Download epithelial cell - David Geffen School of Medicine at UCLA

Epithelial Tissue
Elena Stark, MD, PhD
Stephen Schettler, PhD
Department of Pathology & Laboratory Medicine
David Geffen School of Medicine at UCLA
Contributor: Paul Frank DGSOM Class of 2014
THIS FILE HAS COPYRIGHTED MATERIAL. IT IS INTENDED FOR YOUR
USE ONLY. DO NOT DISTRIBUTE OR SHARE IT. Nongraphical content
and organization Copyright 2012 M.E. Stark, MD, PhD.
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1. Epithelial Tissue – Introduction
2. Epithelia – Types
2.1 Epithelia – Types – Simple Squamous
2.1/2.2 Epithelia – Types – Simple Squamous/Simple Cuboidal – Visualization
2.2 Epithelia – Types – Simple Cuboidal
2.3 Epithelia – Types – Simple Columnar
2.4 Epithelia – Types – Stratified Squamous
2.5 Epithelia – Types – Pseudostratified
2.6 Epithelia – Types – Transitional
2.7 Epithelia – Types – Recap; Next Topics
3. Epithelia – Cell Surfaces
3.1 Epithelia – Cell Surfaces – Lateral Surface
3.1.1 Epithelia – Cell Surfaces – Lateral Surface – Zonulae Adherentes
3.1.2 Epithelia – Cell Surfaces – Lateral Surface – Desmosomes
3.1.3 Epithelia – Cell Surfaces – Lateral Surface – Tight Junctions
3.1.4 Epithelia – Cell Surfaces – Lateral Surface – Gap Junctions
3.2 Epithelia – Cell Surfaces – Cell Junctions, Image
3.3 Epithelia – Cell Surfaces – Recap; Next Topics
4. Epithelia – Basement Membrane
4.1 Epithelia – Basement Membrane – Hemidesmosomes
4.2 Epithelia – Basement Membrane – Recap; Next Topics
5. Epithelia – Exocrine Glands
5.1 Epithelia – Exocrine Glands – Shape of Duct
5.2 Epithelia – Exocrine Glands – Shape of Secretory Portion
5.3 Epithelia – Exocrine Glands – Mode of Secretion
5.4 Epithelia – Exocrine Glands – Type of Secretion
5.5 Epithelia – Exocrine Glands – Adenocarcinoma
5.6 Epithelia – Exocrine Glands – Recap; Next Topics
6. Epithelia – Endothelium and Mesothelium
7. Epithelial Tissue – Image Sources
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1. Epithelial Tissue – Introduction
Epithelia are present in all organs of the body, in conjunction with the other 3 types
of tissue. Epithelia usually sit on a layer of connective tissue (which usually sits on
muscle tissue) with nerves interspersed throughout.
The term epithelia (singular: epithelium) refers to specialized
cells that:
a. Cover all the surfaces of the body (ie, skin, gums)
b. Line the lumen, or internal cavity, of hollow organs
(ie, lining of blood vessels, lining of the digestive tract).
As you will see throughout your medical school career, all
organs of the body are compartmentalized and lined by
something. In almost every case, the something is a layer of
epithelial cells.
As an example, the image at right shows a cross-section of
the wall of the heart, which is formed by the endocardium,
myocardium, and the epicardium.
The external surface of the heart (the surface exposed to the
chest cavity) is covered by a layer of cells (epithelium) that
covers and constitutes part of the epicardium
.
Meanwhile, the internal cavity of the heart – called the lumen –
is exposed to the blood being pumped through the heart.
The lumen is lined by a layer of cells (epithelium) that
covers and constitutes part of the endocardium
,
this particular epithelium has a special name: endothelium
(more on this later).
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1. Epithelial Tissue – Introduction
To illustrate the general concept, here is another example of how epithelia line and
cover organs. Also note other tissues in between layers of epithelia, such as
connective tissue layers, muscle layers, and nerves interspersed throughout.
Don’t worry about knowing which is which at this point.
The figure illustrates the concept of epithelia
lining and covering organs. (We do NOT
expect you to know which layer is which now,
we will study all those layers and structures in
detail in Block 3.) The point here is to show
you that the digestive tract is lined by an
epithelium
and covered by another
epithelium
.
Between these epithelia we observe
connective tissue; and sandwiched in
between are layers of muscle (depicted as the
darker red layers); with nerves (depicted as
yellow lines or meshes) interspersed.
The figure is a portion of the small intestine.
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1. Epithelial Tissue – Introduction
Remembering from the module “Introduction to Histological Tissues,” epithelia have abundant
cells
and a very small amount of extracellular material (ECM) .
This is a high magnification (about x600) of the epithelium in the respiratory tract.
(Ignore the i.ds in the image.)
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2. Epithelia – Types
There are different types of epithelia.
Try to think of how the structure of each type of epithelium may relate to its function.
Epithelia are typically classified by the following
criteria:
– Number of layers of cells
– Shape of the cells
If there is only 1 layer of cells, the epithelium is
called simple. If there are more, the epithelium is
called stratified.
Stratified epithelia are named for the shape of the
cells on the top layer.
There are 3 possible shapes of epithelial cells:
– Squamous (flat-shaped)
– Cuboidal (square-cube-shaped)
– Columnar (column-shaped)
Notice that all 3 of these illustrations (to the right) are
of simple epithelia (ie, only 1 layer of cells).
Under the cell layer, there are other layers (here in
dark orange and pink) – most likely of connective
tissue.
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2. Epithelia – Types
Simple or stratified is combined with the cell shape to name the different epithelia.
Thus we have:
• Simple squamous: 1 layer of flat cells
• Simple cuboidal: 1 layer of square-cube-shaped cells
• Simple columnar: 1 layer of column-shaped cells
• Stratified squamous: 2 or more layers of cells where the top layer has flat cells
Stratified cuboidal and stratified columnar epithelia do exist, but are rare (they will be presented
in context, as they arise in later blocks).
Note: there are 2 other types of epithelia: transitional and pseudostratified. These epithelia are
NOT described as simple or stratified. (We will talk about them later in this module.)
Simple squamous
Simple cuboidal
Simple columnar
Stratified squamous
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2.1 Epithelia – Types – Simple Squamous
Now that we have the basic concept of the different types of epithelia, let’s look at them in histological
slides. We will take these tissues 1 at a time. This is very important. Start training your eye to real
histological images, understanding what you are observing.
Simple squamous: 1 layer of flat cells
Examples of locations:
• Lining of blood and lymph vessels, and heart
• Air sacs of lungs
FORM FITS FUNCTION: The relatively thinness of
this epithelium allows oxygen to diffuse through and
into the underlying tissue; nutrients can be rapidly
transported across, as well. In addition, it is possible
for some types of cells to “crawl” between the spaces
that exist between the cells of a simple squamous
epithelium.
ORIENT YOURSELF: To visualize the relationship
between the figure and the image, take the flat sheet
of cells (the figure at the top), mentally roll it into a
cylinder, and look through the cylinder – like a
telescope. The bottom image shows a cross-section of
a capillary, which is always lined with a simple
squamous epithelium, called endothelium.
(More on endothelium in later slides.)
A cell
Image: cross-section of a capillary.
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2.1/2.2 Epithelia – Types – Simple Squamous/Simple Cuboidal – Visualization
How to visualize the relationship between a flat sheet of cells and a cross-section.
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2.2 Epithelia – Types – Simple Cuboidal
Simple cuboidal epithelium is very similar to simple squamous, the only difference being that the cells
form a barrier that is a bit thicker.
Simple cuboidal: 1 layer of square-cube-shaped cells
Examples of locations:
• Nephrons
(tubules forming the internal architecture of kidneys)
• Some glands and their ducts
These epithelia line structures where secretion and/or
absorption take place.
FORM FITS FUNCTION: Cuboidal cells are actively
involved in transporting substances into or out of a
lumen, or into or out of the extracellular environment.
As such, their cuboidal shape is a function of the
additional cellular machinery required to carry out their
role.
ORIENT YOURSELF: To visualize the relationship
between the figure and the image, take the flat sheet
of cells (the figure at the top), mentally roll it into a
cylinder, and look through the cylinder – like a
telescope. The bottom image shows a cross-section of
a portion of a nephron, which is the functional unit of
the kidneys (to be covered in Block 2).
A cell
Image: cross-section of a portion of a
nephron.
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2.3 Epithelia – Types – Simple Columnar
Simple columnar epithelium is very similar to simple cuboidal, the only difference being that the cells
form a barrier that is even thicker.
Simple columnar: 1 layer of column-shaped cells
Examples of locations:
• Lining of most of the digestive tract
• Lining of the gallbladder
• Some glands
• Lining of small bronchi
• Lining of the uterine tubes
• Lining of portions of the uterus
FORM FITS FUNCTION: These epithelia are lining
structures where absorption and/or secretion of
substances take place.
ORIENT YOURSELF: The bottom image shows a
portion of the wall of a villus, which is a finger-like
projection found in the wall of the small intestine.
A cell
Longitudinal section.
Make note of the plane in the illustration (above).
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2.4 Epithelia – Types – Stratified Squamous
Stratified squamous epithelium usually contains many cell layers. The theme with stratified epithelia in
general is that they protect the underlying tissue.
