00:02
Well, it is important that we start off by learning
something about how connective tissue is classified,
how it is named. All connective tissue arises
in the embryo from mesenchyme or mesoderm.
00:22
Sometimes that mesenchyme differentiates into
what we call mucous connective tissue in the
umbilical cord and I will talk about that
later on during this lecture. But mesenchyme
developing into the mesoderm then provides
all the necessary cells and components of
connective tissues. We will learn later on
that mesenchyme also gives rise to smooth
muscle and muscle to do with the cardiovascular
system and the urogenital system and serous
membrane. So mesenchyme is not just restricted
to providing connective tissue. Now, in the
adult, you can divide connective tissue into
three basic components or three basic levels
of classification. The most common one is
just ordinary connective tissue such as lamina
propria, which is loose connective tissue
or dense irregular connective tissue like
the dermis of skin or regular connective tissue
like a tendon. Sometimes we classify connective
tissues as being supporting such as cartilage
and bone. And there are also very special connective
tissues, adipose tissue, which stores fat,
stores energy, reticular tissue, we are going
to talk about during our talks on the lymphatic
system and of course blood as well.
02:01
Here is an image of a developing part of the
body. It is embryonic. And the reason why I want
you to have a look at this very carefully
is because I want to show you a mesenchyme cell.
02:14
A mesenchyme cell is a very very thin
elongated nucleus and it consists of cytoplasm
that we cannot see because that cytoplasm
is spread out throughout the connective tissue,
very very thin. All we see a very spindle-shaped
nucleus. Now that mesenchyme cell is going to give
rise to all the connective tissue cells that
we will learn about. In this case, it is an
undifferentiated cell. But during development,
as we see here, it can be told to develop
into a tissue. Here that mesenchyme cell is
being told, you will form bone. So that mesenchyme
cell starts to differentiate and form an osteoblast.
And here we see some osteoblasts lying on
the surface of bone that has been created.
Those osteoblasts are lying down bone matrix
and you see lots of little dark nuclei belonging
to these osteoblasts and they are sitting
on top of the surface of newly formed bone.
And when those bone cells of secreted matrix they
surround themselves and then they sit there
and maintain that matrix as osteocytes. And
we will talk more about bone development,
bone growth, bone formation and the role of
these osteoblasts and osteocytes in a later
lecture. But the key point is mesenchyme cells
are undifferentiated and they can be told
to differentiate further into the osteoblast
and then other connective tissue cells as
well.
04:07
Here is a slide, a summary of all the sorts
of cells in connective tissue that we will
cover in this lecture and in a subsequent
lecture on cartilage and bone.
04:21
A fibroblast, a mesothelial cell, endothelial cells that line
blood vessels and also chondroblasts and chondrocytes
that form a cartilage and as we saw previously,
the osteoblast and the osteocytes in bone.
04:40
Let's look at some of these classifications
of connective tissue in a little bit more
detail. Here is a section through loose
connective tissue or areolar connective tissue.
04:53
You know it is called areolar connective tissue because
in the very early days, when the early anatomists
and histologists tried to describe these connective
tissues, when they peel the skin away from
an animal, the gap between the dermis and
the hypodermis accumulate a little air bubbles.
05:12
So they called it is aerated connective tissue
or areolar tissue. That is where it gets its
name from. But have a look at this picture,
you can see at the very top, some rather dense
stained connective tissue, collagen. It happens
to be the wall of a blood vessel, but underneath
you can see very sparse cells wandering through
this loose connective tissue. That clear space
dominates and that clear space is the ground
substance. Sometimes you can see fine little
collagen fibres running through this particular
connective tissue. So this is loose connective
tissue such as we saw in the lamina propria.
Here is the lamina propria again just to remind
you of what it looks like. It supports the
epithelium, the basement membrane is the cement
or the structure that holds that epithelium
bound to the underlying lamina propria or
the connective tissue. And if you look very
carefully in this slide, in the lamina propria,
you can make out some very fine little red
dots and red fibres. These are going to be
both collagen and elastic tissue that we will
talk about later on. This gives the lamina
propria support, structure and strength, cohesion,
but it also gives the lamina propria flexibility
because epithelial serves moves. They are mobile
and they are only mobile because of the looseness
of the underlying connective tissue and the
components of the fibres within that lamina
propria. Here is another example of areolar
tissue or loose connective tissue.
07:07
Here is a sweat gland shown here and just around
the sweat gland, you have very loose pieces of
connective tissue. This is now dense irregular
connective tissue. Notice how different it
is to what you saw previously with the lamina
propria. The nuclei you see here are mainly
nuclei belonging to either endothelial
cells of capillaries going to the surface
of the skin, this happens to be the epidermis
sitting on the dermis. Or they're the nuclei of may
be some fibroblasts that the collagen here
dominates. You can see collagen that sectioned
longitudinally and you can say collagen that
section transversely. Now this provides a dermis
with enormous strength because the collagen
is arranged in lots of different networks
and in lots of different directions that prevents
the skin from tearing. It gives the skin
strength and also some mobility. The epidermis
is cemented directly onto this dermis. Now
there is are too many cells there to do with
the immune system and yet skin is an epithelium.
08:30
It is a very specialised epithelium and that
is because the epidermis that is labelled
here has special components that prevent pathogens
from invading through the epidermis into the
underlying dermis. The waterproofing layer
of the keratin, the lipid and the protein
layers that are insoluble that are on the outside
and inside of the membrane of the stratum
corneum, and the Langerhans cells or the
antigen presenting cells within the epidermis
are all there to be a barrier to invading
pathogens. So in the case of dermis, you do
not see the sorts of immune cells or presence
of lots of nuclei that are under surveillance
or sentry duty as you do in the lamina propria
underneath an epithelium.
09:25
Here is an example of dense regular connective
tissue such as the tendon. Here the collagen
bundles are arranged in parallel. As I mentioned
earlier, the job of a tendon is to impart
contraction from a muscle directly onto the
bone. So to do that effectively, the collagen
is arranged parallel. And if you look very
very carefully, you can see fibrocytes.
09:53
These are fibroblasts that they have done their
job laying down the fibres so they are resting
and undergoing just maintenance of the collagen.
The resting fibroblasts we often term a fibrocyte.
10:10
Now in the case of a tendon, these cells can
get another name. They are called a tendinocyte.
10:18
And these collagen bundles, if we can see the
outside surface of the collagen, there would
be a very thin capsule around the collagen
bundles or around the tendon, and the collagen
in this capsule would be arranged less sort
of parallel than you see here. And that capsule
is called the epitendineum. Each bundle of
collagen is wrapped up by endotendineum and
we will come across terminology like that
when we describe the wrappings up of muscle
cells and also nerve cells.