MadSci Network: General Biology

Re: How does the growth plate (cartilage and calcium) make the 'bone' grow?

Date: Wed Aug 1 11:19:56 2007
Posted By: William Gunn, Grad student, The Center for Gene Therapy, Tulane University
Area of science: General Biology
ID: 1183899912.Gb

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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