|MadSci Network: General Biology|
Thanks for your question, Kristina. Your sports teacher has the sequence of events correct, but it's not calcium that causes the bone to grow. There are two processes that are responsible for the hardening of bone during development. They're called endochondral and intramembranous ossification. Endochondral ossification is the process whereby your long marrow-containing bones, such as the bones in your leg or arm, become rigid. Intramembranous ossification is the process whereby the flat bones in your skull become rigid. Endochondral ossification starts in a 6-7 week old embryo with the growth of blood vessels into the cartilaginous skeleton. The blood system carries proteins and hormones that tells the bone forming cells called osteoblasts to surround the cartilage and begin to make bone. At the same time, bone removing cells called osteoclasts begin to eat away the cartilage so that the osteoblasts can replace it with bone. Proteins released by the action of the osteoclasts on cartilage also stimulate the growth of bone. By the time you're born, this has mostly happened, and there's just a little spot in the ends of the bone called the growth plate that still contains cartilage making cells called chondrocytes. This initial bone, called spongy bone, becomes rigid when chondrocytes and osteoblasts change the pH of the surrounding area, causing calcium salts to precipitate on the spongy bone. The spongy bone is removed by osteoclasts from the inside out, and osteoblasts on the outside form a hard coating over the spongy bone called compact bone. Once you're born, the way bones lengthen is by the chondrocytes at the growth plate dividing, so your bones actually grow from the ends, not from the middle. As the chondrocytes divide, the bones lengthen, and the cartilage is replaced with bone as in endochondral ossification. You can read more about the particular proteins involved at Pubmed.
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