Cellular Mechanisms of Damage
Learning Objectives
Students will be able to:
- Describe how particles deposit in different regions of the respiratory tract
- Explain mechanisms by which cells take up particles (phagocytosis, endocytosis)
- Analyze how particle properties (size, composition, surface area) affect toxicity
- Distinguish between cell death pathways (apoptosis vs. necrosis)
- Explain the concept of particle translocation beyond the lung
The Big Question
"Once particles deposit in the lungs, what happens to them at the cellular level? How do cells respond to these foreign invaders?"
Particle Deposition in the Respiratory Tract
The fate of inhaled particles depends on their size and where they deposit in the respiratory system:
| Region | Particle Size | Defense Mechanism | Clearance Time |
|---|---|---|---|
| Nasal/Pharynx | >10 um | Mucociliary clearance, sneezing | Minutes to hours |
| Tracheobronchial | 2-10 um | Mucociliary escalator | Hours to days |
| Alveolar | 0.1-2 um | Macrophage phagocytosis | Days to years |
| Deep alveolar | <0.1 um | Translocation possible | May be permanent |
Cellular Uptake Mechanisms
Phagocytosis
- Primary mechanism for particles >0.5 um
- Performed by alveolar macrophages
- Cell membrane engulfs particle
- Particle enclosed in phagosome
- Fused with lysosome for degradation
Endocytosis
- For ultrafine particles (<100 nm)
- Multiple pathways available
- Can occur in epithelial cells
- May allow particle translocation
- Caveolae-mediated, clathrin-mediated
The Surface Area Paradigm
Particle toxicity often correlates better with surface area than mass:
Why Surface Area Matters
- Surface reactions: Toxic chemicals on particle surfaces contact cellular components
- ROS generation: Surface reactions produce reactive oxygen species
- Ultrafine particles: Same mass of smaller particles has much greater surface area
Example: 1 ug of 100 nm particles has ~60x more surface area than 1 ug of 1 um particles
Cell Death Pathways
Apoptosis (Programmed Cell Death)
- Controlled, orderly process
- Cell shrinkage, DNA fragmentation
- No inflammation triggered
- Cellular contents contained
- Cleared by neighboring cells
Necrosis (Uncontrolled Cell Death)
- Catastrophic membrane failure
- Cell swelling and lysis
- Contents spill into tissue
- Triggers inflammation
- Damages neighboring cells
High doses of toxic particles may overwhelm cellular defenses, shifting from controlled apoptosis to inflammatory necrosis.
Particle Translocation
Beyond the Lung
Research has demonstrated that ultrafine particles can translocate from the lung to other organs:
- Blood circulation: UFPs cross the alveolar-capillary barrier
- Lymphatic system: Particles accumulate in lymph nodes
- Olfactory pathway: Nanoparticles can travel along olfactory nerves to brain
- Target organs: Liver, spleen, heart, brain have all shown particle accumulation
Reference: Oberdorster et al. (2005). Nanotoxicology: An emerging discipline. Environmental Health Perspectives.
Activity: Mechanism Mapping
Create a Cellular Response Diagram
- Diagram components: Create a visual showing a particle depositing on an alveolar surface
-
Include pathways:
- Macrophage phagocytosis pathway
- Epithelial cell endocytosis pathway
- Possible translocation route
- Label cellular responses: Identify where ROS generation, membrane damage, and signaling cascades occur
- Compare outcomes: Show how small vs. large particles might follow different pathways
Key Takeaway
When particles deposit in the lungs, cells respond through specific uptake and defense mechanisms. The particle's size, composition, and surface properties determine which pathways dominate and whether cells can successfully clear the particles or sustain damage. Understanding these cellular mechanisms is essential for predicting health effects and developing protective strategies. In the next lesson, we will explore how cellular damage triggers inflammatory cascades that amplify the body's response.