2. Know the histological classification of epithelia and the characteristics of each type.
3. How are epithelia anchored to underlying connective tissue?
4. Know the components, structure and function of the basal lamina.
5. What are the various types of intercellular junctions, and what functions do they have?
6. What are the functions of the various cell surface modifications?
7. Epithelial cells are highly differentiated to perform specific functions.
How do the
arrangement of organelles (the ultrastructure) of various epithelial
cells relate to their
function?
I. General Functions of Epithelia
II. Major Features of Epithelial Tissue
IV. Classification of Epithelial Tissue
V. Structure-Function Relationships
Things you need to know:
Epithelium
Mesothelium
Endothelium
Basal Lamina
Basement Membrane
Intercellular junctions
Terminal bar
Zonula occludens
Zonula adherens
Macula adherens (desmosome)
Hemidesmosome
Gap junctions
Surface specializations
Microvilli
Stereocilia
Cilia, basal bodies
Flagella
Glycocalyx
Keratin
Basolateral processes and folds
Epithelial types
Simple Squamous
Stratified Squamous
Simple Cuboidal
Stratified Cuboidal
Simple Columnar
Stratified Columnar
Pseudostratified columnar, ciliated
Transitional
Specialized epithelia
Goblet cells
Metaplasia
Polarity
Apical surface
Mucous membrane
The purpose of this exercise is to examine and learn to identify the
various types of
epithelia. Notice variations in the shapes of nuclei paralleling those
of the cell. Epithelia
are classified by the shape of the outermost (apical) layer of cells,
and by the number of
layers of cells. In addition to this general classification, there
are several more specialized
types. Although the simple squamous epithelium is conceptually the
easiest, it can be
difficult to distinguish in a section. Therefore, begin by examining
the stratified squamous
epithelium found covering the lip. Some epithelia (such as stratified
cuboidal and
columnar) are difficult to locate in sections and are listed as optional.
Make sure that you
see examples in your atlas and that you could recognize them if you
saw them again. At the
end of the laboratory is a list of additional slides that have good
examples of specific
epithelial types. Observation of these slides is optional. They
are listed only to provide
more practice in identifying and classifying epithelia. More practice
can be otained by
taking advantage of HistoSlideReview.
Note: Complete descriptions for slides are found in the slide description
manual. Some slides have alternative examples, designated with a
letter
after them. In order to see as many examples as possible, and become
used
to the inherent variability of normal tissue, try to look at as
many examples
as you can. Share slides with others in your lab, but make sure
they are
returned to your slide box. You will be held responsible for the
content of
your slide box at the end of the course.
1. Stratified Squamous Epithelium:
Slide 79 is a cross section through the lip. It is covered by
stratified squamous
epithelium on all but the "free (cut) edge". Make sure you can tell
the difference between
surfaces covered by epithelium and cut surfaces. Examine this section
at low power. Both
sides of the lip are covered with stratified squamous epithelium, but
the outside is
keratinized while the inner is not (why?). Look at both sides and determine
which is
which. Note that the non-keratinized surface has cellular (and non-cellular)
debris
associated with it that could be confused with keratin. What is it?
How do you tell that this
is not keratin?
Where does the epithelium begin (basal layer)? What shape are these
cells? Do these
cells look the same as the cells that make up the surface layer? What
about the cells in the
intermediate layers?
2. Simple Columnar Epithelium:
Slide 53 is a cross section of the ileum. At this time you are
not expected to examine
the histology of the intestine. Instead, concentrate only on the simple
columnar epithelial
lining of the lumen. The epithelium covers all the finger-like projections
called villi. These
villi may be cut in a number of different planes of section, making
it difficult to tell exactly
where the epithelium is. First examine the cells covering the tips
of the villi at low power.
They should be a single layer of tall cells. All the cells are not
identical. How many types
are there? How would you describe them? Can you see a basal lamina?
In some regions
the underlying connective tissue is not in direct contact with the
epithelium. These spaces
are artifacts. Where is the nucleus positioned within the cell
for each of the cell types?
Examine areas in which the villi are cut in cross section. How can
you tell you are looking
at a cross section of a tube rather than a cross section of a sphere?
At 20x, examine the outermost (apical) edge of this epithelium. You
should be able to
distinguish the striate (or brush) border. What does this structure
look like at the
ultrastructural (electron microscopic) level? Observe these cells using
the 40x objective.Can you see terminal bars? What is the ultrastructural
correlate of the terminal bar? These
may be very difficult to see, and may require the use of your "histo-imagination".
