Macrophage chemotaxis

I updated my macrophage simulation to use my Python particle module, which I describe here. I was pleased how straight forward it was to effectively re-create my previous simulation from scratch using a few simple commands. There are a few changes I'd like to make so creating new simulations will be even quicker in future.

I created a new Cell class, which has a Membrane and a Cytoskeleton, which are also both classes. The membrane consists of a number of particles (which aren't shown) linked in a loop by spring-like interactions representing the membrane. The cytoskeleton object consists of a central particle and membrane-bound Receptor particles (shown as blue circles below). Each receptor particle is joined to the central particle by an interaction that represents a microtubule.

Simulated macrophage extending towards a chemoattractant.

I also created a Chemoattractant class, which inherits from the Particle object (and is shown as a red particle above). Although chemoattractants objects don't interact with any other particles, it can be dragged around (which is one of the difficulties I had in using my particle module). Receptor objects are activated at a rate inversely proportional to their distance from chemoattractant. The more active a receptor is, the brighter blue they become.

Microtubule length decreases at a fixed rate and increases at a rate dependent on how active the attached receptor is. Microtubule growth also depends on the amount of free tubulin in the cell. The amount of free tubulin decreases as microtubules grow and increases as they shrink. As a result, microtubules are in equilibrium and the total length of microtubules in the cell is approximately constant; if one microtubule grows, others will shrink. This means that the cell elongates towards the chemoattractant, though the cytoskeleton centre tends to stay in the same place.

Future plans

  • In order to get the cell to actually move towards the chemoattractant, I think it will be necessary to create new cytoskeleton centres, probably along the length of the longest microtubule.
  • This will also require merging other centres to prevent a endless increase in particles. Similarly, membrane particles need to be created where the membrane is stretched and removed where it is compressed. I'm not quite sure how new receptors will be created or what will happen to microtubules if receptors are removed.
  • The current cell doesn't look much like a macrophage and forms a bulge rather than extending arms. I think I could avoid this by keeping the volume (or area, as this is a 2D simulation) of the cell constant. I could do this by calculating the area of triangles created by the membrane particles and the cytoskeleton centre, but I think I might create a network of triangles independent of the cytoskeleton.
  • I'd like to see if I can get the macrophage to actually engulf something. This will require adding receptors that can bind on the membrane, but also making the membrane solid; currently objects pass through it quite easily. Also, once the chemoattractant is inside the cell, the microtubules keep growing - they should stop once receptor measure a decreasing concentration.

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