Hoku said:
Thanks for the in depth info, bobze.
I'm actually trying to understand this from an evolutionary perspective. We all know that evolution is caused by genetic mutations. I'm trying to understand the evolutionary relationship between specific bones and the skeletal system as a whole. I would think that each bone has its own gene that can mutate independantly from other bones. I think evidenced for this can be found in whales, as an example. Their vestigial hip bones have evolved useless, even if they DID have legs. But their other bones, that are still useful for them, appropriately evolved with the whale.
But there are commonalities among the entire skeletal system, like density, which varies between species. So my question is, if each bone has its own gene that can mutate independantly, are those individual genes also responsible for their bone density? I'm thinking there must be another gene in the body that is singularly responsible for characteristics like bone density, shared by the entire skeletal system.
Any further insights?
No there isn't single genes which control bones. There is as I pointed out, fields. It is a very, very complex subject
and there isn't really a short answer aside from lots of reading.
As I said before, bones in the embryo are created because of local environmental rules setup by the genes--Not specific "blue print" genes (which unfortunately lots of high school science teachers seem to giving people the impression that genes are really "blue prints").
Here's something really important about evolution. Most genes are pretty conserved, in that the gene codes for a similar protein, across taxa. What differs (and often drastically) and leads to those changes of bauplan you're wondering about is in the
timing and expression of those genes-Specifically during embryological development.
Think about it like this (this is an overly simplified example). Suppose, You and I were two cells on the end of an arm bud in the developing embryo. We "know" our position because of
fields of growth factors (think concentration gradients). Depending on the amount of receptors activated by these fields, then our gene expression will be differential.
Suppose on this arm bud (a 3d "pyramid") you were in a internal proximal, medial spot (inside the pyramid, toward the central line at the bottom). While I was at the "tip" of the pyramid.
Because of the different microgradients each of senses from our local environment, were going to activate different genes.
The gradient I sense leads to a cascade that says "divide and make both your progeny pluripotent stem cells)--Me being on the tip, growth needs to extend the arm.
While the field
you sense activates a cascade which turns on genes that say "Divide and make one of your progeny a pluripotent stem cell and the other a mesenchymal stem cell (one capable of producing chondrocytes--which make cartilage).
By
sensing the local environments around them and
altering gene expression based on the local environment we can go from a single cell, to a person (or whale, or elephant).
) is Endochondral bone formation. The body uses "models" of bone made of hyaline cartilage. As the fetus nears birth, differential gene expression causes recruitment and differentiation of osteoblasts, which lay down a bone collar and form a primary center of ossification (in the diaphesis of the bone). Throughout the life of the organism, continued bone deposition and resorption is done--Called bone remodeling. This serves two purposes, firstly it aligns the collagen (type 2) of the bone in anti-parallel and perpendicular fashions (like plywood) which strengthens the bone. It also allows local regulatory responses to load to add more strength to bones that "need" it. 
