BIOL 221 - Epithelium
Cristofre Martin
Department of Biochemistry
St. George’s University
Lecture objectives
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Students should be able to:
List the various levels of organization in the body.
List the basic tissue types.
Explain functions and locations of epithelial tissues.
Describe the types of secretion from epithelial cells.
List the distinctive characteristics of epithelium.
Describe basic cell shapes, and layering variations.
List locations in the body of the different epithelium.
Discuss mitotic rate in epithelium, and why it makes
epithelium vulnerable to traditional chemotherapy.
• Describe the various cell junctions and their functions.
• Identify slides of epithelium
• Describe human diseases of the epithelium.
Levels of organization in the body
Cell: the smallest functional unit in the body.
Eg. Neuron cell
Tissue: a group of like cells that are similar in both
structure and function. Eg. Lung tissue
Organ: an organized group of tissues,
accomplishing specific functions. Eg. heart
Organ system: group of organs working together.
Eg. Circulatory system
Types of tissue
• Epithelial
– Coverings
– Lining of body surfaces
Eg. skin
• Connective
– Support
Eg. bone, cartilage, ligaments and fat
• Muscle
-location and movement
• Nervous
-control, sensory
Eg. Brain, nerves
Epithelium
• Epithelial tissue is found as a covering and
lining in the body.
It includes:
1. the outer (epidermal) layer of skin
2. the lining of the digestive tract
3. the lining of blood vessels, kidney tubules,
glands and outgrowths of the digestive tract
(liver, gall bladder)
Functions of epithelial tissues:
• Protection from mechanical damage, sun exposure
and bacteria
• Reduction of friction
• Absorption of nutrients by the lining of the small
intestine into the blood
• Filtration of wastes from the blood plasma by the
lining of kidney tubules
• Regulation of water loss
• Sensory surface for reception of stimuli
• Secretion of materials (glandular)
Characteristics of epithelium
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Cells are close together and avascular
Form sheets of cells
One surface is free (luminal surface); the opposing surface rests on a basement
membrane that anchors the cells to the connective tissue
Lateral cell surfaces are in close contact
Apex (free
surface of
cell) (apical
end)
Base of
cell(basal
end)
Epithelial tissues are classified based on
cell shape and the number of cell layers
Squamous – flattened
Cuboidal - cubes
Columnar - columns
Squamous cells have a flat, two dimensional appearance.
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Squamous cells have a flat, two dimensional appearance.
Cuboidal: shaped like a cube: height, width and depth
(when visible) are about the same. The nucleus is
usually centrally located. Most secretory cells of glands
are cuboidal epithelial cells. In addition, the ducts of
most exocrine glands are lined by cuboidal cells as are
most of the tubules in the kidneys.
Columnar: length is greater than width; nucleus is usually located toward
the base of the cell. They have the largest cytoplasmic volumes of all
epithelia. Cells possess the organelle density and energy reserves to
engage in secretory or absorptive functions. Simple columnar epithelia
with microvilli line the small intestine. Ciliated types are found in small
bronchioles of the respiratory tract and in the fallopian tubes of the female
reproductive tract.
Microilli vs cilia?
Classification based on the number of
layers
• simple
• stratified
• pseudostratified
• transitional
Simple: a single layer; all cells touch the basement membrane and the
outer surface or the lumen. Simple epithelia secrete fluids that lubricate
tissues. Other simple epithelia line body tracts as protective, absorptive, or
secretory cells. As linings in the alveoli(air sacs), kidneys, and blood
vessels, simple squamous types assist in diffusion, osmosis and filtration
phenomena.
Stratified: more than one layer, only the top layer is in contact with the
surface; only the bottom layer rests on the basement membrane. (basal
lamina). Stratified epithelia are found covering the exterior body surfaces
and lining portions of the body tracts where friction phenomena occur. A
multilayered epithelium allows deeper cells to replace the more superficial
cells as they are damaged or cast off from the epithelial free
surface. Although most prevalent in the skin and portions of the digestive
and reproductive tract, stratified epithelia can be found in some large
ducts and tubules.
