5.5.2 Morphogenetic Furrow
1. What Is It?
When a fly's retina develops a linear wave passes through the cells from left to right and in its wake it crystallizing clusters of cells which give rise to the photoreceptors of the eye. This linear wave that passes through the eye is called the morphogenetic furrow. My purpose here is not to reproduce the exact genetic system that causes this like was done for the achaete scute genes. The main reason for this is that I still do not know enough about how that system works. In fact, I am not sure anyone does. However, I have read about it happening and I wanted to see if I could come up with a simple system that would work in this emulator that would produce similar behavior. So what is needed is a group of cells that has a directional, one way wave of differentiation pass through them. Formation of the wave should be able to be induced by its neighboring cells and once it has passed through a group of cells it should not be able to go back through those same cells. Sounds complicated. But as you will see this behavior is very easy to reproduce. So easy in fact that it was not necessary to use a genetic algorithm to do this. The following chromosome was engineered by hand.
2. Example Chromosome
When this simulation starts out each cell is producing a quantity of the receptor Morphogen Producer (Mpd). None of the other proteins are being actively produced, but their genes are on. The Morphogen Ligand (Mlg) is then injected into the cells on the left hand side of the cluster. In a more realistic scenario the cells would be divided into three sections. The middle section would be the one that has the morphogenetic furrow genes turned on. The last section would be stop the progress of the furrow, and the first section would induce the produce of Mlg in the boundary cells using another receptor and ligand combination. Once Mlg is introduced into the cells it initiates the furrow by producing transcription TF_0. TF_0 up regulates G_Mlg and G_Mct. So TF_0 is a positive feedback loop that causes the production of more Mlg. Since Mlg is a diffusible ligand it begins spreading to its neighbor cells and causes up regulation in them as well. It also causes the production of the genetic control Mct. As Mct builds up it eventually exceeds its threshold and switches off the G_Mpd gene. This causes the production of the receptor to stop and prevents the further production of TF_0. Also, since G_Mpd is now off this means that the furrow is one way. As it moves along and switches off the receptors it can no longer go back the other direction. If this gene was not being switched off then since Mgl is diffusible it would effect the cells behind it as well as those in front of it. Eventually TF_0, Mgl and Mct fade away after the wave has passed. Video 1 demonstrates all of these things as they occur.
This experiment is done solely to demonstrate some of the power of this system. While it does produce something that functions similar to the morphogenetic furrow in drosophila it is not an attempt to truly model that system. The important point right now is to show that it can do something of similar complexity. Once I get some more free time I would definitely like to read up more about what is currently known of the mechanism that causes the furrow to form, and see if it would be possible to attempt to model it in the same manner as the achaete scute system was modeled.