The mineralization of bone matrix is rather a
complex process. I have just listed here a
few of the events that happen and probably the
critical event is the first one. For bone
to be mineralized, for the osteoblast to release
matrix vesicles that they developed inside them, there
has to be concentrations of calcium and phosphate
far beyond what is normal in other tissues.
So initially these osteoblasts secrete osteocalcin
and a few other different sorts of proteins
and molecules. And that binds calcium in their
immediate area and by binding extracellular calcium
into that matrix area, the concentration increases.
Osteoblasts sense that and they secrete
alkaline phosphatase and that increases
the concentration of phosphate, which itself
starts a big revolving wheel increasing calcium.
So the process goes on and on and on until
finally those calcium and phosphate levels
far exceed threshold and then the osteoblast
then release their matrix vesicles by exocytosis
and then crystals form. Crystals of hydroxyapatite
crystals and the mineralization begins and
these crystals then disappear and form right
throughout the matrix of the bone.
Far more complicated in that but that
to summarizes some of the main events.
I briefly just want to point out the structure
of a synovial joint. We can return to our
previous section of this growing bone. I pointed
out the two articular surfaces. They are supported
on all the way around the joint by a very
strong ligament. You can see that ligament
in this section. It is the red stained component
at the bottom of the slide and also on the
top of the image. It is collagen, very dense collagen.
The joint cavity is that space between the
two articular surfaces. It is full of synovial
fluid that is secreted by the synovial organ,
very special part of the joint cavity with
a very special epithelial cell that secretes
that synovial fluid and that synovial fluid
gives nutrition to the articular cartilage
as I described in a lecture in this course
on cartilage and it also acts as a lubricant
for the joint. It is a diarthrotic joint.
There are different sorts of joints in the
body that the anatomists would describe to you.
This is a very simple joint that most of us
are familiar with, perhaps the
knee joint or the hip joint.
And finally I just want to mention that bone
is a mineral reservoir. Two hormones secreted
by the parathyroid and also the thyroid control
calcium levels. The parathyroid hormone as
is stated there stimulates osteocytes and
osteoclasts to resorb bone to take up bone
when there are low calcium levels in
the blood. And conversely calcitonin secreted
by the thyroid gland, then inhibits the action
and so there is no more calcium being taken up
from the bone and therefore calcium levels
can be monitored and controlled to be normal.
These are very, very very important hormones and
again I have discussed these hormones in another
lecture in this course on the endocrine system.
So, let me now summarize what I have tried
to describe in this lecture. The list here
is similar to the list I put up at the very start.
I want you to be able to know and understand
the way in which bone forms, from that basic
cartilage model or bone can form purely by
mesenchymal cells differentiating to osteoprogenitor
cells and then becoming osteoblasts and laying
bone down in this membranous component such
as we see when bones of the skull are made.
There is no cartilage template. The cartilage
model that is formed in certain parts of the
body corresponds to regions where long bone
and other bones are going to develop. It is
a part of a skeleton. Part of the skeleton
that is involved with movement and the bones have
muscles attached to them. Those bones go through
endochondral ossification, a process
where the bone is formed from a cartilage model.
But cartilage undergoes waves of proliferation
and then finally they hypertrophy, die, the
matrix becomes calcified, bone is laid down
on that matrix and resolved or retained or
remodeled depending on where the bone is.
It could be spongy bone or in the medullary
cavity. The bone on the diaphysis is produced
by periosteum and I guess it is the form of
intramembranous ossification because that
periosteum being a membrane just lays down
the bony collar, which is going to be compact
bone that forms a diaphysis of the bone. And
I think it is important to remember that the
primary ossification center starts before
birth. It starts to lay down the diaphysis
and the medullary cavity that creates the
zone that is going to develop into the epiphysial
plate that is going to be responsible for
the elongation of the bone after birth until
puberty. And then there is that secondary
ossification center remember that is formed,
that really is just modeling the epiphysial
head of the bone, and mainly it is going to
be spongy bone as you see in the diagram to
the right. And then I've briefly mentioned the
importance of bone in maintaining calcium
levels, the action of parathyroid hormone
and thyroid hormone. And then finally I've briefly
described the structure of a
simple synovial joint.
So thank you very much for listening to this
lecture. I hope you now have some understanding
about the way in which bone forms and the
way in which bone grows.