– One major function
of epithelium
is to form barriers of the body,
lining surfaces and cavities.
But epithelium also
forms glands of the body.
A gland is simply defined
as one or more cells
that make and secrete
a particular product.
The product varies
from gland to gland.
There are two types of glands
found in the body generally.
You have the endocrine glands.
And if you look at the prefix,
you've got "endo-" in there.
Endocrine glands are your
internally secreting glands.
The other type, exocrine glands,
are your externally secreting
glands because they've got
that prefix "exo-" in there.
So let's talk about
each of these.
Endocrine glands include the
organs of the endocrine system.
They're so important in
coordinating processes
of the body that they have
been designated their own organ
system.
Some of the endocrine
glands include the pancreas,
the thyroid, the adrenal
glands, the ovaries and testes,
the pituitary gland.
What these glands have in common
is that they produce hormones.
And hormones are small
messenger molecules
that circulate in the blood.
And through circulating
in the blood,
they can reach parts
of the body far distant
from the actual gland that
secreted that hormone.
Endocrine glands
are ductless glands.
There's no channel that
conducts the hormones
to a particular location.
This is different from
the exocrine glands.
Instead with the
endocrine glands,
hormones are produced in
epithelial cells of the gland
and released directly
into interstitial fluid.
Interstitial fluid– let
me spell that out for you–
is just another way of saying
the extracellular fluid.
So hormones are released
generally through exocytosis
into that extracellular fluid,
and from the extracellular
fluid, those hormones are
taken up into the blood
and circulated
throughout the body.
Endocrine glands are mostly
multicellular organs.
They have a variety
of structures,
which you'll get into when you
get into the endocrine system.
There are some individual
hormone-producing cells,
like the–
they're called the
enteroendocrine cells
of the gut.
And collectively, these
individual hormone-producing
cells are sometimes called
the diffuse endocrine system
because they're scattered
around the body.
Exocrine glands secrete their
products onto body surfaces
or into body cavities.
These include sweat
glands, sebaceous
glands that release oil onto
the surface of your skin.
They include salivary
glands that release saliva
into your mouth, mucous
glands in your intestine
and respiratory tracts,
and the pancreas.
Now, I intentionally
mentioned the pancreas
as an example of both
an endocrine gland
and an exocrine gland because
the pancreas is a bit special.
It's a mixed gland.
It has both endocrine
and exocrine functions.
It produces hormones that
regulate blood sugar levels.
That's its endocrine function.
And it produces enzymes that
are released into the intestine
to digest your food.
That's the exocrine
function of the pancreas.
Exocrine glands can be
either unicellular–
made up of just a single
cell releasing its product–
or they can be multi-cellular–
made up of many cells.
Unicellular glands secrete their
product directly onto a surface
by exocytosis.
Multi-cellular exocrine
glands secrete their product
into channels called ducts.
So the endocrine
glands are ductless.
The exocrine glands
do have ducts.
So for example, in here you're
seeing the secretory cells,
the cells that are actually
secreting the product,
releasing it into the duct,
which carries that product out
to the surface.
Same in the sweat glands.
The secretory cells
are down here.
They release their product
into the duct, which carries
that product up to the surface.
Sebaceous glands
are a bit special.
Sebaceous glands actually
release their product
into the hair follicle,
and then the hair follicle
acts as the duct and
carries the oil up
to the surface of your skin.
Different types
of exocrine glands
produce different products.
This make sense if
you think about it.
Your sweat glands produce
a very different substance
than your salivary
glands, thankfully.
You can classify
exocrine glands according
to kind of the types of
solution that they secrete.
Serous glands secrete a watery
enzyme-containing solution.
So the exocrine cells
of the pancreas,
they're secreting
a watery solution
full of digestive enzymes.
Your sweat glands,
they're releasing
a watery secretion full of
some enzymes and full of salts.
Mucous glands secrete mucins,
which are a little bit oilier
and form a lubricating mucus.
So the goblet cells of
the large intestine,
for example, these
are mucous glands.
More of a thicker, more
viscous, lubricating substance.
There are also mixed
exocrine glands that
contain both types of cells.
So if a gland contains both
serous secretions and mucous
secretions together,
it's referred to
as a mixed exocrine gland.
Salivary glands are
an example of this.
There are cells in the
salivary gland that
secrete enzymes and cells
in the salivary gland
that secrete mucins so your
saliva is a mixture of both.
Goblet cells are unicellular,
mucous, exocrine glands.
They're found among
the columnar epithelium
in the intestinal and
respiratory tracts.
We're looking at a section
of the intestine here.
Goblet cells secrete
mucin via exocytosis,
forming a mucus
that kind of coats
the wall of your
intestinal tract.
They're called goblet cells
because the apical surface
of this cell swells with
vesicles full of mucin,
distorting the columnar
shape of the cells.
Here's the normal
columnar epithelium,
the very column-shaped cells.
The goblet cell, it's been
a little bit distorted
because it's a little bit
swollen at the apical surface.
The mucin also does not
pick up stain as well as
the rest of the cell.
So when you're looking
under the microscope,
goblet cells have a
pale apical region,
like you're seeing
there and there.
There are three possible
ways for exocrine glands
to release their secretions.
In merocrine
secretion, secretions
are released by exocytosis.
Most exocrine glands
use this mode.
It's clean.
It's neat.
You just have a neat
little release of product
as vesicles within the cell
fuse with the plasma membrane.
Secretory cells don't rupture.
The only thing that's
released into the duct
is the secretory product.
In apocrine secretion,
the apical portion
of the secretory cell
actually breaks off.
So here's the basal section.
Here's the apical section.
As the apical section
accumulates vesicles
full of product,
the apical section
breaks down and separates
from the rest of the cell.
Mammary glands actually
use this form of secretion.
In holocrine secretion,
the whole cell ruptures.
Holocrine glands just have
a clump of secretory cells
at the bottom of the duct.
The secretory cells at
the top of the clump
have accumulated so much
product that they just
burst, releasing the
secretion and the cell debris
into the duct.
These dead cells are
replaced by dividing cells
at the bottom of the clump.
Sebaceous glands in
your skin do this.
I like to speculate at least
that this form of secretion
might be part of why
sebaceous glands contribute
to acne because they're
releasing cell debris as well
as their product, and
it's a rich source
of nutrients for the
bacteria that might be
growing in that hair follicle.
OK.
After studying this
video, you should be
able to answer these questions.
You should be able to discuss
how endocrine and exocrine
glands compare, both by
structure and function,
so ductless versus ducted.
You should also be able to
explain what goblet cells are
and distinguish
between the three
types of exocrine secretion.

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