Making things fit together, for a century.

kw: musings, evolution, development

I've been thinking more about The Plausibility of Life by Kirschner and Gerhart. Vertebrate limb development amazes me. There are bones, muscles, tendons, blood vessels, nerves, lymph vessels, and skin. At least these seven systems, maybe more. Whatever size you are, they all fit together (nearly always).

Consider two alternatives (there are likely more). Firstly, that there is some collection of genes that control the development of each of these seven systems separately, seven sets of "this is how big to get" genes. Secondly, that one of the seven has its "how big to get" genes, and the other six each have a set of "how to fit yourself in" genes.

In the first case, suppose a mutation happens, and a child is born with the instructions, "make the forearm muscles 10% longer"? That arm would be just useless. Suppose instead, it were, "make the humerus bone 20% longer"? All the other tissues would either fall short or tear apart during growth of the larger bone.

The second case clearly is better, but which is the master? The experience of some teens provides a folklore clue: growing pains. Many youngsters get pains in their legs during a growth spurt. They are muscle pains, and the muscles are stretched and sore for a number of days until they catch up.

As a matter of fact, the bones are the master. Only they have a set of "how long to get" genes, while other tissues may have "how wide" genes, but otherwise are programmed (genetically) to respond to chemical signals that inform the cells they need to grow faster or to slow down.

Stretch a muscle many times, and it will get longer. Longer muscles need more blood and lymph flow. Lack of Oxygen causes blood vessels to sprout more capillaries, and increased flow in small vessels causes them to enlarge. Similarly, Oxygen saturation beyond a certain point causes some capillaries to degenerate and be re-absorbed. This is the reason you can condition yourself with exercise, but lose condition rapidly when you slack off. Similar signals promote growth of nerves, lymphatics, and so on.

Amazing. Fewer control mechanisms means fewer genes are needed to produce the limb, and mutations that affect limb size affect it in a way that the limb will still be proportional. Not only so, specialized cells are constantly remodeling bones, responding to any persistent changes in the stresses they endure, and all tissues are renewed and remodeled constantly...and they continue to fit.