3. Simple Cuboidal Epithelium:
Slide 115 is a cross section of the kidney. The organ contains
numerous tubules
which are intertwined with each other. Examine the epithelial lining
of the tubes, which
varies from squamous to almost columnar in appearance, although the
cuboidal
organization is the most common. In this example, the basement membrane
is easily seen
because the associated collagen is stained blue.
4. Simple Squamous Epithelium:
Simple squamous epithelium lines every blood vessel, making it present
in virtually
every section. However, because it is so flat, it is often difficult
to see in cross section.
Slide 115 of the kidney has good examples of simple squamous
epithelium which can be
seen lining the glomeruli. Glomeruli look like round globes surrounding
a flower-like
structure which is actually a tuft of capillaries. They are found in
the cortex of the kidney
among the tubules. The globe (called Bowman's capsule) is lined with
simple squamous
epithelium. All you can see are red nuclei protruding into the space.
5. Pseudostratified Epithelium:
In general, a pseudostratified epithelium appears stratified because
the nuclei are on
more than one level. In reality this epithelial layer is of the simple
type, with all cells
touching the basal lamina, but not all reaching the lumen at their
apical poles. Slide 111
of the trachea is an especially good example of this type of epithelium.
Examine the lumen
of the trachea at low power. Find an area with few goblet cells (light
staining mucus
secreting cells). At 20x, examine the epithelium. Notice that there
appears to be several
layers of cells. The pseudostratified nature of the epithelium is not
obvious from inspection
at the light level. You recognize it because of other characteristics
common to the
epithelium that lines the respiratory system, including the presence
of goblet cells that
appear to extend from the base to the surface, and the presence of
cilia on the surfaces of
other cell types. Cilia can be seen at 20x, but are better observed
at 40x. How can you
distinguish among cilia, microvilli and stereocilia at the light microscopic
level? at the
ultrastructural level?
6. Transitional Epithelium:
Transitional epithelium is capable of marked distention, and is restricted
to areas of the
urinary system. Slide 119 is a cross section of the ureter.
The lumen is lined by
transitional epithelium. At first glance, this epithelium may look
like stratified squamous
epithelium. You must carefully examine the uppermost (apical) layer
to make the
distinction. Undistended transitional epithelium has many surface cells
that are not flat, but
are plumb and rounded on the apical surface. Note that not all cells
appear rounded. You
must carefully examine the whole surface before you decide which type
you are viewing.
Slide 120 contains two sections of the urinary bladder, one
prepared in a distended
state and the other in a relaxed or contracted condition. Note the
tremendous stretching
capacity of the epithelial lining, as well as the underlying connective
and muscular tissues,
as you compare the two preparations. Compare stratified squamous epithelium
from slide
79 or from any of the optional slides below with that seen on these
two slides. More
examples can be found in your microfiche. Make sure you are comfortable
distinguishing
them. Slide 173 has 2 round coverslips mounted on it. Both coverslips
have the same
type of cell growing on them - they are both cells that cause breast
tumors in rats. One of
the tumors is metastatic and the other is not. Which one do you think
is metastatic and
why? From what cell type might this tumor originate and why?
In addition to examples of these cells lines, you have an example of
a tumor grown
in a rat injected with the non-metastatic cell line (Slide 174). Observe
this tumor. Can you
tell what kind of cells form the tumor? How are they arranged? How
do these cells
compare to other epithelial cells you have observed in this lab?
Search for this type of epithelium in the coiled ducts of sweat glands
located deep in the
connective tissues of the skin Slides 76, 76-W.
9. Stratified Columnar Epithelium:
This type is extremely rare, but may occasionally be found in some sections
of the male
urethra Slide 121. It is much better appreciated in atlas pictures
and microfiche.
Additional Examples:
Simple Squamous Epithelium:
All sections with blood vessels - best appreciated in the capillaries
and blood vessels of the
heart Slide 26.
Stratified Squamous Epithelium:
Slide 76, 76W (scalp)
Slide 77 (thick skin)
Slide 170 (nipple)
Slides 43 (esophagus)
Slide 164 (cervix)
Simple Columnar:
Slide 54 (ileum)
Slide 71, 71W (gall bladder)
Pseudostratified Columnar Epithelium
Slide 109 (lung)
In addition to the above, slide 3, a cross section through the esophagus
and trachea,
exhibits a number of different epithelial types. See how many you can
find. This slide is
rather complicated. A labeled photograph of it can be found in your
slide descriptions.