Biological complexity: size control in embryology (Introduction)

by David Turell , Wednesday, December 22, 2021, 17:38 (1067 days ago) @ David Turell

All forms use the same genes, so why tiny and big sizes? Controls found:

https://phys.org/news/2021-12-how-do-our-organs-know.html

"Cells of a developing tissue proliferate and organize themselves under the action of signaling molecules, the morphogens. But how do they know what size is appropriate for the living organism to which they belong?

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"In previous studies, Marcos Gonzalez-Gaitan's group, in collaboration with the German team, has shown that these concentration gradients of DPP extend over a larger or smaller area depending on the size of the developing tissue. Thus, the smaller a tissue, the smaller the spread of the DPP gradient from its diffusion source. On the other hand, the larger a tissue, the larger the spread of the DPP morphogen gradient. However, the question remained as to how this concentration gradient scales to the growing size of the future tissue/organ.

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"The scientists collected all this data on DPP in cells belonging to tissues of different sizes in normal flies and in mutants that failed to scale. They found that it is these different individual transport steps that define the extent of the gradient. Thus, in a small tissue, the DPP molecule is mainly spread by diffusion in between cells. Its concentration therefore falls quite rapidly around its source because of degradation, yielding a narrow gradient. On the other hand, in larger tissues, DPP molecules that went inside cells are also highly recycled, making it possible to extend the gradient over a larger area. "We were finally able to propose an unbiased, unified theory of morphogen transport, going down to the key equations of the system and to unravel the mechanism of scaling!" Maria Romanova enthuses."

Comment: what this shows is specific cells of an organ modify DPP to their own purposes with there built in controls in their specifically active DNA genes. This is obviously an irreducibly complex system requiring design all at once to function. It cannot be built stepwise.