Epithelium is one of the 4 primary tissues of the body. It consists of cells
usually arranged in sheets or tubules that are attached to the underlying basement
membrane. The basement membrane, a structure seen with the light microscope,
has been subdivided into a basal lamina (thought to be produced by the epithelium)
and a reticular lamina (produced by connective tissue cells). You will usually
see the basal lamina only with the electron microscope.
Epithelium is found covering the numerous internal and external surfaces of
the body and may also be modified to form glandular structures. As a physician,
it is very important to be able to recognize and identify the various types
of epithelia since they are important in organ identification and essential
in the detection of pathological processes.
In general, the various types of epithelia are classified according to the
shape of the cells and whether or not they are layered. In a simple epithelium
the cells form a single layer and are all attached to the basement membrane.
A stratified epithelium consists of multiple layers of cells in
which only the basal layer is attached to the basement membrane. A simple epithelium
which appears to be stratified is classified as "pseudostratified"
(the nuclei are in different layers, but all the cells are in contact with basement
membrane). A stratified epithelium in which the number of cell layers varies
upon physiological stretching is called "transitional". Epithelial
cells are classified by shape into squamous (flat), cuboidal and
columnar varieties. The name of a stratified epithelium is determined
from the shape of the apical layer of cells.
You should aim to:
- Recognize the presence of an epithelium and understand its defining characteristics.
- Distinguish the different morphological types of epithelium.
- Relate morphology to function where possible.
- Identify ultrastructural features of epithelial cells and their functional
1. Simple Squamous Epithelium
Squamous cells are flattened cells which are wider than they are tall. The
coelomic cavities within the body are covered by a simple squamous epithelium
which is called mesothelium. Slide #2 (Mesentery) is a
piece of mesentery which has been spread flat on the glass and stained with
silver nitrate. The mesentery is a sandwich composed of two sheets of simple
squamous epithelium separated by a relatively thicker layer of connective tissue,
which may contain blood vessels and fat cells. The edges of the squamous cells
can be made out as uneven corrugated black lines due to precipitation of silver
in the intercellular space. The nuclei are stained very lightly, if at all.
By focusing the microscope up and down, both epithelial layers may be brought
into view separately.
Blood and lymphatic vessels are lined with a special type of simple
squamous epithelium called endothelium. Examine slide #70
(nasal mucosa) for blood vessels and locate their endothelial lining.
Blood vessels are usually recognized by their content of red blood cells
(small red staining spheres). In these preparations nuclei of the squamous
endothelial cells are prominent, but cell boundaries cannot be made out
because the cells are so flattened
2. Simple Cuboidal Epithelium
This type of epithelium consists of a single layer of cells which is
thicker than the squamous endothelial cells. Cuboidal epithelial
cells are approximately as wide as they are tall and can be found
in the kidney tubules, glands and ducts. A good example is the thyroid
gland, slide #67 which is composed of follicles that are discrete
areas of colloidal material surrounded by a simple cuboidal epithelium.
Note, that there are some variations in cell thickness.
3. Simple Columnar Epithelium
This epithelium is composed of a single layer of cells which are taller than
they are wide and can be found lining the stomach, intestine, uterus and gall
bladder. Locate the columnar epithelium on slide #3 (rat intestine) noting the presence of mucous-secreting goblet (these
appear as clear, non-stained rounded areas due to extraction of the mucous)
and the brush border (see below: non-motile processes). The cells
are polarized with the nuclei at the basal end (closest to basement membrane).
How could you avoid mistaking simple columnar epithelium for stratified columnar
4. Stratified Cuboidal and Columnar Epithelia
These epithelia can be found in the tubules of the testis, ducts of some glands,
and in regions of the urinary and respiratory tracts. They will be examined
later when you study organ systems.
5. Pseudostratified Columnar Ciliated Epithelium
Pseudostratified ciliated epithelium is typically found lining
the respiratory system and is also found in other tissues such as the
epididymis. Three types of cells make up this epithelium - basal, columnar
and fusiform cells. The first two are easy to see, but fusiform cells
are difficult to demonstrate due to their shape. All cells rest on the
basement membrane, but vary in height, and, as a result, the nuclei are
at different levels giving the appearance of stratification.