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Pseudostratified: all cells touch the basal lamina, but not all reach
the outer surface. There is a perception of layers. Pseudostratified
epithelia are most prevalent in the respiratory tract as ciliated
types.
Transitional: large dome-shaped cells on the
luminal surface in a relaxed state. Name comes from
the ability of this epithelium to accommodate
stretching in the urinary tract as fluid pressures
vary. It is found as a lining of the ureters, urethra,
and bladder.
Nomenclature
The classification or naming system used to identify epithelia is based
on three key criteria:
1. No. of cell layers
2. Cell shapes
3. Surface modifications
The exceptions to this nomenclature are pseudostratified and
transitional epithelia.
If stratified epithelium consists of differently shaped cell in the various
layers (a common condition), the tissue is named according to the
shape of the cells on the free surface.
• Note: there are limits to the
thickness of epithelial tissues, since
they are AVASCULAR and nutrients
must be distributed by diffusion.
• Skin and esophagus are about 30
cells deep.
Epithelial cells of the human body
Epithelial cells of the human body
• Simple squamous lines blood vessels, pleural
and peritoneal cavities
• Stratified squamous non-keratinized lines
moist body surfaces: mouth, esophagus, vagina
• Stratified squamous keratinized- epidermis.
Outer layers lose their nuclei, become flattened,
accumulate keratin. (waterproof)
(Compare water loss of keratinized face and non-keratinized
lips)
Epithelial cells of the human body
• Simple cuboidal - kidney tubules; thyroid follicles….
• Stratified cuboidal - two or more layers of cuboidal
cells. This is found in sweat glands.
• Simple columnar - lines the stomach and intestines.
• Simple columnar with cilia is found in the oviducts/
fallopian tubes.
• Cilia- tiny hair-like structures on the surface of cells
which help move entire cells or particles along the
outer surface of cells.
• Microvilli -increase surface area for absorption.
Brush border- microvillus covered surface of simple
cuboidal or simple columnar epithelium
Epithelial cells of the human body
• Stratified columnar- two or more layers of
cells, the luminal layer is columnar. Found in
excretory ducts of some glands.
• Pseudo-stratified columnar epithelium:
• with cilia (hairlike): found in trachea and
bronchi.
• with stereocilia (bundles of actin filaments):
found in vas deferens and epididymis (male
reproductive tract), inner ear
Cell Junctions
Many cells are held together by junctions, which
vary in structure, depending on their function.
Types of cell junctions:
• Tight junctions (occluding junctions)
• Anchoring junctions
• Communicating junctions
Tight junctions
Tight junctions encircle the cell like a belt
around pants.Tight junctions connect
plasma membranes of adjacent cells in a
sheet.This prevents small molecules
slipping between cells and through the
cellular layer. Nutrients absorbed from the
digestive tract must pass through the cells
to enter the blood.
Tight junctions prevent
substances from
moving between cells
Tight
junction
Intermediate
filaments
Desmosome
Tight junctions are composed of strands of
proteins – mainly claudin and occludin and
associate with actin filaments in the cell.
Gap
junctions
Space
between
cells
Plasma
membranes of
adjacent cells
Extracellular
matrix
Fig. 6-32b
Tight junctions play a
crucial role in the
protection of the
sperm in the testes
and the brain.
For example, tight
junctions of Sertoli
cells form the bloodtestes barrier which
protects developing
sperm.
Tight junctions also
maintain the blood
brain barrier.
Tight
junction
The cells lining the small intestine form a sheet one cell layer thick. One
surface faces the lumen, the opposite surface rests on the basal lamina,
which is in direct contact with connective tissue, containing many blood
vessels.
Anchoring junctions
• Anchoring junctions mechanically attach the
cytoskeleton of a cell to the cytoskeletons of
other cells or to the extracellular matrix.
• Anchoring junctions are most common in
tissue subject to mechanical stress, such as
muscle and the epithelium of the skin.
• Cytoskeletal elements include:
– A. Intermediate filaments: sturdy, long term molecules.
– B. Actin filaments: delicate, often transient molecules.
