as any teacher will tell you when you're
dealing with certain elements that are
being feisty infidelity and basically
not cooperating there's pretty much only
one thing you can do
you gotta keep them separated and
there's a whole system of biological
tissue that's dedicated to doing just
that
creating order where there would
otherwise be total mayhem
because you and pretty much every other
animal is made up of incredibly complex
feisty fidgety systems
that need to be kept apart to some
extent if they're gonna get anything
done
think of it this way say all the middle
schoolers in your town want to have
lunch together
at the same time on taco tuesday if you
crammed everyone into one giant lunch
room you'd have a lot of interesting and
talented people in one place
yes but you'd also never get a handle on
them with
everyone shoved and talking and jostling
and flirting and farting and stepping on
toes and haggling over tater tots it'd
be like a john hughes movie gone
horribly wrong
so what you need is a solid system of
organization like separate lunch lines
for separate groups of kids
or tables that arrange students in
alphabetical order your body is like
that crowded lunchroom it needs
order for it to function it can't have
your liver all up in your brain or
squished between your kidneys
your organs and their systems need their
personal space and that
is where your unsung epithelial tissue
steps in
like a burly gym teacher with a whistle
and a plan
this is the tissue that lines and covers
and generally organizes your body
creating order
from what would be chaos without
epithelial tissue you'd essentially
be a mushy pile of unarticulated goo
when we talk about your epithelial
tissue we're really talking about
two things there's the proper epithelium
which covers and lines your outer and
inner body
and then there is the glandular
epithelium which forms glands and
secretes hormones and other substances
your primary epithelium protects your
whole body
inside and out it's a great organizer
partitioning
everything into separate but connected
units it covers the surface of your body
when it combines with connective tissue
to create skin
but it also lines your body cavities and
coats the internal and external walls of
many of your organs because your body
doesn't just
interact with the outside world through
your skin we and all animals from the
simplest worms on up the tree of life
are really just
tubes corridors of tissue running from a
mouth to an anus
epithelial tissue covers both the inside
and the outside of that
youtube to get a better sense of what i
mean
take a look at this balloon the latex is
like the outer covering of your body in
part made up of epithelial tissue
it separates what's inside the balloon
from the rest of the world now if i
stick my hand in there
you can see how while the tissue still
forms an outer layer it also
folds in on itself creating a continuous
barrier
that lines all of the cavities in a very
similar way the membranes covering your
lungs for example
are actually invaginations of your
epithelium where the tissue that makes
up your
youtube folds to form a cavity just like
this balloon when i push my fingers into
it the epithelium does all this to
protect your deeper layers of tissue
from injury or infection
like for example by lining your stomach
with epithelial cells that produce mucus
so that you don't digest yourself
along with your lungs and all of your
epithelial tissues are avascular
meaning they don't have a blood supply
instead they rely on the blood supply
and the supporting connective tissues
around them for the materials they need
but these tissues come
in different varieties that serve
different purposes and a lot of what
classifies the different types of
epithelium boils down to their shape
and layering that is the shape of the
individual cells
and the number of layers that they form
in and there are three basic
shapes squamous cuboidal and columnar
and they're pretty easy to tell apart
because unlike most terminology you'll
be exposed to in this course their names
actually describe
what they look like squamous cells are
flat their name means scale and they
look kind of squished like fish scales
even the cell's nucleus which gets
darkly stained and is usually easy to
see
is flattened cuboidal cells are you
guessed it cubish shaped
about as tall as they are wide they
absorb nutrients and produce secretions
like sweat
their nucleus is pretty circular
columnar cells are tall and thick and
look like columns and they cushion
underlying tissues
and as if they were cuboidal cells that
got stretched tall their nuclei are
stretched into an ellipse and here's yet
another instance where the form of a
structure relates to its purpose
in this case the shape of each kind of
epithelial cell
correlates with its function for example
squamous cells are flat which makes it
easy for materials like
oxygen to move across them to the other
side so we see these kinds of cells
where absorption or transportation is
most important like
in say the air sacs of your lungs or in
your blood vessels but if the cells that
make up a tissue need to say brew up
hormones or mucus
they'll need all the internal machinery
it takes to make that stuff
and that takes up a lot of space so
those cells can't be flat they've got to
be cuboidal or columnar to accommodate
more room for taking care of business so
that stomach lining that i mentioned for
example is made up of big columnar cells
because they have to make and secrete
mucus but when it comes to what kind of
cells are found where
an important thing to keep in mind is
the fact that cells are biologically
speaking
expensive they take a lot of time and
energy and raw materials to make so in