Stratified squamous: 2 or more layers of cells where
the top layer is formed by flat cells
Examples of locations:
• Skin (shown in the image)
• Lining of the oral cavity and esophagus
• Lining of the vagina
Basal layer : the deepest layer of cells in the
epithelium. These cuboidal-appearing cells are the
stem cell layer for this tissue. They constantly divide to
replenish the lost or damaged layers above them.
FORM FITS FUNCTION: This type of epithelium
serves as a barrier for points of contact between the
outside world and the body, where physical forces or
exposure (to toxic chemicals, radiant energy, or
infectious agents) takes place.
These cells are replaced rapidly in order to maintain
an intact barrier with the outside world.
ORIENT YOURSELF: The bottom image shows skin
cells. (This will be covered in lab, later in Block 1.)
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2.5 Epithelia – Types – Pseudostratified
A
We mentioned earlier that there are 2 other types of
epithelia that are NOT described as simple or stratified.
The first type is pseudostratified epithelium.
A
B
Pseudostratified epithelia are comprised of a single
layer of cells attached to the same
basement
membrane**. The appearance is stratified because
this epithelium is made up of several different types of
cells of varying heights that become superimposed
over each other.
B
Examples of location:
• Upper respiratory tract (most commonly)
(This will be covered in detail in Block 2.)
ORIENT YOURSELF: The image at right shows an
example of pseudostratified epithelium of the trachea.
**All epithelia rest on a layer of tissue called basement
membrane (details later). For now, know that the layer
of cells in every simple epithelium and the deepest
layer of a stratified epithelium attach to a basement
membrane.
A cell
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2.6 Epithelia – Types – Transitional
The second type of epithelium that is NOT simple or stratified is transitional epithelium.
It changes appearance based on distension of the lumen of the organ it lines.
It is a specialized stratified epithelium.
Location:
• Unique to the urinary system (bladder, ureter,
urethra, etc) and appears different depending on
whether the organ it lines is full or empty.
When the organ is distended (full), the
epithelium resembles a simple squamous
epithelium.
When the organ is empty or relaxed, the
epithelium will appear stratified and cuboidal.
One key feature of a transitional epithelium in a
relaxed state is a “scalloped” appearance of the
lumenal layer of cells.
(We will go into more detail in Blocks 2 and 3.)
ORIENT YOURSELF: The top image shows the entire
wall of a full urinary bladder including its transitional
epithelium , which appears very flat because it is
distended (full).
The arrow is on the lumen (space within the bladder).
The bottom image shows a portion of the wall of an
empty urinary bladder. Notice the appearance of the
epithelium, it looks stratified cuboidal.
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2.7 Epithelia – Types – Recap; Next Topics
Topics covered so far:
•
The 6 most common types of epithelia and their very general locations and functions.
•
All epithelia have an abundance of cells and very little ECM. The cells are very close to
one another. In epithelia it is very important that the cells remain very close to one
another because of the tissue functions.
Topics to be covered next:
- Junctions – the structures that attach the cells to each other.
There are different types of junctions between epithelial cells.
- The basement membrane – a layer of connective tissue that epithelia rest on.
(This is a very important structure.)
- Exocrine glands – structures that secrete something to the exterior.
Sometimes epithelia fold to form glands. Glands in the human body are
structures that secrete something either to the exterior or into the bloodstream.
- Two specialized types of simple squamous epithelia – endothelium (plural:
endothelia) and mesothelium (plural: mesothelia).
(We will introduce these in this module, and encounter them again in future blocks.)
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3. Epithelia – Cell Surfaces
Epithelial cells are very close to one another and must remain in tight proximity.
Thus, there are structures between the cells that keep them “attached.”
The figure represents 2 epithelial cells. Here we can
see a simple cuboidal epithelium, but these concepts
apply to all epithelia.
ORIENT YOURSELF:
• Notice that the epithelial cells are “on top” of
another tissue, this is connective tissue – we call it
the basement membrane. (More on this later.)
• The space at the top of the figure would be an
opening or space, called the cavity. It is lined by
epithelial cells. If these cells were in the GI tract,
the space would be the lumen of the GI tract.
The epithelial cells have 3 sides:
1. Apical or lumenal surface
(this particular figure shows finger-like
projections, called cilia or microvilli – depending on
their size – but NOT all epithelial cells have these)
2. Lateral surface
3. Basal surface
Underlying connective tissue.
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3.1 Epithelia – Cell Surfaces – Lateral Surface
Epithelial cells have 3 surfaces. The apical or lumenal surface is closest to the lumen.
The basal surface is closest to the connective tissue underlying the epithelium.
The lateral surface is where the cell junctions are located.
The lateral surface of epithelial cells contains cell
junctions that attach contiguous epithelial cells to
one another.
In histology these junctions are classified as
“stronger” and “weaker”:
1. Stronger junctions hold cells tightly together; they
are called anchoring junctions.
There are 2 types of these:
- Zonulae adherentes
- Desmosomes
2. Weaker junctions assist in holding cells together.
There are 2 types of these:
- Tight junctions (the choice of words is confusing…
but that is what they are called…)
- Gap junctions
Underlying connective tissue.
The following slides elaborate on both strong and
weak junctions.
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3.1.1 Epithelia – Cell Surfaces – Lateral Surface – Zonulae Adherentes
Zonulae adherentes (singular: zonula adherens) are a type of “strong” or anchoring cell junctions.
Zonula Adherens
EPITHELIAL CELL
Actin
filaments
Anchor
proteins
Cadherins
(Ca2+ dependent)
EPITHELIAL CELL
Anchor
proteins
Actin
filaments
Zonulae adherentes
have the structure represented above
and at right:
– Notice the green lines in the figure above,
they represent transmembrane proteins. They go
through the membrane of each epithelial cell, linking the
contiguous epithelial cells. These proteins are cadherins.
They are attached to each cell and link to each other
(cadherin to cadherin) in the intercellular space.
– Cadherins of each epithelial cell also link to anchor
proteins in the cytoplasm, which in turn link to actin
filaments of the cytoskeleton of the cell.
Thus a zonula adherens has the following 3 components:
– Cadherins (transmembrane proteins)
– Anchor proteins
– Actin filaments (these are really part of the
cytoskeleton of the cell, but they contribute to the
zonula adherens)
Underlying connective tissue.
Zonulae adherentes tend to be close to the apical surface, but
are still on the lateral surface.
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3.1.2 Epithelia – Cell Surfaces – Lateral Surface – Desmosomes
Desmosomes are the other type of “strong” or anchoring cell junctions.
Desmosome
EPITHELIAL CELL
Keratin
filaments
Anchor
proteins
Cadherins
(Ca2+ dependent)
EPITHELIAL CELL
Anchor
proteins
Keratin
filaments
Desmosomes
have very similar structure to the
zonulae adherentes:
– Cadherins (transmembrane proteins)
– Anchor proteins
BUT instead of actin filaments they have
– Keratin filaments, also called intermediate
filaments
Keratin is much stronger than actin; thus,
desmosomes are stronger than zonulae
adherentes.
Desmosomes are described as forming button-like
junctions.
They tend to be positioned near a zonula adherens,
although towards the basal surface of the cell;
but still on the lateral surface.
Underlying connective tissue.
CLICK HERE FOR A CLOSER
VIEW OF A DESMOSOME.
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3.1.3 Epithelia – Cell Surfaces – Lateral Surface – Tight Junctions
Tight junctions are “weak” cell junctions.
Tight Junction
EPITHELIAL CELL
EPITHELIAL CELL
Transmembrane
protein
Transmembrane
protein
(Ca2+ dependent)
Tight junctions
Tight junctions are formed when the extracellular
domains of transmembrane proteins (on the lateral
surfaces of adjacent cells) interlock with the help of
calcium.
Some membrane structures (ie, ion channels) are
found only on the apical surface or only on the
basolateral surface. Tight junctions help keep these
structures on the proper surface, thus maintaining the
polarization of the cell.
Tight junctions can block the passage of lumenal
content between cells.
Underlying connective tissue.
The cytoskeleton of the cell does NOT contribute to
tight junctions.
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3.1.4 Epithelia – Cell Surfaces – Lateral Surface – Gap Junctions
Gap junctions are the other type of “weak” cell junctions.
Gap Junction
EPITHELIAL CELL
EPITHELIAL CELL
Connexon
Gap junctions
Gap junctions are formed by transmembrane
proteins, called connexins. Many connexin protein
chains form a connexon.
Gap junctions create a channel whereby cytoplasm
and its water-soluble contents can flow between cells
to couple the cells metabolically and electrically.
The cytoskeleton of the cell does NOT contribute to
the structure of gap junctions.
Underlying connective tissue.
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3.2 Epithelia – Cell Surfaces – Cell Junctions, Image
So far we have used an illustration to show epithelial cell junctions, as these are NOT visible through
the Light Microscope. Here we show a TEM image with several cell junctions.
Legend
TJ = tight junction
(referred to by some authors as zonula occludens)
ZA = zonula adherens
D = desmosome
TEM image of a portion of 2 epithelial cells.
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3.3 Epithelia – Cell Surfaces – Recap; Next Topics
Topics covered so far:
• The 6 most common types of epithelia and their very general
locations and functions.
• The different types of junctions between epithelial cells.
Topics to be covered next:
- The basement membrane – a connective tissue layer underlying an
epithelium.
-Exocrine glands – formed by an epithelium that folds in on itself; the cells
in that epithelium secrete something to the exterior of the body.
