It has a long duct that opens into the duodenum some
distance away. Well, when we look at large glands,
we can see that they have a certain type of
organization. Here is a section through a
large salivary gland. When you look at from
this magnification, it is rather deeply pink
staining and you can identify or you will
able to identify the different types of salivary
glands because the three types of salivary
glands and the pancreas have a preponderance
of a certain sort of secretory product. Some
are purely mucous, some such as a parotid
gland and the pancreas are purely serous. Some are
mixed. And using that criteria, you can distinguish
the three different salivary glands and the
pancreas. But all these large compound glands
are divided into lobes in smaller components
called lobules. Look very carefully at this section
here and you can see large connective tissue
spaces dividing this gland into large segments.
The septa of connective tissue are dividing
these glands into lobes. Smaller segments
of the connective tissue can divide the lobe
into a lobule. So look carefully at this slide
and you can see lots of tiny little divisions.
They are separated by wide spaces here, which
is artifactual caused by shrinkage of the
tissue, but it is a good way of really illustrating
that all these compound glands are dividing
into large components called lobes and each
lobe has subdivisions called lobules. And later
on we are going to learn that ducts and also
blood vessels acts as the secretory units
first of all by coming in through these large
connective tissue spaces between the lobes
and then through the smaller connective tissue
spaces going to each lobule and therefore to
the secretory units.
Let me just diverge slightly and introduce you
to the myoepitheleal cells. These are contractile
cells. And as you can see here, if you look very
very carefully, this is a section through
a sweat gland. And what you see the pilar regions,
the pilar clusters of cells are the secretory
units of the sweat gland. The small darker
little tube you see is the duct portion of
the sweat gland. And I will talk to you in more detail
about these glands when we look at skin.
But what I want to bring your attention to, is the myoepithelial
cell. As I've said, they contract themselves
and they wrap around the secretory units.
They hug the secretory units like an octopus.
And you can see very very fine little red
lines on the outside surfaces of these secretory cells.
They are the processes of the myoepithelial
cells wrapping aorund the secretory units.
Myoepithelial cells are very important in
exocrine glands, because they wrap around the
secretory units and help with the secretion
moving out of the cell into the luminal space.
There are sometimes glands secrete cells. They
have stem cells and the product in these glands
are other cells, to replace cells higher up
in epithelial surfaces, above where the glands
are. They don't produce proteins or mucous.
They produce other cells. If we have a look
at these three images, they illustrate secretory
cells and also stem cells. This picture here
on the left hand side illustrates a number
of finger like projections that protrude into
the lumen of the small intestine. These finger
like projections are called intestinal villi.
And labelled here is a single intestinal villus.
Where that label is pointing is the underlying
connective tissue, the lamina propria. But
on the surface of these villus or villi is
an epithelium and that epithelium is involved
with absorbing material from the lumen of
the duct into various blood vessels and lymphatic
channels that running at lamina propria. But
on the middle section, you can see some red
stained cells, that live in the glandular part
of this epithelial surface, labeled here as
small invaginations into the underlying lamina
propria. That is where the secretory cells
live deep in the glands and not on the surface.
Well besides these secretory cells, there
are also stem cells shown here. A different
stains used to show up clumps of chromosomes
in some of these cells that are undergoing
mytotic activity. Well these stem cells, at
the base of these gland,s produce other cells
that move up along the surface of the little
glands and they differentiate into secretory
cells that you see in the middle section or
they can differentiate into the surface epithileal
cells and be involved in the absorption of
products from the lumen. These stem cells
are very important in epithelia because epithelial
cells are continually being replaced. And they
are replaced because of these stem cells being
present in all the epithelial surfaces that
we are going to come across when we look at
the organ systems in later lectures.