As we age, we grow taller. Logically, our bones, muscles, skin and pretty much everything else must get bigger.
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[li]How does this happen?[/li][li]Presumably, all the internal organs grow proportionally? [/li][li]How does, say, the bone in my left leg know to grow at the same pace as my right leg? [/li][li]Does skin stretch? [/li][li]What determines when you stop growing? [/li][li]Is there anything an individual can do to change the pace and amount of growth?[/li][/ul]
This is one of the biggest questions in developmental biology, and not all the factors are understood. What is becoming clear is that there a number of complex factors involved:
Genetic Control: DNA controls what a cell does. However, the expression of that DNA is regulated by additional factors.
Growth factors: these are hormones that trigger stem cell differentiation
Nearest Neighbours: Cells differentiate differently based on the cells that surround them
Environment: differentiated cells develop differently based on environmental factors - stem cells in a scaffold placed in a heart develop differently to those in a static environment.
In general, organs modify themselves based on environment - an adult kidney transplanted into a child will shrink to an appropriate size, then grow as the body grows. A childs kidney implanted into an adult will increase in size over time as it adapts to the load. Bone/frame growth is regulated by a combination of genetic and environmental factors (if hormone regulation is normal) - malnutrition (for example) can easily override genetic size. Because the body is generally in equilibrium, the same factors have the same influence on symmetric development - legs and arms develop at the same rate. But not always - I have one leg a few cm shorter than another - I get asked if I am injured all the time, because I limp slightly <shrug>. Disease and misuse can easily impact on symmetrical development.
Skin stretches and produces more collagen over time, but rapid stretching can produce stretchmarks - areas of scarring where insufficient skin cells have formed during rapid expansion. Similarly with shrinkage - there can be some shrinkage with time, but rapid shrinkage (as with severe weight loss) can leave excess skin. There is a theory that humans rapid scar formation actually interferes with proper cellular level repair by blocking stem cells from the injury site. Drugs/therapies that gently remove scar tissue and allow cellular regeneration are now being investigated, particularly in the case of nerve damage. But I would not hope for limb regeneration for many, many years - if ever.
Si
Skin will grow (i.e. the cells will multiply) in response to constant stretching. ISTR surgeons implanting balloons under the skin of patients to cause the skin to grow; the intent is to produce extra skin they can harvest for some major, non-urgent surgical procedure they plan to do on that same patient in the near future.