- Two specialized types of epithelia – endothelium (plural: endothelia) and
mesothelium (plural: mesothelia).
(We will introduce these in this module, and encounter them again in
future blocks.)
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4. Epithelia – Basement Membrane
All epithelia rest on a layer of connective tissue, called the basement membrane.
The basement membrane (BM) is a thin fibrous
connective tissue layer between the epithelium
and the underlying supporting tissue .
All epithelia are avascular (ie, they contain NO
blood vessels). In order to receive nutrition from the
blood, epithelia rest superficial to a layer of
supporting tissue which contains blood vessels. The
basement membrane is between the epithelium and
the supporting connective tissue. Blood vessels do
NOT penetrate the basement membrane; however,
it allows transudate (fluid leaked out of the
vasculature) to serve the cells of the overlying
epithelium. The basement membrane controls
permeability to and from the epithelium.
Nerves course through underlying tissue and must
penetrate the basement membrane in order to
innervate the epithelium. Thus, nerves do penetrate
the basement membrane.
Another very important function of the basement
membrane is that it controls epithelial growth,
impeding the epithelium from growing “downward.”
In malignant growth, the epithelium crosses over
the basement membrane.
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4. Epithelia – Basement Membrane
The basement membrane is produced by both the epithelium and the underlying supporting tissue.
The basement membrane is a non-cellular structure produced
by 2 types of cells:
• Epithelial cells that are supported by the basement membrane
• Fibroblasts, which are cells of the underlying supporting tissue
The primary components of the basement membrane are:
• Type IV collagen (unlike other types, collagen type IV forms
flat sheets, NOT cords)
• Heparan sulphate, which is a glycosaminoglycan
• Ground substance, which is a gel-like suspension of
glycoproteins and proteoglycans that includes
• Fibronectin
• Laminin
• Entactin
Laminin is the most common glycoprotein in the basement
membrane. It attaches to:
1- Other components of the basement membrane,
2- Transmembrane proteins in the epithelial cell, called integrins.
Integrins transmit information about external stress to the
nucleus of the epithelial cell. This is important when
responding to and recovering from injury.
BM
A basement membrane (BM) supporting a
pseudostratified epithelium (illustration and
image). Ignore the other i.ds.
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4.1 Epithelia – Basement Membrane – Hemidesmosomes
Hemidesmosomes link epithelial cells to the basement membrane.
Epithelia that experience high abrasive force are
reinforced by junctions called hemidesmosomes,
which are similar to desmosomes in the lateral surface
of adjacent epithelial cells (as previously described).
Refer to the previous slide, as we describe the main
components of a hemidesmosome:
1- Laminin in the basement membrane
2- Anchor proteins
3- Integrin proteins in the epithelial cells that happen to
be attached to keratin of the cytoskeleton of the
epithelial cell.
NOTE: With Light Microssopes basement membranes can be seen if they are thick or if they have been
specifically stained. The basement membrane
in this image is particularly thick.
With EM, the 3 sublayers of a basement membrane can be seen. While the distinction of these
constitutes too much detail for our purposes, know that these layers are called:
• Lamina lucida
• Lamina densa (or basal lamina**)
• Lamina fibroreticularis
**Some authors refer to the basement membrane as the basal lamina (1 of its 3 sublayers).
However, this may lead to confusion, so we will refer to the basement membrane as the basement membrane.
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4.2 Epithelia – Basement Membrane – Recap; Next Topics
Topics covered so far:
•
The 6 most common types of epithelia and their very general locations and
functions.
•
The different types of junctions that attach epithelial cells to each other.
•
The basement membrane, which is a connective tissue layer underlying an
epithelium.
Topics to be covered next:
- Exocrine glands – formed by an epithelium that folds on itself; the cells in that
epithelium secrete something to the exterior.
-Two specialized types of epithelia – endothelium (plural: endothelia) and
mesothelium (plural: mesothelia).
(We will introduce these in this module, and encounter them again in future
blocks.)
27
5. Epithelia – Exocrine Glands
Glands in the human body are structures that secrete something either into the exterior (then
called exocrine glands) or into the bloodstream (then called endocrine glands).
Only exocrine glands are presented here (however, we will discuss endocrine glands in Block 3).
Epithelia can invaginate, or fold inward, to form
exocrine glands. For example, mammary
glands, digestive glands, and sweat glands all
arise from epithelial tissue.
These glands secrete their products into ducts
that ultimately lead to the exterior of the body
(and NOT into the blood stream).
Can you see a folded epithelium in the figure?
28
5. Epithelia – Exocrine Glands
Exocrine glands are formed by epithelial cells that specialize in secreting some substance
to the exterior.
Exocrine glands are formed when the cells in an
epithelium proliferate and invaginate.
The gland has a space in the center called the
lumen.
The cells closest to the surface, in conjunction
with the corresponding lumen, form the duct of
the gland .
The rest of the gland is the secretory portion .
Observe the apical surface of the cells
in the gland.
Observe the lateral surface of the cells
in the gland.
Observe the basal surface of the cells
in the gland.
The gland is surrounded by blood capillaries
.
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5.1 Epithelia – Exocrine Glands – Shape of the Duct
Exocrine glands are classified in a number of ways according to the:
1- Shape of the duct
2- Shape of the secretory portion
3- Mode of secretion
4- Type of secretion
Exocrine glands are classified, as the following, according to the shape of their duct:
•Short
•Long and coiled
(We will elaborate in lab.)
Short duct
Long coiled duct
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5.1 Epithelia – Exocrine Glands – Shape of the Duct
Notice how a cut through a coiled duct looks on a slide.
Keep this in mind when looking at virtual slides in lab.
The illustration does NOT match perfectly, as this is only to show the concept.
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5.2 Epithelia – Exocrine Glands – Shape of the Secretory Portion
Exocrine glands are classified in a number of ways according to the:
1- Shape of the duct
2- Shape of the secretory portion
3- Mode of secretion
4- Type of secretion
Exocrine glands can be described, as the following, according to the anatomical arrangement of
the gland cells – ie, the shape of the secretory portion:
•Acinar or alveolar (glands of this type resemble bunches of grapes)
•Tubular (glands of this type are simple tube-like, U-shaped portion)
Acinar gland.
Acinar gland
cross-section.
;
Tubular gland.
In the images, note the difference between a cross-section and a longitudinal section.
Tubular gland
longitudinal or
sagittal section.
;
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5.3 Epithelia – Exocrine Glands – Mode of Secretion
Exocrine glands are classified in a number of ways according to the:
1- Shape of the duct
2- Shape of the secretory portion
3- Mode of secretion
4- Type of secretion
Exocrine glands can also be described according to the gland cells’ mode of secretion
(there are 3 modes):
1- Merocrine – the secretory product is released from the cell via exocytosis
Merocrine
33
5.3 Epithelia – Exocrine Glands – Mode of Secretion
Exocrine glands are classified in a number of ways according to the:
1- Shape of the duct
2- Shape of the secretory portion
3- Mode of secretion
4- Type of secretion
Exocrine glands can also be described according to the gland cells’ mode of secretion
(there are 3 modes):
2- Apocrine – the apical portion of the cell, which contains the secretory product, breaks away
from the rest of the cell
Apocrine
34
5.3 Epithelia – Exocrine Glands – Mode of Secretion
Exocrine glands are classified in a number of ways according to the:
1- Shape of the duct
2- Shape of the secretory portion
3- Mode of secretion
4- Type of secretion
Exocrine glands can also be described according to the gland cells’ mode of secretion
(there are 3 modes):
3- Holocrine – the entire cell breaks apart, beginning with the rupture of the plasma membrane;
the secretory product is released into the duct (along with fragments of the dead cell)
Holocrine
35
5.4 Epithelia – Exocrine Glands – Type of Secretion
Exocrine glands are classified in a number of ways according to the:
1- Shape of the duct
2- Shape of the secretory portion
3- Mode of secretion
4- Type of secretion
Finally, exocrine glands are classified, as the
following, according to the type of secretion:
•Mucous glands – secrete mucus, which is rich
in mucigens (large glycosolated proteins that
absorb water and, as a result, produce a thick
gel-like material).
•Serous glands – secrete a viscous fluid rich
in proteins (that are often enzymes) not as
thick as mucus.
Other exocrine glands are specialized in
secreting substances like sweat (sweat
glands), sebum/oil (sebaceous glands), or milk
(mammary glands).
(We will cover these in more detail as we proceed
through the different systems in Blocks 2 and 3.)
In general with use of H&E, serous cells
stain darker than mucous cells
. The exact shade of color
is dependent on the sample, as there can be variations – depending on the composition of the secretion,
and on the degree in which the sample “takes the stain in.”
In this course, we will give you examples that are clearly lighter (for mucous cells) and clearly darker (for
serous cells).
36
5.5 Epithelia – Exocrine Glands – Adenocarcinoma
Tumors developing from glandular epithelium are called adenocarcinomas. Because there are
epithelial glands in many organs – such as the stomach, uterus, colon, pancreas, liver, prostate,
breast, and thyroid – we refer to adenocarcinomas in these organs when the epithelial cells in their
respective glands become neoplastic.
In some adenocarcinomas, the epithelial cells
have a fairly normal morphology; but they
secrete abnormal amounts of mucus.
In other adenocarcinomas, the epithelial cells
form very atypical glands that have abnormal
cell morphology.
