In this lecture, I am going to talk about
bone. But I am going to concentrate on mature
bone and the function of mature bone. In a
later lecture, I am going to talk about bone
formation and bone growth. Bone is a very
specialized connective tissue. Like all connective
tissues, it is composed of cells, fibres and
matrix. But in the case of bone, the matrix
is calcified. And because of that it enables
bone to have some very important functions
in the body. It can protect vital organs of
the body, the thoracic cavity protects the heart
and the lungs. The cranium protects the brain.
Bone always forms the skeletal components
of the body and in unison with the muscle
affects movement of the body. It is also a
very important mineral reserve for both calcium
and phosphate and it provides a compartment
for bone marrow to form blood cells. I am
going to talk about these latter functions
in a lecture later on. Let us look at really
what I want you to understand with this initial
lecture on bone. First of all, I want you
to appreciate that there are two types of bone,
compact bone and spongy bone. And I will describe
these two types during this lecture.
The very fundamental structural unit of bone is
the osteon and I will also describe that and its
something that you really should understand
because it will explain to you really how
bone forms its functional role in the body.
And equally important you must understand
how bone cells receive their nutrients. As
I said earlier, the matrix is very hard.
It is calcified. Unlike other connective tissues
such as loose connective tissue or even cartilage,
there is no aqueous component of the extracellular
matrix. So nutrients including oxygen cannot
diffuse through that matrix. So I am going
to explain in this lecture, how bone cells
get their nutrients from the blood when they
are embedded in this very firm calcified matrix.
So again let us look at what characterizes
bone. First of all, it is vascular. Unlike
cartilage, it has a blood supply. It has
a very large blood supply. If you break
a bone, it bleeds. Again like connective tissues,
it has cells, fibres and matrix.
The cells are called osteocytes and these osteocytes
live within the bone matrix and often you
see just a small clear space. This is a lacuna
and a lacuna is really a space that is occupied
by the bone cell or the osteocyte. Normally
the osteocyte would occupy the entire space
but during processing of bone, often the cell
is completely lost from its territorial area
in the bone matrix and just the small little
space or a lacuna remains. Sometimes however
in some of these little lacunar spaces, you
can see remnants of the osteocytes.
Again, like cartilage, the most important
functional part of bone is the matrix.