places where you lose a lot of cells
like your outer skin or in your mouth
you have more squamous cells because
they're smaller and flatter and
therefore
cheaper practically disposable rather
than big expensive cuboidal or columnar
ones
which brings me to the other trait that
we use to classify epithelial tissue
it's layering a simple epithelium has
only one layer of cells
a stratified type has multiple layers
set on top of each other like the bricks
and mortar of a wall
and pseudostratified epithelium is
mostly just one layer but the cells can
be different
shapes and sizes and the nuclei can be
at lots of different levels so it looks
sort of messy and multi-layered
even though it really isn't and when we
describe a type of epithelial tissue
like in a lab setting
we cite both its shape and its layering
you can think of a tissue's first name
as its number of layers and its last
name as the shape of its cells for
example a simple squamous epithelium
refers to a single layer of flat
scale-like cells like the lining of the
air sacs deep in your lungs a stratified
cuboidal tissue meanwhile would have
layers of cube-shaped cells
like the linings of the ducts that leak
sweat and spit when you put the shape of
a cell together with its type of
layering
you can begin to see how both traits
inform the function
of your epithelial tissue let's go back
to those squamous cells because they're
thin like scales it takes many layers of
them to form a tissue that's thick
enough to offer protection
so you end up with a really dense stack
of cells that
on an individual basis are small and
cheap to make that's why when i like
scratch my hand or hit the inside of my
mouth with a toothbrush i can lose a
couple of layers
no big deal those squamous cells are a
dime a dozen there's still lots of
layers left
plus epithelial tissue regenerates
really quickly
but if you say get tossed off a moving
motorcycle you lose a lot
more layers and if your road rash is
really bad
you can scrape all the way through all
of those squamous cells down to the
nerves and the blood and all the
underlying connective tissue
plowing through a lot more expensive
cells and wind up with a real like can
you please get me to the hospital i need
to go to the hospital kind of problem of
course when we talk about epithelial
tissue protecting you it's not always
protecting you from the outside world it
also creates order among all of those
rambunctious seventh graders that are
your
organs and here it's important to note
that all of your epithelial cells are
polar
meaning they have distinct sides the
apical or upper side is exposed to
either the outside of your body or
whatever internal cavity it's lining the
basal side or
inner surface is tightly attached to the
basement membrane
a thin layer of mostly collagen fibers
that helps hold the epithelium together
and anchors it to the next deeper layer
your connective tissue many of these
boundaries that the cells form aren't
absolute instead they're selectively
permeable
allowing for some level of absorption
filtration and excretion of substances
the tissue lining your small intestines
for instance is what allows you to
absorb nutrients through diffusion and
active transport so that's pretty
important
and all of your urinary waste gets
filtered through a different epithelial
lining in your kidneys so by now you're
probably starting to get it every
interaction that your body has with the
rest of the physical universe is made
possible somehow
by your epithelium but that is not all
remember your glams are
also made up largely of epithelial
tissue so it
also plays a big role in facilitating
all of your secretions
from sweat and mucus to hormones and
enzymes this glandular epithelium forms
two different kinds of glands your
endocrine glands
the ones that secrete hormones right
into your bloodstream or to nearby cells
and your exocrine glands the type that
secrete their juices into tubes or ducts
that lead to the outside of the body or
the inside of your tube
rather than right into the blood the
hormone thyroxine for example is
secreted by an
endocrine gland your thyroid and it
needs to be distributed throughout the
entire body so that it can stimulate the
metabolism in all of your cells some
example of exocrine secretions would be
sweat saliva mucous stomach acid and
milk if you're lactating all those
secretions go right into ducts where
they're ferried to an epithelial surface
which could be your outer layer of skin
in the case of your sweat
or the edge of your stomach lining if
it's your stomach acid so hey
the system works and it's due in large
part to the humorless gym teacher that
is your epithelial tissue it may not be
a ton of fun
but darn it it gets results today you
learned how your unsung epithelial
tissue creates the inner and outer
boundaries that keep you alive
we looked at how proper epithelial
tissue is classified by both layering
symbol or stratified and shape squamous
cuboidal or columnar
and how the structure of these tissue
types match their function
we also talked about how epithelial
cells are polar having both apical and
basal sides
and are selectively permeable and lastly
we took a brief look at how our
glandular epithelial tissue forms both
our endocrine
and exocrine glands thanks for watching
especially to all of our supplemental
subscribers who make crash course
possible
to find out how you can become a
supporter just go to support.com this
episode was written by kathleen yale
edited by blake di bestino and our
consultant is dr brandon jackson
our director and editor is nicholas
jenkins the script supervisor is sara
meismer the sound designer is michael
laronda and the graphics team
is thought cafe