•Normal glandular epithelium
in the endocervix
•Adenocarcinoma
(Observe the difference in the glandular epithelial cells;
don’t worry about the details at this point, you’ll learn
them next year, just focus on the concept for now).
37
5.6 Epithelia – Exocrine Glands – Recap; Next Topics
Topics covered so far:
•
The 6 most common types of epithelia and their very general locations and
functions.
•
The different types of junctions that attach epithelial cells to each other.
•
The basement membrane, which is a connective tissue layer underlying an
epithelium.
•
Exocrine glands – mucous glands, serous glands; sweat glands,
sebaceous glands, and mammary glands – are formed by an epithelium
that folds on itself; the cells in that epithelium secrete something to the
exterior.
Topics to be covered next:
- Two specialized types of epithelia – endothelium (plural: endothelia) and
mesothelium
(plural: mesothelia) – these are considered membranes.
(We will give examples in Blocks 2 and 3; as here, only the concept is important.)
38
6. Epithelia – Endothelium and Mesothelium
Endothelium (plural: endothelia) and mesothelium (plural: mesothelia) are simple squamous epithelia
that are specialized. Remember: simple squamous epithelia are formed by 1 layer of flat cells;
and line and cover organs.
Endothelium is the simple squamous
epithelium lining that is specific to:
the walls of the heart, blood vessels
(arteries, veins, and capillaries), and
lymphatic vessels.
Endothelium :
Notice that in arteries, veins, and
capillaries, the most internal layer (lining)
is an endothelium.
(Ignore the labels; just focus on the
concept.)
39
6. Epithelia – Endothelium and Mesothelium
Endothelium (plural: endothelia) and mesothelium (plural: mesothelia) are simple squamous epithelia
that are specialized. Remember: simple squamous epithelia are formed by 1 layer of flat cells;
and line and cover organs.
Mesothelium is a simple squamous epithelium that covers certain
organs in the body and lines internal body cavities (lines CAVITIES;
NOT organs).
Cells forming mesothelia often secrete a substance called serous
(viscous) fluid; thus, mesothelia are also referred to as serous
membranes.
Visualize the thoracic cavity lined by a mesothelium. The thoracic wall is
internally lined by this tissue, which goes on to cover the outer aspect of
the organs in the thoracic cavity – much like a blanket draping over the
organs. Thus, the internal thoracic wall, the lungs, and heart are all
covered by this mesothelium. Similarly, organs in the abdomen and
pelvis are covered by a mesothelium. (If this is not clear, ask in lab.)
The image at top shows a portion of an ovary – the pink part is the
organ, the whitish-blue portion on the right side of the image is the pelvic
cavity.
Notice that the outermost layer (covering) of the ovary is
a mesothelium .
The bottom figure illustrates the location of mesothelium
lining
the thoracic wall and covering the lungs (real images shown in lab).
(Ignore the labels; just focus on the concept.)
Image: www.faculty.une.edu
40
7. Epithelial Tissue – Image Sources
1.
Mescher AL. Junqueira’s Basic Histology. 12th ed.
McGraw-Hill Medical; August 28, 2009.
2.
Young B, et al. Wheater’s Functional Histology. 5th ed.
Churchill Livingstone, Elsevier Limited; March 14, 2006.
3.
Young B, et al. Wheater’s Basic Pathology. 5th ed.
Churchill Livingstone, Elsevier Limited; December 15, 2009, 2010.
4.
Gartner LP, Hiatt JL. Color Textbook of Histology. 3rd ed.
W.B. Saunders Company; November 17, 2006.
5.
University of New England, Biddeford & Portland, Maine; Westbrook College of
Health Professions. University of New England Web site.
http://www.faculty.une.edu. 2011.
6.
University of Connecticut Health Center; Radiology, Department of Diagnostic
Imaging and Therapeutics. University of Connecticut Health Center Web site.
http://www.radiology.uchc.edu. 2011.
41
42
BACK
1. Epithelial Tissue – Introduction
Epithelia are present in all organs of the body, in conjunction with the other 3 types
of tissue. Epithelia usually sit on a layer of connective tissue (which usually sits on
muscle tissue) with nerves interspersed throughout.
The term epithelia (singular: epithelium) refers to specialized
cells that:
a. Cover all the surfaces of the body (ie, skin, gums)
b. Line the lumen, or internal cavity, of hollow organs
(ie, lining of blood vessels, lining of the digestive tract)
As you will see throughout your medical school career, all
organs of the body are compartmentalized and lined by
something. In almost every case, the something is a layer of
epithelial cells.
As an example, the image at right shows a cross-section of
the wall of the heart, which is formed by the endocardium,
myocardium, and the epicardium.
The external surface of the heart (the surface exposed to the
chest cavity) is covered by a layer of cells (epithelium) that
covers and constitutes part of the epicardium .
Meanwhile, the internal cavity of the heart – called the lumen –
is exposed to the blood being pumped through the heart.
The lumen is lined by a layer of cells (epithelium) that
covers and constitutes part of the endocardium
,
this particular epithelium has a special name: endothelium
(more on this later).
43
BACK
1. Epithelial Tissue – Introduction
Epithelia are present in all organs of the body, in conjunction with the other 3 types
of tissue. Epithelia usually sit on a layer of connective tissue (which usually sits on
muscle tissue) with nerves interspersed throughout.
The term epithelia (singular: epithelium) refers to specialized
cells that:
a. Cover all the surfaces of the body (ie, skin, gums)
b. Line the lumen, or internal cavity, of hollow organs
(ie, lining of blood vessels, lining of the digestive tract)
As you will see throughout your medical school career, all
organs of the body are compartmentalized and lined by
something. In almost every case, the something is a layer of
epithelial cells.
As an example, the image at right shows a cross-section of
the wall of the heart, which is formed by the endocardium,
myocardium, and the epicardium.
The external surface of the heart (the surface exposed to the
chest cavity) is covered by a layer of cells (epithelium) that
covers and constitutes part of the epicardium .
Meanwhile, the internal cavity of the heart – called the lumen –
is exposed to the blood being pumped through the heart.
The lumen is lined by a layer of cells (epithelium) that
covers and constitutes part of the endocardium
,
this particular epithelium has a special name: endothelium
(more on this later).
44
BACK
1. Epithelial Tissue – Introduction
To illustrate the general concept, here is another example of how epithelia line and
cover organs. Also note other tissues in between layers of epithelia, such as
connective tissue layers, muscle layers, and nerves interspersed throughout.
Don’t worry about knowing which is which at this point.
The figure illustrates the concept of epithelia
lining and covering organs. (We do NOT
expect you to know which layer is which now,
we will study all those layers and structures in
detail in Block 3.) The point here is to show
you that the digestive tract is lined by an
epithelium
and covered by another
epithelium
.
Between these epithelia we observe
connective tissue; and sandwiched in
between are layers of muscle (depicted as the
darker red layers); with nerves (depicted as
yellow lines or meshes) interspersed.
The figure is a portion of the small intestine.
45
BACK
1. Epithelial Tissue – Introduction
To illustrate the general concept, here is another example of how epithelia line and
cover organs. Also note other tissues in between layers of epithelia, such as
connective tissue layers, muscle layers, and nerves interspersed throughout.
Don’t worry about knowing which is which at this point.
The figure illustrates the concept of epithelia
lining and covering organs. (We do NOT
expect you to know which layer is which now,
we will study all those layers and structures in
detail in Block 3.) The point here is to show
you that the digestive tract is lined by an
epithelium
and covered by another
epithelium
.
Between these epithelia we observe
connective tissue; and sandwiched in
between are layers of muscle (depicted as the
darker red layers); with nerves (depicted as
yellow lines or meshes) interspersed.
The figure is a portion of the small intestine.
46
BACK
2. Epithelia – Types
There are different types of epithelia.
Try to think of how the structure of each type of epithelium may relate to its function.
Epithelia are typically classified by the following
criteria:
– Number of layers of cells
– Shape of the cells
If there is only 1 layer of cells, the epithelium is
called simple. If there are more, the epithelium is
called stratified.
Stratified epithelia are named for the shape of the
cells on the top layer.
There are 3 possible shapes of epithelial cells:
– Squamous (flat-shaped)
– Cuboidal (square-cube-shaped)
– Columnar (column-shaped)
Notice that all 3 of these illustrations (to the right) are
of simple epithelia (ie, only 1 layer of cells).
Under the cell layer, there are other layers (here in
dark orange and pink) – most likely of connective
tissue.
47
BACK
2. Epithelia – Types
There are different types of epithelia.
Try to think of how the structure of each type of epithelium may relate to its function.
Epithelia are typically classified by the following
criteria:
– Number of layers of cells
– Shape of the cells
If there is only 1 layer of cells, the epithelium is
called simple. If there are more, the epithelium is
called stratified.
Stratified epithelia are named for the shape of the
cells on the top layer.
There are 3 possible shapes of epithelial cells:
– Squamous (flat-shaped)
– Cuboidal (square-cube-shaped)
– Columnar (column-shaped)
Notice that all 3 of these illustrations (to the right) are
of simple epithelia (ie, only 1 layer of cells).
Under the cell layer, there are other layers (here in
dark orange and pink) – most likely of connective
tissue.
48
BACK
2. Epithelia – Types
There are different types of epithelia.
