For now, let me explain the term mucosa and
also serosa. Remember when I showed you the
image of the stomach mucosa, I asked you to
remember that word, that term, mucosa, because
in subsequent lectures, I am going to use
the term mucosa and also serosa quite often.
So it is very important to understand what
I mean about these terms and they relate to
glandular epithelium. They relate to the glands
that are just being describing, the exocrine
glands. Well you are familiar with this section
on the left hand side. It is a section through
the intestinal villus. And it shows the epithelial
surface sitting on the lamina propria.
Remember the intestinal villus is a finger like projection
that protrudes into the lumen of the intestine.
Well, it is actually an example of a mucosa. A mucosa
lines cavities that are connected to the outside
of the body, such as the intestine. We will
see mucosas also in the respiratory passages,
and in the urogenital system. Again, epithelial
surfaces that line cavities exposed to the
outside of the body. So when we look at an
endothelium later on or the epithelium lining
a blood vessel, we do not call them mucosa.
Why? Because blood vessels are not connected
to the outside of the body. On the right hand
side, this is a very low magnification of
a section through a part of the small intestine
and those long extended processes are actually
gross anatomical components of the small intestine
and where the label of mucosa is shows these
villi under very very low magnification. But
remember, the villus, the epithelium and the
underlying lamina propria plus sometimes a
little bit of a little muscle layer you might
see there is called the mucosa. It is facing
in towards the lumen. It is a cavity of a
tube that is connected to the outside of the
body, which is what the gut is. Here is another
example of the mucosa. This is a stomach mucosa
that I asked you to recall earlier on the lecture.
So now you know what the mucosa means. It
is labeled here under low magnification on
the right hand side, underneath it's supported
by what we call a submucosa, and I will talk
about that term later on when we look at the
description of the structure of the gut wall.
But again it is just to emphasize that the
mucosa is the surface epithelium and supporting
connective tissue or lamina propria, which
we do not see a lot here because the mucosa is
dominanted by secretory cells. But it's facing
the lumen and it is in a tube that is exposed
to the outside of the body through the esophagus
and the other parts of the gut. Well let's look
at the serosa. A serosa is an epithelial surface
that lines cavities through the body that
are closed. They are not exposed to the exterior
of the body. Cavity such as the peritoneal
cavity, the thoracic cavity, the pleural cavity,
the pericardial cavity. All those surfaces
are lined by what we called a serosa, a very
thin simple squamous epithelium that is found
on the surfaces of these cavities. On the
right hand side, it is actually part of the
large intestine or the colon. If you look
inside of the lumen and you see the many folds
stained very purple in this section, that
is the mucosal surface in contact with the lumen.
It is a mucosa because it is lining
the cavity of the gut here, which is exposed
to the outside of the body, but on the other
side of the gut wall, the serosa is exposed
to the abdominal cavity, the peritoneal cavity.
So that is what is called a serosa.
When I lectured about epithelium, I mentioned that
one function of simple squamous epithelium
is to act as a lubricating surface. Well, often
in these cavities, there is a serous secretion
or watery secretion that helps lubricate these
cavities and helps create a friction free
environment because often these cavitiies
have contents moving all time. The lungs increasing
and decreasing during breathing, the pumping
of the heart inside the pericardial cavity
requires a friction free environment, a surface
that is lubricated by the serous secreting
membranes or cells.
Well in summary then, make sure you understand what
a gland is. It is a secretory unit, derived
from epithelial cells. Make sure you are aware
and can identify the difference between serous
and mucous secreting acini, or little clusters
of secretory cells and mucous and serous secreting
glands. Know the different ways in which the
secretion product is delivered to the luminal
surface and know how glands are named, simpler
compound. And how they're classified by the way
they appearm, are they unicellular or multicellular.
And finally know and appreciate that ducts
are very important in some glands because
they can modify the products, in other glands,
they merely convey the product to the surface
or to where the secretory product is active.
So, I hope you enjoyed this account of exocrine
glands. I hope you now know something
about these glands and I hope you also look
forward to learning in a later lecture about
endocrine glands and also paracrine glands.
Thank you very much for listening.