but also the arrangement of the collagen
within the bone matrix.
So let us get on now to look at how osteocytes
do receive their nutrients. Well, on this image,
it is really a high magnification of an osteon.
It has a central osteonal canal or Haversian
canal and let us just go through the main
points about the osteons that I mentioned
earlier. The canal is lined by endosteal cells.
Endosteal cells line also all the spicule
bone, all the trabecular bone that we see
up in the cancellous and spongy regions of the
bone. Endosteal cells line the internal medullary
cavity, and periosteal cells line the outside
capsule of bone, the periosteum. So bone is
never naked. If it is naked, if it is bare,
the bone dies. And that is because if you
look at this image, when blood vessels, blood
capillaries pass up through the Haversian
canal and all the nutrients leak out of these
capillaries into the Haversian canal, they
then get pumped across into the very fine
canaliculi by the endosteal cells. And because all
the osteocytes are in contact with these endosteal
cells, and are linked together through these
little canaliculi, the nutrients once pumped
across the surface of these endosteal cells
into the canaliculi can then be accessed by
all the osteocytes. Even those are long long
way away from the Haversian canal.
So they receive their nutrients via endosteal cells.
And as I mentioned before, if these endosteal
cells die, as sometimes happens in bone cancers,
then the bone is naked. The nutrients cannot
be pumped across into the canals and, therefore,
the osteocytes cannot receive nutrients and
so die. So it is important for you to understand
now how these bone cells get their nutrition
even if they are embedded in this
very hard calcified matrix.
Well, let us just review what we have covered
so far in looking at the structure of mature
bone. Remember there are two types of bone
histologically, spongy or cancellous bone and
hard or compact bone. And remember that all
those bony spicules you see and even the outside
capsular bone, they are all lined by either
endosteal cells or periosteal cells.
And the important thing about that is that if you break
a bone, those endosteal cells and the periosteal
cells can initiate bone formation and bone
growth and bone repair. So not only are those
endosteal cells important for providing nutrition
to the osteocytes embedded in the lamellae,
those endosteal cells are also important for
repairing bone should it be injured.
Bone is never naked because of that reason, because
the importance of these endosteal cells to
provide nutrition to the bone. So that will
cover mainly the structure of mature bone
and how these bone cells get their nutrition.
In a later lecture, I will explain how bone
is formed and then how bone grows.