Try to think of how the structure of each type of epithelium may relate to its function.
Epithelia are typically classified by the following
criteria:
– Number of layers of cells
– Shape of the cells
If there is only 1 layer of cells, the epithelium is
called simple. If there are more, the epithelium is
called stratified.
Stratified epithelia are named for the shape of the
cells on the top layer.
There are 3 possible shapes of epithelial cells:
– Squamous (flat-shaped)
– Cuboidal (square-cube-shaped)
– Columnar (column-shaped)
Notice that all 3 of these illustrations (to the right) are
of simple epithelia (ie, only 1 layer of cells).
Under the cell layer, there are other layers (here in
dark orange and pink) – most likely of connective
tissue.
49
BACK
2.1 Epithelia – Types – Simple Squamous
Now that we have the basic concept of the different types of epithelia, let’s look at them in histological
slides. We will take these tissues 1 at a time. This is very important. Start training your eye to real
histological images, understanding what you are observing.
Simple squamous: 1 layer of flat cells
Examples of locations:
• Lining of blood and lymph vessels, and heart
• Air sacs of lungs
FORM FITS FUNCTION: The relatively thinness of
this epithelium allows oxygen to diffuse through and
into the underlying tissue; nutrients can be rapidly
transported across, as well. In addition, it is possible
for some types of cells to “crawl” between the spaces
that exist between the cells of a simple squamous
epithelium.
ORIENT YOURSELF: To visualize the relationship
between the figure and the image, take the flat sheet
of cells (the figure at the top), mentally roll it into a
cylinder, and look through the cylinder – like a
telescope. The bottom image shows a cross-section of
a capillary, which is always lined with a simple
squamous epithelium, called endothelium.
(More on endothelium in later slides.)
A cell
Image: cross-section of a capillary.
50
BACK
2.2 Epithelia – Types – Simple Cuboidal
Simple cuboidal epithelium is very similar to simple squamous, the only difference being that the cells
form a barrier that is a bit thicker.
Simple cuboidal: 1 layer of square-cube-shaped cells
Examples of locations:
• Nephrons
(tubules forming the internal architecture of kidneys)
• Some glands and their ducts
These epithelia line structures where secretion and/or
absorption take place.
FORM FITS FUNCTION: Cuboidal cells are actively
involved in transporting substances into or out of a
lumen, or into or out of the extracellular environment.
As such, their cuboidal shape is a function of the
additional cellular machinery required to carry out their
role.
ORIENT YOURSELF: To visualize the relationship
between the figure and the image, take the flat sheet
of cells (the figure at the top), mentally roll it into a
cylinder, and look through the cylinder – like a
telescope. The bottom image shows a cross-section of
a portion of a nephron, which is the functional unit of
the kidneys (to be covered in Block 2).
A cell
Image: cross-section of a portion of a
nephron.
51
BACK
2.3 Epithelia – Types – Simple Columnar
Simple columnar epithelium is very similar to simple cuboidal, the only difference being that the cells
form a barrier that is even thicker.
Simple columnar: 1 layer of column-shaped cells
Examples of locations:
• Lining of most of the digestive tract
• Lining of the gallbladder
• Some glands
• Lining of small bronchi
• Lining of the uterine tubes
• Lining of portions of the uterus
FORM FITS FUNCTION: These epithelia are lining
structures where absorption and/or secretion of
substances take place.
ORIENT YOURSELF: The bottom image shows a
portion of the wall of a villus, which is a finger-like
projection found in the wall of the small intestine.
A cell
Longitudinal section.
Make note of the plane in the illustration (above).
52
BACK
2.4 Epithelia – Types – Stratified Squamous
Stratified squamous epithelium usually contains many cell layers. The theme with stratified epithelia in
general is that they protect the underlying tissue.
Stratified squamous: 2 or more layers of cells where
the top layer is formed by flat cells
Examples of locations:
• Skin (shown in the image)
• Lining of the oral cavity and esophagus
• Lining of the vagina
Basal layer : the deepest layer of cells in the
epithelium. These cuboidal-appearing cells are the
stem cell layer for this tissue. They constantly divide to
replenish the lost or damaged layers above them.
FORM FITS FUNCTION: This type of epithelium
serves as a barrier for points of contact between the
outside world and the body, where physical forces or
exposure (to toxic chemicals, radiant energy, or
infectious agents) takes place.
These cells are replaced rapidly in order to maintain
an intact barrier with the outside world.
ORIENT YOURSELF: The bottom image shows skin
cells. (This will be covered in lab, later in Block 1.)
53
BACK
2.4 Epithelia – Types – Stratified Squamous
Stratified squamous epithelium usually contains many cell layers. The theme with stratified epithelia in
general is that they protect the underlying tissue.
Stratified squamous: 2 or more layers of cells where
the top layer is formed by flat cells
Examples of locations:
• Skin (shown in the image)
• Lining of the oral cavity and esophagus
• Lining of the vagina
Basal layer : the deepest layer of cells in the
epithelium. These cuboidal-appearing cells are the
stem cell layer for this tissue. They constantly divide to
replenish the lost or damaged layers above them.
FORM FITS FUNCTION: This type of epithelium
serves as a barrier for points of contact between the
outside world and the body, where physical forces or
exposure (to toxic chemicals, radiant energy, or
infectious agents) takes place.
These cells are replaced rapidly in order to maintain
an intact barrier with the outside world.
ORIENT YOURSELF: The bottom image shows skin
cells. (This will be covered in lab, later in Block 1.)
54
BACK
2.5 Epithelia – Types – Pseudostratified
A
We mentioned earlier that there are 2 other types of
epithelia that are NOT described as simple or stratified.
The first type is pseudostratified epithelium.
A
B
Pseudostratified epithelia are comprised of a single
layer of cells attached to the same
basement
membrane**. The appearance is stratified because
this epithelium is made up of several different types of
cells of varying heights that become superimposed
over each other.
B
Examples of location:
• Upper respiratory tract (most commonly)
(This will be covered in detail in Block 2.)
ORIENT YOURSELF: The image at right shows an
example of pseudostratified epithelium of the trachea.
**All epithelia rest on a layer of tissue called basement
membrane (details later). For now, know that the layer
of cells in every simple epithelium and the deepest
layer of a stratified epithelium attach to a basement
membrane.
A cell
55
BACK
2.5 Epithelia – Types – Pseudostratified
A
We mentioned earlier that there are 2 other types of
epithelia that are NOT described as simple or stratified.
The first type is pseudostratified epithelium.
A
B
Pseudostratified epithelia are comprised of a single
layer of cells attached to the same
basement
membrane**. The appearance is stratified because
this epithelium is made up of several different types of
cells of varying heights that become superimposed
over each other.
B
Examples of location:
• Upper respiratory tract (most commonly)
(This will be covered in detail in Block 2.)
ORIENT YOURSELF: The image at right shows an
example of pseudostratified epithelium of the trachea.
**All epithelia rest on a layer of tissue called basement
membrane (details later). For now, know that the layer
of cells in every simple epithelium and the deepest
layer of a stratified epithelium attach to a basement
membrane.
A cell
56
BACK
2.6 Epithelia – Types – Transitional
The second type of epithelium that is NOT simple or stratified is transitional epithelium.
It changes appearance based on distension of the lumen of the organ it lines.
It is a specialized stratified epithelium.
Location:
• Unique to the urinary system (bladder, ureter,
urethra, etc) and appears different depending on
whether the organ it lines is full or empty.
When the organ is distended (full), the
epithelium resembles a simple squamous
epithelium.
When the organ is empty or relaxed, the
epithelium will appear stratified and cuboidal.
One key feature of a transitional epithelium in a
relaxed state is a “scalloped” appearance of the
lumenal layer of cells.
(We will go into more detail in Blocks 2 and 3.)
ORIENT YOURSELF: The top image shows the entire
wall of a full urinary bladder including its transitional
epithelium , which appears very flat because it is
distended (full).
The arrow is on the lumen (space within the bladder).
The bottom image shows a portion of the wall of an
empty urinary bladder. Notice the appearance of the
epithelium, it looks stratified cuboidal.
57
BACK
3. Epithelia – Cell Surfaces
Epithelial cells are very close to one another and must remain in tight proximity.
Thus, there are structures between the cells that keep them “attached.”
The figure represents 2 epithelial cells. Here we can
see a simple cuboidal epithelium, but these concepts
apply to all epithelia.
ORIENT YOURSELF:
• Notice that the epithelial cells are “on top” of
another tissue, this is connective tissue – we call it
the basement membrane. (More on this later.)
• The space at the top of the figure would be an
opening or space, called the cavity. It is lined by
epithelial cells. If these cells were in the GI tract,
the space would be the lumen of the GI tract.
The epithelial cells have 3 sides:
1. Apical or lumenal surface
(this particular figure shows finger-like
projections, called cilia or microvilli – depending on
their size – but NOT all epithelial cells have these)
2. Lateral surface
3. Basal surface
Underlying connective tissue.
58
BACK
3. Epithelia – Cell Surfaces
Epithelial cells are very close to one another and must remain in tight proximity.
Thus, there are structures between the cells that keep them “attached.”
The figure represents 2 epithelial cells. Here we can
see a simple cuboidal epithelium, but these concepts
apply to all epithelia.
ORIENT YOURSELF:
• Notice that the epithelial cells are “on top” of
another tissue, this is connective tissue – we call it
the basement membrane. (More on this later.)
