Glandular Epithelium

Glandular Epithelium


– 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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About the Author: John Markowski

11 Comments

  1. Hey can you do a video, where you show a lot of pictures of Glandular Epithelium and you describe them ? please..
    Like that we will be able to find them when we see a picture of one..
    By the way, is a great video.. Thank you .

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