Epithelial Tissue

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    Yeah!  You're starting NEW anatomy and physiology material!  Congratulations on making it through all the review material!   Epithelial tissue is described in chapter 5, as are the other types of tissues.   Also, since epithelial tissue is one of the main tissues in skin, we will be discussing skin this week as well.

This page contains:

bulletgeneral information about epithelial tissue
bulletcharacteristics of epithelial tissue
bulletdescription of the functions of epithelial tissue
bulletdescription of the types of epithelial tissue
bulletfeatures of epithelial cells
bulletglands

For another web site that goes in depth into epithelial tissue, check out this one from Vanderbilt or the HistoWeb (where you'll have to go to the epithelial page; note that the first simple squamous photo is a look from on top, not a cross section).

General information about epithelial tissue

Tissues are groups of cells that lie together to accomplish a common function.  There are four main types of tissues:  epithelial, connective, muscular, and nervous.  All of these tissues are found in our bodies, but epithelial tissue has a special function-- it must cover all the surfaces of the body.   Therefore, it is found in our skin, and it is also found covering all the surfaces of the openings (each one is called a lumen) within our bodies.

Take a look at this photograph... It was taken of a section through a real human cadaver (male) off the NPAC Visisble Human Viewer website:

abdcrossxn.jpg (95174 bytes)

Do you see the lumen of the stomach?  Do you see the lumen of the aorta?  Do you see the lumina (spaces within blood vessels) in the liver?  How about the lumina within the arms (blood vessels)?...

...Each of those spaces has to be lined with epithelial tissue!  Got it?  I hope so!  (note:  you can also see the skin on the outside of the entire body)

Characteristics of epithelial tissue:

    Epithelial tissue lies on a basement membrane.   That means that underlying the cells that are the cellular component of the epithelial tissue, there is a layer of acellular ("a-" means not, so "acellular" means not cellular) material.  This basement membrane can be thought of as a sticky layer to keep the epithelial cells attached to whatever underlies them.  The bottom edge of the epithelial tissue abuts the basement membrane; this bottom edge is called the basal surface.  The edge of the epithelial tissue that faces the lumen (or the outside world) is called the apical surface.

Other characteristics:

  1. Cells within this tissue readily divide to make more cells.  This helps this tissue recover after any sort of abrasions occur.
  2. This tissue does not have any vasculature.  This means that there are no blood vessels within it.  This should make sense, since epithelial tissue is likely to get damaged by material moving against it-- and you don't want to bleed everytime something bangs into your skin or everytime you swallow something rough.
  3. The cells within this tissue are firmly attached to each other.  As a border-tissue, if the cells weren't adherent to one another, it would be a leaky border.   This would be no good-- liquids from inside of us would drip out!  Yuck!   So the cells all make the type of junctions with each other called tight junctions.  Your book describes these on page 65 and in table 3.1 on page 66.

Here's a drawing of all of these characteristics:wpe2.jpg (18576 bytes)

    In this drawing, the cells (all dark blue with visible nuclei) are tightly opposed to one another, held together with tight junctions (yellow).  The cells are stuck onto a basement membrane (pink).  The side of the tissue facing the lumen is the apical surface of the epithelial tissue; the side against the basement membrane is the basal surface.

 

Functions of epithelial tissue:

    Since epithelial tissue marks the border between our bodies and some kind of space, the epithelial tissue must be able to handle whatever it encounters within that space.  If the space is our external world (that the skin has to come into contact with), the epithelial tissue must be pretty hearty.  If the space is a tiny little air passage deep within your lung, not much besides air would ever get in there and the epithelial tissue does not have to offer protection.

    Also, as a bordering tissue, it may also have to allow materials to pass through it.  Think again about epithelial tissue in the deep air passageways within the lungs-- oxygen has to be able to freely cross this epithelial tissue to enter our bodies.