• The space at the top of the figure would be an
opening or space, called the cavity. It is lined by
epithelial cells. If these cells were in the GI tract,
the space would be the lumen of the GI tract.
The epithelial cells have 3 sides:
1. Apical or lumenal surface
(this particular figure shows finger-like
projections, called cilia or microvilli – depending on
their size – but NOT all epithelial cells have these)
2. Lateral surface
3. Basal surface
Underlying connective tissue.
59
BACK
3. Epithelia – Cell Surfaces
Epithelial cells are very close to one another and must remain in tight proximity.
Thus, there are structures between the cells that keep them “attached.”
The figure represents 2 epithelial cells. Here we can
see a simple cuboidal epithelium, but these concepts
apply to all epithelia.
ORIENT YOURSELF:
• Notice that the epithelial cells are “on top” of
another tissue, this is connective tissue – we call it
the basement membrane. (More on this later.)
• The space at the top of the figure would be an
opening or space, called the cavity. It is lined by
epithelial cells. If these cells were in the GI tract,
the space would be the lumen of the GI tract.
The epithelial cells have 3 sides:
1. Apical or lumenal surface
(this particular figure shows finger-like
projections, called cilia or microvilli – depending on
their size – but NOT all epithelial cells have these)
2. Lateral surface
3. Basal surface
Underlying connective tissue.
60
BACK
3. Epithelia – Cell Surfaces
Epithelial cells are very close to one another and must remain in tight proximity.
Thus, there are structures between the cells that keep them “attached.”
The figure represents 2 epithelial cells. Here we can
see a simple cuboidal epithelium, but these concepts
apply to all epithelia.
ORIENT YOURSELF:
• Notice that the epithelial cells are “on top” of
another tissue, this is connective tissue – we call it
the basement membrane. (More on this later.)
• The space at the top of the figure would be an
opening or space, called the cavity. It is lined by
epithelial cells. If these cells were in the GI tract,
the space would be the lumen of the GI tract.
The epithelial cells have 3 sides:
1. Apical or lumenal surface
(this particular figure shows finger-like
projections, called cilia or microvilli – depending on
their size – but NOT all epithelial cells have these)
2. Lateral surface
3. Basal surface
Underlying connective tissue.
61
BACK
3. Epithelia – Cell Surfaces
Epithelial cells are very close to one another and must remain in tight proximity.
Thus, there are structures between the cells that keep them “attached.”
The figure represents 2 epithelial cells. Here we can
see a simple cuboidal epithelium, but these concepts
apply to all epithelia.
ORIENT YOURSELF:
• Notice that the epithelial cells are “on top” of
another tissue, this is connective tissue – we call it
the basement membrane. (More on this later.)
• The space at the top of the figure would be an
opening or space, called the cavity. It is lined by
epithelial cells. If these cells were in the GI tract,
the space would be the lumen of the GI tract.
The epithelial cells have 3 sides:
1. Apical or lumenal surface
(this particular figure shows finger-like
projections, called cilia or microvilli – depending on
their size – but NOT all epithelial cells have these)
2. Lateral surface
3. Basal surface
Underlying connective tissue.
62
BACK
3.1 Epithelia – Cell Surfaces – Lateral Surface
Epithelial cells have 3 surfaces. The apical or lumenal surface is closest to the lumen.
The basal surface is closest to the connective tissue underlying the epithelium.
The lateral surface is where the cell junctions are located.
The lateral surface of epithelial cells contains cell
junctions that attach contiguous epithelial cells to
one another.
In histology these junctions are classified as
“stronger” and “weaker”:
1. Stronger junctions hold cells tightly together; they
are called anchoring junctions.
There are 2 types of these:
- Zonulae adherentes
- Desmosomes
2. Weaker junctions assist in holding cells together.
There are 2 types of these:
- Tight junctions (the choice of words is confusing…
but that is what they are called…)
- Gap junctions
Underlying connective tissue.
The following slides elaborate on both strong and
weak junctions.
63
BACK
3.1 Epithelia – Cell Surfaces – Lateral Surface
Epithelial cells have 3 surfaces. The apical or lumenal surface is closest to the lumen.
The basal surface is closest to the connective tissue underlying the epithelium.
The lateral surface is where the cell junctions are located.
The lateral surface of epithelial cells contains cell
junctions that attach contiguous epithelial cells to
one another.
In histology these junctions are classified as
“stronger” and “weaker”:
1. Stronger junctions hold cells tightly together; they
are called anchoring junctions.
There are 2 types of these:
- Zonulae adherentes
- Desmosomes
2. Weaker junctions assist in holding cells together.
There are 2 types of these:
- Tight junctions (the choice of words is confusing…
but that is what they are called…)
- Gap junctions
Underlying connective tissue.
The following slides elaborate on both strong and
weak junctions.
64
BACK
3.1 Epithelia – Cell Surfaces – Lateral Surface
Epithelial cells have 3 surfaces. The apical or lumenal surface is closest to the lumen.
The basal surface is closest to the connective tissue underlying the epithelium.
The lateral surface is where the cell junctions are located.
The lateral surface of epithelial cells contains cell
junctions that attach contiguous epithelial cells to
one another.
In histology these junctions are classified as
“stronger” and “weaker”:
1. Stronger junctions hold cells tightly together; they
are called anchoring junctions.
There are 2 types of these:
- Zonulae adherentes
- Desmosomes
2. Weaker junctions assist in holding cells together.
There are 2 types of these:
- Tight junctions (the choice of words is confusing…
but that is what they are called…)
- Gap junctions
Underlying connective tissue.
The following slides elaborate on both strong and
weak junctions.
65
BACK
3.1 Epithelia – Cell Surfaces – Lateral Surface
Epithelial cells have 3 surfaces. The apical or lumenal surface is closest to the lumen.
The basal surface is closest to the connective tissue underlying the epithelium.
The lateral surface is where the cell junctions are located.
The lateral surface of epithelial cells contains cell
junctions that attach contiguous epithelial cells to
one another.
In histology these junctions are classified as
“stronger” and “weaker”:
1. Stronger junctions hold cells tightly together; they
are called anchoring junctions.
There are 2 types of these:
- Zonulae adherentes
- Desmosomes
2. Weaker junctions assist in holding cells together.
There are 2 types of these:
- Tight junctions (the choice of words is confusing…
but that is what they are called…)
- Gap junctions
Underlying connective tissue.
The following slides elaborate on both strong and
weak junctions.
66
BACK
3.1.1 Epithelia – Cell Surfaces – Lateral Surface – Zonulae Adherentes
Zonulae adherentes (singular: zonula adherens) are a type of “strong” or anchoring cell junctions.
Zonula Adherens
EPITHELIAL CELL
Actin
filaments
Anchor
proteins
Cadherins
(Ca2+ dependent)
EPITHELIAL CELL
Anchor
proteins
Actin
filaments
Zonulae adherentes
have the structure represented above
and at right:
– Notice the green lines in the figure above,
they represent transmembrane proteins. They go
through the membrane of each epithelial cell, linking the
contiguous epithelial cells. These proteins are cadherins.
They are attached to each cell and link to each other
(cadherin to cadherin) in the intercellular space.
– Cadherins of each epithelial cell also link to anchor
proteins in the cytoplasm, which in turn link to actin
filaments of the cytoskeleton of the cell.
Thus a zonula adherens has the following 3 components:
– Cadherins (transmembrane proteins)
– Anchor proteins
– Actin filaments (these are really part of the
cytoskeleton of the cell, but they contribute to the
zonula adherens)
Underlying connective tissue.
Zonulae adherentes tend to be close to the apical surface, but
are still on the lateral surface.
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3.1.2 Epithelia – Cell Surfaces – Lateral Surface – Desmosomes
Desmosomes are the other type of “strong” or anchoring cell junctions.
Desmosome
EPITHELIAL CELL
Keratin
filaments
Anchor
proteins
Cadherins
(Ca2+ dependent)
EPITHELIAL CELL
Anchor
proteins
Keratin
filaments
Desmosomes
have very similar structure to the
zonulae adherentes:
– Cadherins (transmembrane proteins)
– Anchor proteins
BUT instead of actin filaments they have
– Keratin filaments, also called intermediate
filaments
Keratin is much stronger than actin; thus,
desmosomes are stronger than zonulae
adherentes.
Desmosomes are described as forming button-like
junctions.
They tend to be positioned near a zonula adherens,
although towards the basal surface of the cell;
but still on the lateral surface.
Underlying connective tissue.
CLICK HERE FOR A CLOSER
VIEW OF A DESMOSOME.
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3.1.2 Epithelia – Cell Surfaces – Lateral Surface – Desmosomes
Desmosomes
are consist
one of the
of “strong” or anchoring cell junctions.
Desmosomes
oftypes
3 components:
Desmosome
Cadherin molecules (CAMs)
are in the lateral plasma membranes
EPITHELIAL
CELLcells; they
CELLthe narrow
of the adjacent
bind to oneEPITHELIAL
another across
Cadherins
intercellular
space.
in the figure.
Anchor These are green
Keratin
Anchor
Keratin
filaments
proteins
proteins
filaments
(Ca2+ dependent)
Anchor proteins are located
on the cytoplasmic face of the lateral
plasma
membrane
of the
cells; they
to the
Desmosomes
have
veryadjacent
similar structure
toconnect
the
cadherin
molecule. These are yellow in the figure.
zonulae
adherentes:
– Cadherins
(trans-membrane
proteins)
Keratin
filaments (intermediate
filaments)
attach to the anchor
– anchor
proteins.
