The Organs of our Body changes its Size: A Mystery

The smallest fish in the world till date is the Paedocypris which measures only 7 millimeters in length. This is nothing compared to the 9 meters of the whale shark. The small fish have many of the same genes and the same anatomy with the shark, but the dorsal and caudal fins, gills, stomach and heart, are thousands of times smaller! How do organs and tissues of this small fish stop growing very quickly?

 A multidisciplinary team led by scientists from the University of Geneva (UNIGE), Switzerland, and the Max Planck Institute for the Physics of Complex Systems (MPIPKS), Germany, was able to answer this fundamental question by studying its physics and using mathematical equations. Cells of a developing tissue proliferate and organize themselves under the action of signaling molecules, the morphogens. The research was done among the fruit fly Drosophila. In Drosophila, the morphogen Decapentaplegic (DPP), diffuses from a localized source within the developing tissue and then forms decreasing concentration gradients  as it moves away from the source. 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.