So how does the bone grow?
Having in a fetus have this
primary ossification center, initiating bone
growth or bone formation, how does that template
that cartilage then grow in length and get
to the length of the bone that we see in the
mature adult? What happens, just at the beginning
of birth or at birth, you get the invasion
or the same process occurring up at the epiphysis
end of the bone or of the developing bone.
Here you see a section. You see the epiphysial
cartilage that represents that cartilage component
in the previous sections I have showed you
whereby there was this hypertrophied zone of cartilage
cells and there was absorption away from that
down the diaphysis of the bone creating that
medullary cavity. By the time, birth has occurred
and all the way through now until adulthood,
two things happen. You get a secondary ossification
center. You can see that to the right of the
epiphysial cartilage. That secondary ossification
center develops exactly the same way, suddenly
the chondrocytes hypertrophy. They die.
They become calcified. The matrix is calcified
and bone is laid down and then resolved to
some significant degree. This area, this secondary
ossification center will develop mostly into
spongy bone. Remember that spongy bone we
saw on the previous section through the femur
I showed you towards the start of this lecture.
One difference of the secondary ossification
center though is that there is no periosteum
on the external sides. It remains cartilage
because that part of the cartilage and the
more distal part of that cartilage is going
to be the articular surface of this particular
bone. You can see in the far right of this
slide another articular surface of articular
cartilage. And then what happens is that the
bone is going to elongate and widen, but mainly
elongate. And it elongates because that epiphysial
cartilage component starts to undergo the
growth where cartilage cells go through a
similar transition through what they did before
and bone is being laid down. So just look
at those details a little bit more.
This summarizes what I described in the previous
slide. Down the bottom the question is what
reminds is its epithelial cartilage template
that is going to be the structure that allows
the bone to elongate. And when you look this
emphysial plate and you see the processes
different zone, on the far right-hand side
is cartilage. You can see the cells there
are just typical cartilage that results on the
reserve cartilage. Then you can see where
there's higher concentration of chondrocytes.
They have divided. They are proliferating and
I call those two zones the reserve zone and the
proliferation zone, the running away zone
and I will describe why I name that in a moment.
Then you've got this zone of hypertrophy of cartilage,
the calcification of the matrix and then resorption bone
is laid down and then resorbed as I described previously.
Now that running away zone that I
mentioned is because when the cartilage
matrix is calcified and bone is deposited
on it and then resorbed to create the diaphysis,
the cavity, those two zones reserves in the
proliferation run away from that calcified
zone. They keep dividing and moving away, getting
away from that calcified area consequent in
the bone grows in length or the developing
bone grows in length. Same thing happens
on the other surface, the secondary ossification
center. Cells are continually dividing to
run away from the zone of calcified matrix.
So the bone elongates. It also thickens, gets wider.
But bone being laid down form the
periosteal collar and then eat their way
inside to create a large medullary cavity.
The secondary ossification center that I described
earlier is mainly occurring not to lengthen
the bone, that is done by the epiphysial plate
region of proximal to the medullary cavity.
The secondary ossification center is mainly
to do with the moulding of the shape of the
epiphysial head of the bone that I have described
again in the start of this lecture and the
spongy bone that I showed you. And this shows
you some of the processes, little spicules
of spongy bone is formed on the cartilage,
calcified spicules and is continually removed
or remodeled. The bone is red staining here.
The cartilage is of very pale stain. You see
it here in more detail and it just summarize
on the left-hand side some of the processes
that I've just described as a review for you.
Later on, at the end of puberty, suddenly all
those reserve cartilage cells will suddenly
start to disappear. And there will be fusion,
there will be a closure of this epiphysial
plate and there's just a small evidence of
the little line going through there in the adult
that you can sometimes see. It is called the
epiphysial closure. No more growth of the
bone occurs. That is all done by this process.