Theseproteins
are blue in the figure.
– BUT instead of actin filaments they have
keratin filaments (also called intermediate
filaments). Keratin is much stronger than
actin thus desmosomes are stronger than
zonulae adherentes.
Desmosomes are described as forming button-like
junctions.
They tend to be positioned near a zonula adherens
but towards the basal surface of the cell (still on the
lateral surface though).
Underlying connective tissue.
CLICK HERE FOR A CLOSER
VIEW OF A DESMOSOME.
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3.1.3 Epithelia – Cell Surfaces – Lateral Surface – Tight Junctions
Tight junctions are “weak” cell junctions.
Tight Junction
EPITHELIAL CELL
EPITHELIAL CELL
Transmembrane
protein
Transmembrane
protein
(Ca2+ dependent)
Tight junctions
Tight junctions are formed when the extracellular
domains of transmembrane proteins (on the lateral
surfaces of adjacent cells) interlock with the help of
calcium.
Some membrane structures (ie, ion channels) are
found only on the apical surface or only on the
basolateral surface. Tight junctions help keep these
structures on the proper surface, thus maintaining the
polarization of the cell.
Tight junctions can block the passage of lumenal
content between cells.
Underlying connective tissue.
The cytoskeleton of the cell does NOT contribute to
tight junctions.
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3.1.4 Epithelia – Cell Surfaces – Lateral Surface – Gap Junctions
Gap junctions are the other type of “weak” cell junctions.
Gap Junction
EPITHELIAL CELL
EPITHELIAL CELL
Connexon
Gap junctions
Gap junctions are formed by transmembrane
proteins, called connexins. Many connexin protein
chains form a connexon.
Gap junctions create a channel whereby cytoplasm
and its water-soluble contents can flow between cells
to couple the cells metabolically and electrically.
The cytoskeleton of the cell does NOT contribute to
the structure of gap junctions.
Underlying connective tissue.
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4. Epithelia – Basement Membrane
All epithelia rest on a layer of connective tissue, called the basement membrane.
The basement membrane (BM) is a thin fibrous
connective tissue layer between the epithelium
and the underlying supporting tissue .
All epithelia are avascular (ie, they contain NO
blood vessels). In order to receive nutrition from the
blood, epithelia rest superficial to a layer of
supporting tissue which contains blood vessels. The
basement membrane is between the epithelium and
the supporting connective tissue. Blood vessels do
NOT penetrate the basement membrane; however,
it allows transudate (fluid leaked out of the
vasculature) to serve the cells of the overlying
epithelium. The basement membrane controls
permeability to and from the epithelium.
Nerves course through underlying tissue and must
penetrate the basement membrane in order to
innervate the epithelium. Thus, nerves do penetrate
the basement membrane.
Another very important function of the basement
membrane is that it controls epithelial growth,
impeding the epithelium from growing “downward.”
In malignant growth, the epithelium crosses over
the basement membrane.
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4. Epithelia – Basement Membrane
All epithelia rest on a layer of connective tissue, called the basement membrane.
The basement membrane (BM) is a thin fibrous
connective tissue layer between the epithelium
and the underlying supporting tissue .
All epithelia are avascular (ie, they contain NO
blood vessels). In order to receive nutrition from the
blood, epithelia rest superficial to a layer of
supporting tissue which contains blood vessels. The
basement membrane is between the epithelium and
the supporting connective tissue. Blood vessels do
NOT penetrate the basement membrane; however,
it allows transudate (fluid leaked out of the
vasculature) to serve the cells of the overlying
epithelium. The basement membrane controls
permeability to and from the epithelium.
Nerves course through underlying tissue and must
penetrate the basement membrane in order to
innervate the epithelium. Thus, nerves do penetrate
the basement membrane.
Another very important function of the basement
membrane is that it controls epithelial growth,
impeding the epithelium from growing “downward.”
In malignant growth, the epithelium crosses over
the basement membrane.
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4.1 Epithelia – Basement Membrane – Hemidesmosomes
Hemidesmosomes link epithelial cells to the basement membrane.
Epithelia that experience high abrasive force are
reinforced by junctions called hemidesmosomes,
which are similar to desmosomes in the lateral surface
of adjacent epithelial cells (as previously described).
Refer to the previous slide, as we describe the main
components of a hemidesmosome:
1- Laminin in the basement membrane
2- Anchor proteins
3- Integrin proteins in the epithelial cells that happen to
be attached to keratin of the cytoskeleton of the
epithelial cell
NOTE: With Light Microssopes basement membranes can be seen if they are thick or if they have been
specifically stained. The basement membrane
in this image is particularly thick.
With EM, the 3 sublayers of a basement membrane can be seen. While the distinction of these
constitutes too much detail for our purposes, know that these layers are called:
• Lamina lucida
• Lamina densa (or basal lamina**)
• Lamina fibroreticularis
**Some authors refer to the basement membrane as the basal lamina (1 of its 3 sublayers).
However, this may lead to confusion, so we will refer to the basement membrane as the basement membrane.
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5. Epithelia – Exocrine Glands
Exocrine glands are formed by epithelial cells that specialize in secreting some substance
to the exterior.
Exocrine glands are formed when the cells in an
epithelium proliferate and invaginate.
The gland has a space in the center called the
lumen.
The cells closest to the surface, in conjunction
with the corresponding lumen, form the duct of
the gland .
The rest of the gland is the secretory portion .
Observe the apical surface of the cells
in the gland.
Observe the lateral surface of the cells
in the gland.
Observe the basal surface of the cells
in the gland.
The gland is surrounded by blood capillaries
.
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5. Epithelia – Exocrine Glands
Exocrine glands are formed by epithelial cells that specialize in secreting some substance
to the exterior.
Exocrine glands are formed when the cells in an
epithelium proliferate and invaginate.
The gland has a space in the center called the
lumen.
The cells closest to the surface, in conjunction
with the corresponding lumen, form the duct of
the gland .
The rest of the gland is the secretory portion .
Observe the apical surface of the cells
in the gland.
Observe the lateral surface of the cells
in the gland.
Observe the basal surface of the cells
in the gland.
The gland is surrounded by blood capillaries
.
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5. Epithelia – Exocrine Glands
Exocrine glands are formed by epithelial cells that specialize in secreting some substance
to the exterior.
Exocrine glands are formed when the cells in an
epithelium proliferate and invaginate.
The gland has a space in the center called the
lumen.
The cells closest to the surface, in conjunction
with the corresponding lumen, form the duct of
the gland .
The rest of the gland is the secretory portion .
Observe the apical surface of the cells
in the gland.
Observe the lateral surface of the cells
in the gland.
Observe the basal surface of the cells
in the gland.
The gland is surrounded by blood capillaries
.
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BACK
5. Epithelia – Exocrine Glands
Exocrine glands are formed by epithelial cells that specialize in secreting some substance
to the exterior.
Exocrine glands are formed when the cells in an
epithelium proliferate and invaginate.
The gland has a space in the center called the
lumen.
The cells closest to the surface, in conjunction
with the corresponding lumen, form the duct of
the gland .
The rest of the gland is the secretory portion .
Observe the apical surface of the cells
in the gland.
Observe the lateral surface of the cells
in the gland.
Observe the basal surface of the cells
in the gland.
The gland is surrounded by blood capillaries
.
78
BACK
5. Epithelia – Exocrine Glands
Exocrine glands are formed by epithelial cells that specialize in secreting some substance
to the exterior.
Exocrine glands are formed when the cells in an
epithelium proliferate and invaginate.
The gland has a space in the center called the
lumen.
The cells closest to the surface, in conjunction
with the corresponding lumen, form the duct of
the gland .
The rest of the gland is the secretory portion .
Observe the apical surface of the cells
in the gland.
Observe the lateral surface of the cells
in the gland.
Observe the basal surface of the cells
in the gland.
The gland is surrounded by blood capillaries
.
79
BACK
5. Epithelia – Exocrine Glands
Exocrine glands are formed by epithelial cells that specialize in secreting some substance
to the exterior.
Exocrine glands are formed when the cells in an
epithelium proliferate and invaginate.
The gland has a space in the center called the
lumen.
The cells closest to the surface, in conjunction
with the corresponding lumen, form the duct of
the gland .
The rest of the gland is the secretory portion .
Observe the apical surface of the cells
in the gland.
Observe the lateral surface of the cells
in the gland.
Observe the basal surface of the cells
in the gland.
The gland is surrounded by blood capillaries
.
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BACK
5.4 Epithelia – Exocrine Glands – Type of Secretion
Exocrine glands are classified in a number of ways according to the:
1- Shape of the duct
2- Shape of the secretory portion
3- Mode of secretion
4- Type of secretion
Finally, exocrine glands are classified, as the
following, according to the type of secretion:
•Mucous glands – secrete mucus, which is rich
in mucigens (large glycosolated proteins that
absorb water and, as a result, produce a thick
gel-like material).
•Serous glands – secrete a viscous fluid rich
in proteins (that are often enzymes) not as
thick as mucus.
Other exocrine glands are specialized in
secreting substances like sweat (sweat
glands), sebum/oil (sebaceous glands), or milk
(mammary glands).