Types of Anchoring Junctions
• Cadherin-mediated junctions
– Desmosomes
– Adherens junctions
– Integrin-mediated junctions
hemidesmosomes
Cadherin mediated junctions
Cadherins are single pass
transmembrane glycoproteins* that
attach adjacent cells.
Desmosomes connect adjacent cells
and are anchored to intermediate
filaments.
Adherens junctions also attach
surrounding cells and are anchored to
actin filaments.
*Glycoprotein: protein +saccharide side chain.
In desmosomes, the
cytoplasmic end of the
cadherin is connected to
intermediate filaments.
The other end projects
outward from the plasma
membrane, joining
directly with a cadherin
protruding from an
adjacent cell, binding the
cells together.
(Note the reinforcing plaque.)
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Fig. 6-32c
Desmosome
1
µm
In adherens
junctions, cadherins
attach to an actin
framework which
forms a less stable
link than
intermediate
filaments.
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Integrin mediated
junctions
Hemidesmosomes anchor epithelial
cells to the underlying basement
membrane.
Integrins are composed of two different
subunits that extend outward from the
basement membrane.
Together these units bind a protein
component of the extracellular matrix
like two hands grasping a pole.
Keratin filaments
and
hemidesmosomes
connect epithelial
cells to the basal
lamina.
Human disease such as
epidermolysis bullosa simplex
cause severe blistering of the
skin and are the result of
mutations in keratin genes
that are expressed in the
basal cell layer of the
epidermis.
Epidermolysis bullosa simplex
(EBS) is characterized by
fragility of the skin that results
in non-scarring blisters of the
skin caused by little or no
trauma.
EBS disease is characterized by
rupturing of cells in the basal layer of
the epidermis between the nucleus and
the hemidesmosomes which connect
the keratin filaments to the basal
lamina.
Communicating Junctions
• Communicating junctions link the cytoplasm of
adjacent cells.
• They are open channels for the rapid flow of ions
and small molecules.
• These junctions are formed by connexons,
complexes of 6 identical transmembrane
proteins. The proteins are arranged in a circle,
creating a channel that protrudes from the cell
surface.
• When the channels of two adjacent cells align
perfectly, a gap junction is formed.
Connexons hold two cells
slightly apart, hence the term
“gap junction”. Connexons can
respond to the level of ions in a
cell, closing a channel when
appropriate. Gap junctions are
present in heart muscle and
play a significant role in signal
transmission there.
Recall: only tight junctions
involve direct contact of
adjoining cell membranes.
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Fig. 6-32d
Gap junction
Glands
• Exocrine glands have ducts.
Secretions of liver, salivary glands,
pancreas, empty into the GI tract;
mammary, sebaceous, and sweat
glands empty externally.
• Endocrine glands are ductless and
empty secretions directly into the
blood.
Glands of the skin
Sebaceous glands
-Clumps of epithelial tissue distributed within
dermis and all over the body
-Secrete an oily, fat-based substance into hair
follicle pore that serves to bring it to the surface
-Lubricates and waterproofs the skin
Eccrine sweat glands
-concentrated on hands and soles of
feet and forehead, secrete sweat
(water and NaCl) to cool body
-also secrete sweat during fear or
strong emotion.
-composed of tubes of cuboidal cells
Merocrine secretion: Cells that secrete products via the merocrine method
form membrane-bound secretory vesicles internal to the cell. These are moved to
the apical surface where the vesicles coalesce with the membrane on the apical
surface to release the product. Most common mechanism of secretion. Example,
eccrine sweat glands
Aprocrine secretion: The apical portions of cells are pinched off and lost during
the secretory process. This results in a secretory product that contains a variety
of molecular components including those of the membrane. Example, mammary
glands, sweat pheromones in some animals (possibly humans).
Holocrine secretion: Involves death of the cell. The secretory cell is released
and as it breaks apart, the contents of the cell become the secretory product.
Example, sebaceous glands
Epithelium and mitosis
• Epithelium has a very high mitotic rate.
• Anything that interferes with mitosis in the body
affects the epithelium.
• Ex., Anti-mitotic chemotherapy often causes hair
loss, skin lesions, loss of GI epithelium
• High mitotic rate and exposure to radiation
such as UV light makes skin cells prone to
cancer (mutation)