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Easy Way to Develop Limbs; An Example of Morphogenesis
Topic Started: Jan 25 2011, 03:15 PM (257 Views)
Rhob
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Here's an "easy" way to develop limbs and other appendages.

The starting point is something called lateral inhibition. This is where a genetic marker is activated in a cell, which then suppresses its activation in surrounding cells. Lateral inhibition is an extremely common mechanism in life on Earth.

A developing organism, starting out as a ball of cells, has a gene which causes lateral inhibition in cells on its outer surface. The gene is first turned on in a cell essentially "chosen" at random. It's subsequently activated in cells that are beyond a certain distance away from this first cell and from each other.

Another ingredient is differential growth. As the name suggests, this process involves some parts of the developing organism growing more/faster than others. What this can do for us is let the areas that aren't laterally inhibited undergo positive differential growth, giving rise to projections on the surface of the organism.

Polarity is a third ingredient. This involves a gradient of a gene product going from one end of the organism to the other. Bilaterally symmetrical organisms arise from the interaction of at least two polarities: one running from top to bottom (dorso-ventral polarity) and another running from front to back (antero-posterior polarity). With only one polarity, some kind of radial symmetry will develop.

Polarity is useful for keeping the projections confined to a certain area along or around the organism. The number of projections is then controlled by how wide the lateral inhibition reaches and how large the organism is when it's first activated. At that point, a framework is in place for true limbs to develop. Basically all that's left is more growth and development of muscles and skeletal elements.

Hopefully this will help others in realistically creating new organisms. Questions and comments welcome!
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