(We will cover these in more detail as we proceed
through the different systems in Blocks 2 and 3.)
In general with use of H&E, serous cells
stain darker than mucous cells
. The exact shade of color
is dependent on the sample, as there can be variations – depending on the composition of the secretion,
and on the degree in which the sample “takes the stain in.”
In this course, we will give you examples that are clearly lighter (for mucous cells) and clearly darker (for
serous cells).
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5.4 Epithelia – Exocrine Glands – Type of Secretion
Exocrine glands are classified in a number of ways according to the:
1- Shape of the duct
2- Shape of the secretory portion
3- Mode of secretion
4- Type of secretion
Finally, exocrine glands are classified, as the
following, according to the type of secretion:
•Mucous glands – secrete mucus, which is rich
in mucigens (large glycosolated proteins that
absorb water and, as a result, produce a thick
gel-like material).
•Serous glands – secrete a viscous fluid rich
in proteins (that are often enzymes) not as
thick as mucus.
Other exocrine glands are specialized in
secreting substances like sweat (sweat
glands), sebum/oil (sebaceous glands), or milk
(mammary glands).
(We will cover these in more detail as we proceed
through the different systems in Blocks 2 and 3.)
In general with use of H&E, serous cells
stain darker than mucous cells
. The exact shade of color
is dependent on the sample, as there can be variations – depending on the composition of the secretion,
and on the degree in which the sample “takes the stain in.”
In this course, we will give you examples that are clearly lighter (for mucous cells) and clearly darker (for
serous cells).
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6. Epithelia – Endothelium and Mesothelium
Endothelium (plural: endothelia) and mesothelium (plural: mesothelia) are simple squamous epithelia
that are specialized. Remember: simple squamous epithelia are formed by 1 layer of flat cells;
and line and cover organs.
Endothelium is the simple squamous
epithelium lining that is specific to:
the walls of the heart, blood vessels
(arteries, veins, and capillaries), and
lymphatic vessels.
Endothelium :
Notice that in arteries, veins, and
capillaries, the most internal layer (lining)
is an endothelium.
(Ignore the labels; just focus on the
concept.)
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6. Epithelia – Endothelium and Mesothelium
Endothelia and mesothelia are simple squamous epithelia that are specialized. Remember simple
squamous epithelia are formed by one layer of flat cells. Also remember we said epithelia line and
cover organs. ****OMIT THIS SLIDE****
Mesothelium is a simple squamous
epithelium that covers certain organs in
the body and lines internal body cavities.
Cells forming mesothelia often secrete a
substance called serous (viscous) fluid,
thus mesothelia are also referred to as
“serous membranes”.
Visualize the thoracic cavity lined by a
mesothelium. The thoracic wall is
internally lined by this tissue which goes
on to cover the outer aspect of the
organs in the thoracic cavity. Thus the
internal thoracic wall, the lungs and heart
are covered by this mesothelium. Ask in
lab if this is not clear.
Mesothelium: ignore the labels and
focus on the concept. Notice that the
outermost layer (covering) of the
ovary (top image) and the lining of the
thoracic wall and covering of the lungs
(bottom image) are mesothelia.
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1. Epithelial Tissue – Introduction
Remembering from the module “Introduction to Histological Tissues,” epithelia have abundant
cells
and a very small amount of extracellular material (ECM) .
This is a high magnification (about x600) of the epithelium in the respiratory tract.
(Ignore the i.ds in the image.)
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1. Epithelial Tissue – Introduction
Remembering from the module “Introduction to Histological Tissues,” epithelia have abundant
cells
and a very small amount of extracellular material (ECM) .
This is a high magnification (about x600) of the epithelium in the respiratory tract.
(Ignore the i.ds in the image.)
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5.2 Epithelia – Exocrine Glands – Shape of the Secretory Portion
Exocrine glands are classified in a number of ways according to the:
1- Shape of the duct
2- Shape of the secretory portion
3- Mode of secretion
4- Type of secretion
Exocrine glands can be described, as the following, according to the anatomical arrangement of
the gland cells – ie, the shape of the secretory portion:
•Acinar or alveolar (glands of this type resemble bunches of grapes)
•Tubular (glands of this type are simple tube-like, U-shaped portion)
Acinar gland.
Acinar gland
cross-section.
;
Tubular gland.
In the images, note the difference between a cross-section and a longitudinal section.
Tubular gland
longitudinal or
sagittal section.
;
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5.2 Epithelia – Exocrine Glands – Shape of the Secretory Portion
Exocrine glands are classified in a number of ways according to the:
1- Shape of the duct
2- Shape of the secretory portion
3- Mode of secretion
4- Type of secretion
Exocrine glands can be described, as the following, according to the anatomical arrangement of
the gland cells – ie, the shape of the secretory portion:
•Acinar or alveolar (glands of this type resemble bunches of grapes)
•Tubular (glands of this type are simple tube-like, U-shaped portion)
Acinar gland.
Acinar gland
cross-section.
;
Tubular gland.
In the images, note the difference between a cross-section and a longitudinal section.
Tubular gland
longitudinal or
sagittal section.
;
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6. Epithelia – Endothelium and Mesothelium
Endothelium (plural: endothelia) and mesothelium (plural: mesothelia) are simple squamous epithelia
that are specialized. Remember: simple squamous epithelia are formed by 1 layer of flat cells;
and line and cover organs.
Mesothelium is a simple squamous epithelium that covers certain
organs in the body and lines internal body cavities (lines CAVITIES;
NOT organs).
Cells forming mesothelia often secrete a substance called serous
(viscous) fluid; thus, mesothelia are also referred to as serous
membranes.
Visualize the thoracic cavity lined by a mesothelium. The thoracic wall is
internally lined by this tissue, which goes on to cover the outer aspect of
the organs in the thoracic cavity – much like a blanket draping over the
organs. Thus, the internal thoracic wall, the lungs, and heart are all
covered by this mesothelium. Similarly, organs in the abdomen and
pelvis are covered by a mesothelium. (If this is not clear, ask in lab.)
The image at top shows a portion of an ovary – the pink part is the
organ, the whitish-blue portion on the right side of the image is the pelvic
cavity.
Notice that the outermost layer (covering) of the ovary is
a mesothelium .
The bottom figure illustrates the location of mesothelium
lining
the thoracic wall and covering the lungs (real images shown in lab).
(Ignore the labels; just focus on the concept.)
Image: www.faculty.une.edu
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6. Epithelia – Endothelium and Mesothelium
Endothelium (plural: endothelia) and mesothelium (plural: mesothelia) are simple squamous epithelia
that are specialized. Remember: simple squamous epithelia are formed by 1 layer of flat cells;
and line and cover organs.
Mesothelium is a simple squamous epithelium that covers certain
organs in the body and lines internal body cavities (lines CAVITIES;
NOT organs).
Cells forming mesothelia often secrete a substance called serous
(viscous) fluid; thus, mesothelia are also referred to as serous
membranes.
Visualize the thoracic cavity lined by a mesothelium. The thoracic wall is
internally lined by this tissue, which goes on to cover the outer aspect of
the organs in the thoracic cavity – much like a blanket draping over the
organs. Thus, the internal thoracic wall, the lungs, and heart are all
covered by this mesothelium. Similarly, organs in the abdomen and
pelvis are covered by a mesothelium. (If this is not clear, ask in lab.)
The image at top shows a portion of an ovary – the pink part is the
organ, the whitish-blue portion on the right side of the image is the pelvic
cavity.
Notice that the outermost layer (covering) of the ovary is
a mesothelium .
The bottom figure illustrates the location of mesothelium
lining
the thoracic wall and covering the lungs (real images shown in lab).
(Ignore the labels; just focus on the concept.)
Image: www.faculty.une.edu
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5.5 Epithelia – Exocrine Glands – Adenocarcinoma
Tumors developing from glandular epithelium are called adenocarcinomas. Because there are
epithelial glands in many organs – such as the stomach, uterus, colon, pancreas, liver, prostate,
breast, and thyroid – we refer to adenocarcinomas in these organs when the epithelial cells in their
respective glands become neoplastic.
In some adenocarcinomas, the epithelial cells
have a fairly normal morphology; but they
secrete abnormal amounts of mucus.
In other adenocarcinomas, the epithelial cells
form very atypical glands that have abnormal
cell morphology.
•Normal glandular epithelium
in the endocervix
•Adenocarcinoma
(Observe the difference in the glandular epithelial cells;
don’t worry about the details at this point, you’ll learn
them next year, just focus on the concept for now).
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5.5 Epithelia – Exocrine Glands – Adenocarcinoma
Tumors developing from glandular epithelium are called adenocarcinomas. Because there are
epithelial glands in many organs – such as the stomach, uterus, colon, pancreas, liver, prostate,
breast, and thyroid – we refer to adenocarcinomas in these organs when the epithelial cells in their
respective glands become neoplastic.
In some adenocarcinomas, the epithelial cells
have a fairly normal morphology; but they
secrete abnormal amounts of mucus.
In other adenocarcinomas, the epithelial cells
form very atypical glands that have abnormal
cell morphology.
•Normal glandular epithelium
in the endocervix
•Adenocarcinoma
(Observe the difference in the glandular epithelial cells;
don’t worry about the details at this point, you’ll learn
them next year, just focus on the concept for now).
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