|MadSci Network: Development|
Thanks for your question, Alicia. Bone growth is actually a somewhat complex process, an overview of which can be found at this site and also here.
The initial pattern of the skeleton(the shape of the ribs and skull and arms and legs) is formed as a cartilage scaffold when the organism is still very small, and doesn't turn into actual bone until after birth, so there's quite a bit of growing of the cartilage that happens first, then the bone cells lay down on top of the already established shape, and continue growing from there. The bones grow not only lengthwise, but also grow in diameter and develop thicker or thinner walls according to the instructions given to the cells that form and shape bone, called osteoclasts and osteoblasts. If the cartilage for the legs becomes curved, as can happen in some genetic disorders or with malnourishment, the bones that are supposed to be straight will become curved, and likewise the ribs can become malformed if the cartilage scaffold is misshapen.
If you want to know how the cartilage that makes up the ribs forms in the shape it does in the first place, that's a more difficult question to answer, because the shape of the cartilage is a result of a very complex process called morphogenesis. Morphogenesis is essentially the process by which an organism develops from a ball of identical cells, into the varied shapes and sizes of cells that make up a complete organism. Initially, the fertilized oocyte simply divides in half, making cells identical to itself, but around the 7th division, the cells begin to form patterns and take on specific roles. One of the roles that some cells take on is the role or skeletal formation, and these cells grow into the shape that we recognize as a skeleton by listening to the protein signals emitted by other cells, which serve a similar function as lights along a dark path, guiding the cells which will eventually form the cartilage scaffold into their proper places.
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