What is Degranulation?
Watch Out For My Granules!
Each mast cell holds hundreds of little granules that contain chemicals. In normal circumstances, mast cells release these chemicals when they perceive that the body is under attack from invading germs, viruses or parasites. These chemicals have a toxic effect on the invader and usually kill them, which is really what we want.
However, these chemicals also have a moderate negative impact on healthy tissues. So, although mast cells are good for us as they protect us from foreign invasion, there is a bit of collateral damage when mast cells perform their normal, everyday duty. Nothing that we can't tolerate, but some damage nevertheless.
The release of the chemical granules by the mast cell when it receives a signal that we are invaded by nasties is called ‘degranulation'.
Too Much Of A Good Thing
Now, in mastocytosis, we have a mast cell “population explosion”. Due to a genetic defect, our body keeps creating mast cells and disregards the signal to stop when enough mast cells have been produced. So, we have an overabundance of these critters. And since mast cells are very difficult to kill, that overabundance increases day by day.
Mast cells are good, in normal circumstances. But you know what they say about too much of a good thing …
If a normal amount of mast cell causes a certain level of collateral damage on our tissues and nerves, imagine what happens when we have an oversupply of mast cells degranulating.
OK, then. Watch this short video and see an animation of a mast cell degranulating.
Mast cells have like little antennas on their surface, called receptors. These receptors are waiting for a signal that tells the mast cell that our bodies are under attack. You can see the Y-shaped receptors (in blue) on the cell membrane (the thin envelope which separates the inside of the cell from the outside world) in light green.
These Y-shaped receptors, which in this case are called antibodies, tunnel through the cell membrane into the cytoplasm (the cell innards) and and are able to transmit signals from the outside of the cell to the inside of the cell. They are like the mast cell ‘detonators'.
Through the transparent cell membrane, you can see the nucleus of the cell (in purple) and the chemical granules (mediators) in red. In the video, these granules contain histamine. But be aware that the cell contains around 200 other chemical mediators which are also dispersed on activation of the mast cell receptors. The other more frequently quoted mediators are tryptase, heparin, prostaglandin D2, serotonin, leukotrienes and proteinases … loads of heavy duty, sometimes toxic material to assist in the killing of the invader.
- at 00:01 here comes the antigen (the bad guy with bad intentions, in bright green)
- at 00:12 the antigen binds with the antibody and the antibody signals to the cell that it's under attack and it is time to degranulate
- at 00:15 degranulation starts
- at 00:17 the cell membrane disintegrates, releasing the payload of mediators
- at 00:20 the cell has dispersed the mediators, the cytoplasm debris and the nucleus will be eaten up by garbage collectors later
Also, while the video depicts a total degranulation, mast cells can also selectively secrete some biochemical mediators and retain others within the cell. And most importantly, in the case of mastocytosis, the mast cell often degranulates spontaneously without being ‘detonated' by an antigen.
Here's a video of a real mast cell degranulating.
If you read the uploader's comments on Youtube, you'll find their answer the following question about mast cell death after degranulation:
Mast cells did not apoptosis after degranulation. Cells were alive and restored the granules again. We have observed these phenomena before.