00:01
Well I mentioned earlier that the osteon is the
functional and structural unit of the compact
bone. So let us have a look at what the structure
of the osteon is. Here again is a section
cut through decalcified bone. All the mineral
contents have been removed and all that is
left is the organic content. I mentioned earlier
on, that these little osteocytes or bone cells
live in the matrix and they are often sitting
in these lacunar spaces that are really filled
with the cell itself. The lacunar spaces are really
artifact. They represent where the cell has
been lost during the processing of the tissue
for you to view using a microscope.
01:02
Well, there are some very large circular structures you see
also within the compact bone and these are referred
to as the osteonal canal or often Haversian
canals. Now these are very very important
because they are actually channels running
through the compact bone, running up and down
the compact bone that carry blood vessels
in them. Remember I asked you to recall or
imagine blood vessels entering into the diaphysis
of bone and running up and down that compact
bone. Well they run up and down through these osteoneocanals.
I referred it was being Haversian canals earlier
named after Havers, the person who first described
these canals. We tend now to refer to them
as osteonal canals, which is why I use the
label here and not the Haversian canal or
Haversian system. Now besides blood vessels running
up and down these canals, there are also very
fine nerves and lymphatic tissue. Importantly,
these Haversian canals or osteonal canals
are lined by endosteum. It is very difficult
to see sometimes because the endosteum cells
tend to be lost during processing. What you
also see, if you look very very carefully is
that the osteocytes seem to be arranged around
each of these osteonal canals in shades or
lamellae. We call these lamellae. We call
them concentric lamellae because they appear
to be in a circular pattern around the osteonal
canal. And this represents the why and which
bone was formed earlier on. And I will explain
that in a later lecture. It is hard to see
these concentric lamellae because just of
the nature of the staining in an H&E section,
but you will see an image of these using a ground
section in a moment when they are lot clearer.
03:26
The important point to understand is that
if you have a look at both the osteonal canal
shown here, some of the osteocytes sitting
in the lamellae are a long long way away from the
blood vessel, embedded in calcified matrix.
So one of the things we really need to understand
is how these bone cells are going to get their
nutrition. Or they get the nutrition because
you cannot see them here I'll show you an image
later on. They get their nutrition because
of the little fine canals running from the
osteonal canal towards the osteocytes, little
tiny canals called canaliculi. Now sometimes
if you look at an image such as the one I
have shown you here, you can see there are
paler regions of the matrix, where the lamellae
do not seem to be the nice concentric pattern
around the osteonal canal. These are called
interstitial lamellae. And I have not labelled
them here yet because I will talk more about
them when we talk about bone growth in a
later lecture. But it does indicate to you
that bone again is a very active tissue because
bone is constantly remodeling itself. And these
interstitial lamellae represent areas that
were once Haversian canals and concentric
lamellae, but have been overgrown by further
development of bone tissue.
05:10
Now, here is a lovely slide. It is a wonderful
slide because it is of a ground section of
bone. A ground section means that the bone
was ground up, filed down to be a very very thin section
and it illuminated in light and viewed with a
light microscope. So the light refracts through
the ground section and it gives you a lot
more detail than you saw in the decalcified
section. The three large clearer circles are
again the osteonal canal or the Haversian
canal. You can see clearly now, concentric
lamellae, sheaths of cells wrapping around
these canals. And you can see the osteocytes
as very tiny dark structures. They are living
there. They have been removed, but the light
is refracted differently here and so they
appear as little dark shadows. And more importantly,
if you look very closely, you can see these
fine canaliculus, running through the matrix.
Sometimes the matrix appears to be rather
uneven. The lamellae seem to get lost and
that is because they're interstitial lamellae,
again evidence that bone has been
remodeled in this region.
06:50
Here is a higher magnification taken of an
osteonal canal. And it really defines what the
osteon is. The osteon consists of the osteonal canal,
which you see in the center of the slide where
just towards the bottom left corner of the
image that osteonal canal remember, is lined
by endosteal cells, squamous epithelial cells.
That osteonal canal also carries with it blood
vessels, blood capillaries and the lamellae,
you can see one, two, three circular sheaths
or lamellae of cells around the osteonal
canal. That defines an osteon.
07:48
The Haversion canal or the osteonal canal and subsequent
layers or sheaths of concentric lamellae and
their osteocytes. And you can also very clearly
see here, the very very little fine lines representing
the very fine canals or fine canaliculi. So
it is important that you appreciate the
basic structure of compact bone being these
osteons. Well here is a model of an osteon, which
shows you blood vessels and little fine nerves
in the center of the osteonal canal.
08:31
In the model, it shows you in yellow and very pale
coloured concentric rings or concentric lamellae.
08:39
And it shows you some osteocytes embedded in these
lamellae, kind of like little spiders. You can see little
tiny tendicles. They represent the canaliculi,
little fine canals as I have mentioned before
in the bone matrix. Now what you need to appreciate
also, is that in these canaliculi lie long
processes of the osteocytes. So they sit there,
in the lacunar space and they extend very long
cell processes through these canaliculi. And
as you can see in this slide, in the diagram
of the model, they actually are in contact
with neighboring osteocytes. We will learn
in a moment that they are also in contact
with the endosteal cell that lines the Haversian
canal or at least the inner osteocytes in the
inner concentric lamellae are all in contact
through the cell processes with the endosteal
cells. On the right-hand side, you see there's kind
of wool or a rope. And I've put that there to
illustrate that in different concentric lamellae,
the collagen is arranged differently. It is
arranged obliquely in all sorts of directions
in different levels of the concentric lamellae.
So in one lamellae, they may be running in
one direction and in immediately adjacent lamellae,
the collagen may be running in an oblique
direction. And this is an ideal setup for bone
to be able to withstand compressive forces.
10:30
It is very very hard to compress or squash
that rope because the arrangement of the rope,
similarly is very hard to compress bone because
of not only the hardness of the matrix,
but also the arrangement of the collagen
within the bone matrix.