00:00
Again, like cartilage, the most important
functional part of bone is the matrix.
00:02
Now, there are different types of bone. If we
look at this image, this is a section through
the very top part of the femur or the bone
in our thigh that articulates with our hip
joint and even at this gross level you can
see that there is some area of the bone that
is compact on the very outside of the bone.
It is compact, very very hard. But towards
the articular surface, the bone is very spongy.
We call it spongy bone or we can also call
it cancellous bone. And those little bony
components you see within the cancellous bone
or the spongy bone are often referred to as
bony spicules. Bones are also classified as
being long bones, short bones such as in the
wrist, the hand joints, flat bones such as
in the cranium or irregular bones that we
also find in the wrist joint. These are gross
anatomy classifications. Really what we want
to concentrate today is the histological classification,
and that is spongy bone or cancellous bone
and compact bone. If you look very very carefully
at this section of the spongy bone, you can
see that the trabecula or the spicules of
bone are all entitled in certain directions.
The job of these spicules of bone is to impart
the force of gravity and forces generated
by movement down the shaft of the bone, down
the compact part of the bone. And these can
be rearranged. Bone is a very very active
tissue. If you undergo weightlifting program
or go into weightlessness in space, where the
force is changed on the spicules of bone,
then the bone changes the directions and the
components of the spicules and the amount
of the spicules to either withstand the increased
load on the bone or if the load disappears,
these spicules also disappear. But the key
point is bone is a very very active tissue.
Here is a structure of a long bone.
02:45
Again it is the bone in our thigh. It articulates
with our pelvis at the hip joint. And really
there are three or four major gross anatomy
components of the bone. We call the shaft
of the bone, the diaphysis. Towards the articular
end of the bone, we call that part the epiphysis
and the region just between the two in other
words, the end of the shaft of the bone and
the beginning of the epiphysis, we call the
metaphysis. So these are gross anatomical
definitions. But I want you to ask you to
do something during this lecture. I want you
to look at this bone and remember something
forming. I want you to imagine that if you
looked at this bone in real life, some of you
may have looked at bones in the gross anatomy
lab or other occasions. If you look very
closely at this bone, you would see that there
are some very very small holes in the bone.
These very small holes represent where blood
vessels enter the bone. I mentioned earlier
that bone is very vascular. So I want you
to imagine blood vessels coming into the bone
at about the level of the diaphysis.
04:18
And I want you to imagine that these blood vessels
travel up and down the compact bone that I
pointed out earlier, and also travel into the
internal part of the bone, which is hollow.
04:32
We call it the medullary cavity. And I want
you to also imagine blood vessels passing
up the compact bone and supplying the spongy
bone, supplying that enormous surface area of bone
with blood and therefore nutrients. So if you
can imagine that, I am going to ask you to
recall that imagination later on when we talk
about how bone receives its nutrition.
05:04
Well, here is an H&E section taken of bone. It
happens to be decalcified. All the matrix is taken
out and we can see some of the organic components
of the bone. And what I want you to focus on
in this slide, are the images of the periosteum,
the capsule around bone. Cartilage has a capsule
or at least hyalin cartilage and elastic cartilage
has a capsule, but that is called the perichondrium.
05:42
Well the periosteum has an outer fibrous layer,
which is really just dense connective tissue.
05:52
But also lying very very close to the bone
itself, is a very thin osteogenic layer. And
that is a very important layer because that
layer persists during our life. And if we break
a bone, then that layer can be activated
to help repair that break. On the outside
part of both of these slides, both of these
images, you can see skeletal muscle, and that
skeletal muscle is going to insert into the
bone via a tendon. Sometimes when you look
into the bone matrix, you can see the osteocytes
sitting in their lacuna. But also you can
see these lacuna that seem to have dark
pink contents. They are called Sharpey's fibres.
06:47
They are not cells. They are not the osteocytes,
but they are in fact collagen bundles, collagen
fibres that insert into the bone from the
tendon. And of course that is very important because
when the muscle contracts and the force of
contraction is imparted through the tendon
onto the bone by having this very strong cement or
association with the bone, the tendon can then
easily pass on that force of contraction onto
the bone through the Sharpey's fibres and
affect movement. Here is a ligament. In the
middle of the section, we have the periodontal
ligament. This periodontal ligament attaches
the tooth to the bony socket, the alveolar
bone and you can see in part where the collagen
from the periodontal ligament inserts into
the compact bone. Again these are
called Sharpey's fibres.
07:56
Well let us go back to the section of the femur,
the thigh bone. Try and point out yourself
an area where this compact bone and try and
point out to yourself where there is spongy or
cancellous bone. But we really need to understand
that the bone has always cavities inside them
and surfaces and we really need to understand
what covers these surfaces. At the very top,
the articulating surface of bone has articular
cartilage over it or hyalin cartilage, and
this forms the articular joint. I have already
spoken about periosteum, which forms the capsule
around the bone. It ends of course at the
articular cartilage because articulate cartilage
does not have any periosteum around it nor
is it covered by perichondrium. It is a friction
free surface, lubricated by synovial fluid.
Now within the bone, covering all those little
spicules of bone that I described earlier,
is the very very fine fine specialized connective tissue.
09:22
It is called endosteum. And I want you to remember
that these endosteal cells are very important
because again they can be stimulated to lay
down bone, to repair bone, and they also have
a very important function to do with nutrition
of bone that I will talk to you about later
on in this lecture. When you look at the diaphysis
part of the long bone, it is hollow.
10:01
I have explained earlier that it has a medullary
cavity within it. And during very early life,
this medullary cavity and also within all
the spaces between the spicules or trabecular
bone in the spongy area, this area is dominated
by red bone marrow, the haemopoietic organ
that produces all our blood cells. And of course
that will be the subject of a later lecture.
10:32
Once all our bone cells are formed, our bone
marrow begins to start to disappear. And it
only remain in certain parts of bones particularly
in ribs. And when it disappears, as it does
in the case of these long bones, it is replaced
by fatty tissue and we call that yellow bone
marrow. Because it has a yellow appearance,
that fatty appearance.