6. Stratified Squamous Epithelium
Stratified squamous epithelium is found in the skin and in the
passageways leading to the exterior, such as the mouth, anus and vagina.
Examine the esophagus (slide #50) and note the epithelium
lining the lumen. In the odd-numbered boxes, it is cut in cross-section
and the epithelium lines the lumenal surface of the tube; in the even-numbered
boxes it is a longitudinal piece of tissue with the epithelium on
one surface. By visual inspection, most of the tissue is pink. The epithelium
will be the most basophilic material in the slide. Inspection of the epithelial
layer at low-power indicates that the basal surface of the epithelium
is indented by connective tissue papillae. If these papillae are
cut in cross-section, they appear as circular areas within the epithelium.
Using the 10x objective, observe the many layers of cells. With this
and the 40x objective, examine the changes in shape the cells undergo
from the basal to the lumenal layers. New cells are formed in basal areas of
the epithelium, are pushed towards the free surface eventually and are sloughed
off. If the epithelium is subject to a great deal of abrasion, such as in the
skin, the outer cells undergo a special type of transformation. Keratin
is formed in these cells as they migrate towards the surface. The nuclei disappear
and the cells become dehydrated. This process will be discussed in the lecture
Slide #43 (Thick skin), is a section through the plantar
surface of a foot. The pink material is connective tissue and the epithelium
is the blue layer next to it. The thick outer layer is composed of the
keratinized cells mentioned above. It may be partially or completely detached
in some slides. If so, examine your neighbor's slide.
7. Transitional Epithelium
This epithelium is found in urinary tract. Its appearance varies depending
on whether it is in a stretched or relaxed state. Examine slide #76
(human bladder) and slide #9 (rat bladder) made from
relaxed bladders. The cells in the outermost layer characteristically
have a large amount of cytoplasm, which may give the cell a swollen, balloon
shape appearance ("cap" cells). In slide #9, many cells
in the outer layer appear to be covered by a dense-layer of material which
is not part of the epithelium.
Specializations of Epithelial Cells
Epithelial cells have polarity in terms of their morphology and orientation.
The plasma membrane nearest the basement membrane is the basal membrane;
the apical membrane lines the free surface, and the lateral membrane is
between adjacent cells. The various membranes may be modified to perform
specialized functions. Projections from the cell-surface may be found
on different cells depending on their function. These can be visualized
in the light microscope but their structure is only apparent in electron
micrographs (E.M.s). Examine the E.M.s in your textbook. Non-motile processes
called microvilli may project from the surface of cells. Microvilli
vary in size from extremely small processes which can only be seen with
the electron microscope, to larger structures which can be seen with the
light microscope. In the small intestine (slide #3), individual
microvilli are not seen but the brush border they form is visible.
Explain why. What is the function of microvilli?
Motile processes called cilia are present in the pseudostratified
epithelium of the trachea (slide #72). Observe these and then study
the E.M.s noting the characteristic 9 + 2 microtubule arrangement. Notice
in the thin section EM of a simple columnar ciliated epithelium
numerous cilia cut in both cross section and longitudinal section
can be observed at the apical membrane. The cilia are attached to basal
bodies in the apical cytoplasm
Thin section EM of a tight junction on the lateral
membrane of adjacent epithelial cells. Using this procedure, the tight
junction appears as punctate membrane fusions.
In freeze-fracture EM, these membrane fusions appear as intramembrane
ridges. The function of tight junctions is to seal the
intercellular space between epithelial cells and prevent transepithelial
diffusion. Tight junctions maintain epithelial cell surface polarity.
Desmosomes are observed on the lateral membrane between
epithelial cells. They are involved in cell-cell adhesion and are abundant
in organs that undergo severe mechanical stress such as the skin. Hemidesmosomes
are found on the epithelial basal membrane and anchor the epithelium
to the basal lamina.
Gap junctions appear as plaques of tightly packed
intramembrane particles in freeze-fracture electron microscopy. For comparison,
the intramembrane particles of the adjacent plasma membrane (labelled
M in the picture) are less densly packed. Gap junctions are involved in