Chapter 4.2.1 Introduction to Epithelial Tissues BIO201

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okay in this video we're going to talk

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about the epithelial tissue so

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epithelial tissue also called epithelium

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is basically a sheet of cells that are

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all tightly bound together

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and these cover body surfaces or

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cavities there's two major forms we have

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covering and lining epithelia that you

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find let's say

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you know covering your whole body like

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skin or lining the internal surfaces of

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your body like

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all on your respiratory tract or all

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along the inside of your digestive tract

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these would form the walls

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of those spaces now the glandular

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epithelia

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are involved with lining the the

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ducts of a gland things like salivary

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glands or pancreas

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have epithelial tissues that line those

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ducts and they even

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make part of this secretion that we'll

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talk about here in a minute

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some of the major functions of

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epithelial tissues are to protect

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absorb to filter to excrete

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an excretion is elimination of wastes to

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secrete

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you know saliva obvious secretion and

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some epithelia are involved with sensory

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reception so they're used to pick up

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information in your body

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now the epithelial tissues of our body

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have five distinguishing characteristics

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we say they have polarity

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specialized contacts they're supported

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by connective tissue

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and this is because epithelial tissues

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are avascular so a meaning lacking

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vascular means blood

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blood vessels so avascular means that

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epithelial tissues do not have blood

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vessels within them

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this is why they're supported by an

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underlying connective tissue

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that's because the connective tissues

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can bring in the blood vessels and

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nutrients that can

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basically nourish those epithelia but

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epithelial tissues are

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innervated which means they get nerve

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supply so that you do find nerves within

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epithelial tissues

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also epithelial tissues need have a high

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regenerative capacity

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um this is because they're involved with

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protection and

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uh they're you know exposed to a lot of

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damaging factors so they need to be able

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to regenerate pretty frequently

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so polarity refers to the fact that

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epithelial tissues have a top and a

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bottom

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the top is what we refer to the apical

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surface surface now the apical surface

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is this

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upper free side or you can also call

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this the side that's exposed to your

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surface or cavity

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so an example of where you'd find this

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would be if you look at your skin

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the side of the cells you're looking at

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when you're viewing your skin

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externally is the apical surface of

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those epithelial cells

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and then deeper those epithelial cells

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would be attached by their basal surface

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to an underlying basal lamina of

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basement membrane

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and then that would be attached to an

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underlying connective tissue

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now both surfaces differ in their

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structure and function because the

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apical surface is involved with

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secretion and protection and absorption

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whereas the basal surface is involved

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with attachment

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and also transportation of certain

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materials

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epithelial tissues need to have lots of

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specialized contacts

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so going back to when we talk about the

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intercellular junctions of chapter three

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we said we had tight junctions and

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desmosomes remember tight junctions were

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involved with waterproofing so that

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water can't pass between cells

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and desmosomes were involved with very

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strongly holding cells together

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so we find that epithelial tissues have

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a lot of these because we really need

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those

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cells to be tightly bound together and

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not pass

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substances between them now connective

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tissues all

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support epithelial tissues and

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what we find is that the epithelial

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sheets are basically these long mats of

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cells that are all interconnected

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are supported by an underlying

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connective tissue at some point

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now uh we have this reticular lamina

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that's deep to the basal lamina and it's

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basically a network of collagen fibers

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that

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these epithelial cells attach to this is

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part of the basement membrane so the

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basement membrane is made of the basal

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and reticular laminas

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it's basically something that reinforces

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the epithelial sheet

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think of this as like the velcro for the

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cells to attach to

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this base membrane also resists

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stretching and tearing and it defines

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the epithelial boundary

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because just deep to basement membrane

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that's when you have the underlying

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connective tissue

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that would basically help to support and

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nourish the epithelium

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now we need an underlying connective

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tissue because epithelial tissues are

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avascular

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which means that they don't have blood

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vessels within them but they do have a

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lot of nerve supply

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so because of the fact that epithelial

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tissues don't have blood vessels within

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them

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they need to be nourished by diffusion

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or you know sort of

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movement of particles from an underlying

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connective tissue

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now these epithelial do have nerve

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fibers and these are involved with

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receiving information in different areas

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of your body

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now epithelial tissues also have a high

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regenerative capacity

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this is because they are exposed to

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friction

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or hostile substances which can result

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in damage

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and therefore must be replaced and it

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requires an adequate amount of nutrients

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as well as cell division to adequately

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replace those epithelia

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what something else that stimulates the

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regeneration of epithelial tissues is

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this

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loss of apical basal polarity or broken

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lateral contacts that way the tissue

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knows that

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um you know there are spaces that need

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to be filled in

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with new cells now what we'll do next is

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move on and talk about the

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classification of epithelial tissues

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so all epithelial tissues have two names

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the first part of the name

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indicates the number of cell layers so

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if we say it's a simple epithelial

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tissue it means it has a single layer of

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cells

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if it's a stratified epithelial tissue

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it has two or more cell layers thick

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an example this would be skin where you

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have you know probably over

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30 cell layers thick so we would call a

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stratified epithelium

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you'd find an example of simple

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epithelium like in your respiratory path

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uh passages specifically the alveoli

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where you need to absorb rapid

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absorption of gases

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now another name here would indicate the

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shape of the cells

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so squamous means that the cells look

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kind of flattened or scale like

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cuboidal means they look box like like a

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cube and columnar means they're taller

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than they are

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wide or column like so we do that is we

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can mix

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these the combinations of these words

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here so we can have simple

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squamous epithelium or stratified

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cuboidal epithelium

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and that tells you basically the number

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of cells and the shape of those cells

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in the stratified epithelia the shape

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can vary in each layer so

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ultimately we classify the stratified

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epithelium

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based on the shape of the cells at the

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apical surface

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of the epithelium and i'll give you guys

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an example here in a minute

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so you can see here is a classic example

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of simple squamous epithelium

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we can tell it's simple squamous because

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you have one layer of cells here

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and they're flattened which means

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they're squamous so we said it's a

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simple squamous

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now we can compare this down here to a

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stratified epithelium

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and if you look first of all you can

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tell this is stratified because you have

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many cell layers that are stacked

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upon each other now you might wonder

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okay well these cell shapes can kind of

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vary the closer you get to the surface

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and you might say okay these ones kind

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of look cube shape so then you might be

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inclined to say stratified cube oil

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where you have

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you know many layers of cube shaped

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cells however the stratified epithelial

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tissues

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uh that are named for the shape of the

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cells

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at the apical surface you can see that

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apically we see that these cells are

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more flattened

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so we would call this type of epithelium

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actually stratified squamous which is

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basically like what you'd find in your

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skin

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now remember squamous is flattened

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cuboid is cube shaped and columnar is

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more column shaped

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you'll find that when you look in a

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microscope

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they're not going to be in their

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three-dimensional views they're going to

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see more two-dimensional like this

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so we start looking at microscopy images

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here pretty soon

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you'll see cells that are shaped like

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this we see there's squamous or

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cube shaped cuboidal or columnar down

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here which are more column shaped

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but in reality they're actually

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three-dimensional structures here