In list format, the functions of epithelial tissues (as taken from Table 5.1 on page 136) are:

  1. protection-- as a barrier between the outer world (or inner spaces) and our bodies.
  2. secretion-- when our bodies need to release material, like hormones into the blood, this tissue has to allow for such material to pass through.  Often, it is the cells within the epithelial tissue that make the material for secretion.
  3. absorption-- epithelial tissue facing our digestive tract has to be very good at absorbing nutrients from the digestive tract lumen in order for us to get what we need from what we eat.
  4. excretion-- epithelial tissue even lines the excretory lumina, like the tracts from the kidneys through to the urethra.

Types of epithelial tissue:

Most of the types of epithelial tissue are shown in this figure... click on the type to see a short description!  (note that the top edge, toward the label, is the apical edge, while the bottom, toward the pink basement membrane, is the basal edge)

wpe3.jpg (28249 bytes)

Simple epithelium offers little protection... it has only one row of cells that make it up.  But if that row is made up of columnar cells, the protection is greater.

Simple Squamous Simple Cuboidal Simple Columnar
 

So thin that it offers almost no protection; it functions to allow materials to pass through.

View an example (apical is down in this photo)!

 

This offers some protection, but it is more prevalent when material must be secreted & absorbed.

 

View an example!

 

Although only a simple epithelium, this offers some protection of underlying tissue.  It is found where some protection is necessary and where secretion occurs.

View an example!

Stratified epithelium is layers of cells.  This always offers protection.  Pseudostratified epithelium looks stratified at a first glance (because the nuclei are not all at one level), but upon further inspection, one notices that pseudostratified (columnar) epithelium is made up of cells which all reach both the basal and the apical edges of the tissue.

Stratified Squamous Stratified Cuboidal Pseudostratified
 

This tissue is usually made up of so many layers of cells that it offers the most protection to the underlying tissue (that's why we find it in skin).

View an example!

 

This is typically only found in areas where secretion is important (like in glands), but where a little bit more protection is needed than just simple cuboidal.

 

This tissue offers protection and is usually found in places where mucus must get secreted, since it has many mucus-secreting cells called goblet cells.

View an example!

Other types of epithelium:  stratified columnar (not very common) and transitional epithelium (found lining the bladder & seen in these 3 images).  These will be discussed in lab.

Features of epithelial cells:

bulletcolumnar and pseudostratified are commonly found to be ciliated.  The cilia can help move things along within the lumen.  For example, in this photo, apical is up, and all the material above the cilia are actually immature sperm cells.
bulletgoblet cells (mucus-secreting cells) are commonly found in columnar and pseudostratified epithelium-- view some goblet cells secreting mucus (pink stuff).
bullettight junctions are found between them

Glands:

At this point in time, only concern yourself with exocrine glands.  Exocrine glands are those that are directly connected to the epithelial apical surface via a pore; for this reason, they secrete material directly onto the apical surface of the epithelium.  Therefore, an exocrine gland dips down below the surface, but it is still lined with epithelial tissue since it faces a space.  One example of an exocrine gland is a sweat gland.

Shape of a glandThere is specific terminology about the appearance of an exocrine gland

You'll notice in Figure 5.10 (page 142) and in these links that an exocrine gland can have a simple shape or a compound shape-- the difference between these two shapes is that the major duct to the surface is unbranched in a simple gland, whereas it is branched in the compound gland.

You should also notice in Figure 5.10 that a gland can be branched.  A branched gland just means that if you look at the deepest end of a duct, its tip is branched into several secretory endings.

Finally, the shape of the secretory regions of the gland can be rounded, called alveolar, or elongate, called tubular.  If it is a tubular gland, and the tubular end bends around a bit, it is called a coiled tubular gland.

Secretions of a glandThere is specific terminology related to exactly what a gland secretes

merocrine glands:  only dissolved materials are secreted (via exocytosis)

apocrine glands:  pieces/chunks of cells get pinched off in the secretion

holocrine glands:  entire cells are secreted

Just get familiar with these terms... they'll come back soon as you start to learn about some specific glands.  OK?

2011 STCC Foundation Press
written by Dawn A. Tamarkin, Ph.D.