalling complex (DISC) 3. Direct activation of caspase-8 4. Caspase-8 → caspase-3/7 activation (or amplification via mitochondrial pathway) ### Comparative Table | Feature | Intrinsic Pathway | Extrinsic Pathway | |---------|-------------------|-------------------| | **Trigger** | Intracellular stress (DNA damage, hypoxia) | Extracellular ligands (Fas, TNF, TRAIL) | | **Initiator Caspase** | Caspase-9 | Caspase-8 | | **Defining Event** | **MOMP & cytochrome c release** | Death receptor clustering & DISC formation | | **Bcl-2 Family Role** | Central (Bax, Bak, Bcl-2, Bcl-xL) | Minimal (only in Type II cells) | | **Speed** | Slower (requires mitochondrial events) | Faster (direct caspase-8 activation) | | **p53 Involvement** | Often involved | Usually not involved | **High-Yield:** MOMP is the **hallmark and obligatory step** of the intrinsic pathway. Without mitochondrial involvement, you are in the extrinsic pathway. **Clinical Pearl:** In Type II apoptotic cells (e.g., hepatocytes, T lymphocytes), extrinsic pathway signals can amplify through the intrinsic pathway via caspase-8 cleavage of Bid → tBid → MOMP. However, the defining feature of the intrinsic pathway remains MOMP itself. **Mnemonic:** **MOMP = Mitochondrial Intrinsic** — remember that mitochondrial outer membrane permeabilization is the signature event of the intrinsic pathway. [cite:Robbins 10e Ch 7]