We have already discussed how graded morphogens can give polarity to an embryo (e.g. Bicoid) or to a tissue (e.g. Shh and TGFbs). This covers much of developmental patterning. In this section I will summarize some other types of patterns that we see in development, both repeated patterns, like spots and stripes, as well as tissue shape and size patterning (called morphometrics). To start out, I will cover a simple mathematical model that has been used to help explain stripe and spot patterns that we see in many tissues. But it's important to keep in mind that 1) this is only a model and does not approach the true complexity of actual biological systems and 2) it does not explain all stripe and spot patterns - for example the Drosophila segmentation pattern, which is set up by graded morphogens.
- 7.1: Turing Patterns to Generate Stripes and Spots
- The Turing "reaction-diffusion" model uses a two-protein system to generate a pattern of regularly-spaced spots, that can be converted to stripes with a third external force. In this model, there is one activating protein that activates both itself and an inhibitory protein, that only inhibits the activator. By itself, transient expression of the activating protein would only produce a pattern of "both proteins off" or "spot of inhibitor on".
- 7.2: A Turing-like Model for Generating Stripes in Digit Development (Rivera and Ramirez)
- If you think about the growth of your limbs, you imagine a tiny tissue with a little bone (that will become the humerus or femur) growing right in the middle. This bone is specified early on by Sox9, a transcription factor also involved in sex-determination. As the tissue grows longer and wider, two parallel bones appear (the radius and ulna or the tibia and fibula). The tissue grows longer and wider still, and five parallel bones appear - the metacarpals and eventually phalanges.
- 7.3: Lateral Inhibition in Nervous System Patterning
- Drosophila neuroblast formation differs in one very important way from a traditional Turing pattern - each neuroblast arises in isolation from other neuroblasts. This patterning is not over an entire tissue, but is super local, occuring only over a 6-7 cell cluster with only a single cell becoming a neuron.
- 7.4: Size and Shape
- The final type of developmental patterning that evolution can act on is the size and shape of tissues or organs. These are generally considered "morphometric" scaling issues and are classified as "allometric" changes. Morphometrics is the study of how a continuous geometry (like the outer surface of a body) can be warped. Allometry studies this in the context of evolution and development.
Thumbnail: An example of a natural Turing pattern on a giant pufferfish. (CC BY-SA 3.0).