EPITHELIAL TISSUES
You’d be lost (dead) without them.
WHAT ARE EPITHELIA?
A. simple definition - "a layer of cells with a free surface"
B. better definition - single or multiple layers of cells
characterized by,
1. a layer or layers composed of closely aggregated,
polyhedral cells
2. one side of cell layer(s) has a free surface
3. little intercellular substance between cells
4. cells cohere (stick) strongly to each other
5. cells form a sheet that covers a surface
In embryonic terms we can say that epithelia are derived
from all 3 major germ layers, i.e. ectoderm, mesoderm,
endoderm.
Functions of Epithelia
A. Covering and lining surfaces (a barrier)
Examples:
a. skin
b. epithelial cells (endothelium) lining blood vessels,
c. mesothelium of peritoneal cavity (coelom)
FUNCTIONS OF EPITHELIA
B. Regulation of materials and sensory information passing
between, into, or out of organs/tissues
1. absorption (e.g. tall columnar epithelium of intestine)
2. secretion (e.g. epithelial lining of glands)
3. sensation (e.g. sensory cells, neuroepithelium - taste buds)
4. lubrication (e.g. mucus secreting epithelium of digestive tract)
C. In some cases contractility (e.g. myoepithelium - often
associated with glands such as sweat and mammary glands)
D. Protection from the external environment
BASIC CELL SHAPES FOUND IN EPITHELIA
A. squamous - flat - very thin (flat) in cross-section
B. cuboidal - appear square in cross-section
C. columnar - appear rectangular (tall, column shaped) in cross-section
1. NOTE THAT HOW AN EPITHLIUM LOOKS IN HISTOLOGICAL SECTIONS DEPENDS ON
THE ORIENTATION OF THE EPITHELIUM WHEN SECTIONS ARE CUT. A CUBOIDAL
EPITHELIUM CAN LOOK COLUMNAR, ETC.
2. ALSO NOTE THAT THE THREE SHAPES GIVEN FOR EPITHELIA ARE DERIVED FROM
THEIR SHAPE AS THEY ARE SEEN IN HISTOLOGICAL CROSS-SECTIONS.
IN REALITY, THE CELLS ARE REALLY POLYHEDRAL
(MANY SIDED) IN THREE DIMENSIONS
3. There are also intermediate shapes of some epithelial cell types that lie
between the major types described above.
4. The shape of the nucleus often corresponds to cell shape.
Squamous epithelial cells, nucleus flattened - long
axis parallel to basement membrane
Cuboidal epithelial cells, nucleus - spherical
Columnar - nucleus oblong, long axis perpendicular
to the basement membrane
5. The shape of the nucleus is sometimes important in distinguishing
different types of epithelia since often the cell borders are indistinct despite
staining.
CLASSIFICATION OF EPITHELIA
A. Simple epithelia - epithelia consisting of one layer of cells
1. Simple squamous - one layer of flattened cells covering a
surface, e.g. endothelium of blood vessels.
2. Simple cuboidal - one layer of cuboidal cells covering a surface,
e.g. epithelial lining of proximal and distal convoluted tubules in
kidney.
3. Simple columnar - one layer of columnar cells covering a
surface, e.g. epithelial lining of small intestine.
a. Simple “low” columnar
b. Simple “tall” columnar
B. Stratified epithelial types- more than one layer
1. keratinized, stratified, squamous epithelium - e.g. skin. Actually, usually only the
cells at or near the free surface are squamous. Deeper cells tend to be cuboidal,
columnar or polyhedral.
2. non-keratinized, stratified squamous (mucosal) epithelium - e.g. line wet cavities
like the mouth, nasal passages and vagina. This epithelium often (but not always)
has glands associated with it. As above, only the cells at or near the free surface
are squamous.
3. stratified columnar epithelium - rare - e.g. parts of urethra, some ducts of the
paratid gland. Only the cells at the free surface are columnar.
4. stratified cuboidal epithelium - rare, found in some glandular ducts e.g. sweat
glands, sub-maxillary glands.
5. transitional epithelium - intermediate form of cells. Cells are often binucleate,
cells at free surface may bulge into adjacent lumen, e.g. inner lining of urinary
bladder.
6. pseudostratified epithelium - Looks stratified due to varying positions of cell
nuclei, but all cells are in contact with the basement membrane, e.g. ciliated,
pseudostratified, columnar epithelium of respiratory tract.
Types of epithelia - examples
Kidney - Bowman’s capsule
http://www.echt.chm.msu.edu/courseware/blockII/Pathology/RenalArchitecture_2.html
Types of epithelia - examples
Types of epithelia - examples
http://www.lab.anhb.uwa.edu.au/mb140/CorePages/Epithelia/Epithel.htm#labduo
Types of epithelia - examples
Cornea
Non-keratinized
http://www.anatomy.dal.ca/html/Human%20Histology/DHD/Lab2/Laboratory2.html
Inner lip epithelium
http://www.anatomy.dal.ca/html/Human%20Histology/DHD/Lab2/Laboratory2.html
http://www.lab.anhb.uwa.edu.au/mb140/CorePages/Epithelia/Epithel.htm#labduo
Types of epithelia - examples
Keratinized
http://www.lab.anhb.uwa.edu.au/mb140/CorePages/Integumentary/Integum.htm
http://www.mhhe.com/biosci/ap/histology_mh/stratepi.html
Types of epithelia - examples
www.mhhe.com/biosci/ ap/histology_mh/stratcuc.html
Glandular duct
http://hist.class.kmu.edu.tw/Basic/Epithelial/Stratified/Cuboidal/ep2c3h48_2.h
tm
Types of epithelia - examples
http://www.lab.anhb.uwa.edu.au/mb140/CorePages/Epithelia/Epithel.htm#labduo
Types of epithelia - examples
http://www.lab.anhb.uwa.edu.au/mb140/CorePages/Epithelia/Epithel.htm#labduo
Types of epithelia - examples
http://www.lab.anhb.uwa.edu.au/mb140/CorePages/Epithelia/Epithel.htm#labduo
http://www.udel.edu/Biology/Wags/histopage/colorpage/cep/ceptd.GIF
GENERAL CHARACTERISTICS OF EPITHELIA AND WHY
EPITHELIA HAVE THESE CHARACTERISTICS
I. The basement membrane is visible with the light microscope, particularly
when silver impregnation or the PAS (Periodic Acid Schiff stain - stains
carbohydrates) methods are used. Staining properties are due to the basement
membrane having a high glycoprotein content. The basement membrane stains
dark purple in the preparation of kidney tubules below.
http://www.mc.vanderbilt.edu/histo/BasicTissue/Epith.char.html
GENERAL CHARACTERISTICS OF EPITHELIA AND WHY
EPITHELIA HAVE THESE CHARACTERISTICS
There is more to the basement membrane than just a layer of glycoprotein.
A. When examined with the electron microscope, an epithelium has a basal lamina
(50 - 80 nm, not resolved by light microscope)
1. Found wherever an epithelium contacts connective tissue. Visible only
at EM level. Consists of granular layer of thin fibrils that is composed of
a. collagen,
b. a glycoprotein called laminin, and
c. other large molecules called proteoglycans.
2. Forms a partial barrier (semipermeable) to
diffusion between epithelium and underlying
tissue (usually connective tissue).
3. Evidence suggests that the basal lamina is
formed by the epithelial cells rather than the
connective tissue.
http://www.med.uiuc.edu/histo/small/atlas/image/tem38a/16250a1.htm
GENERAL CHARACTERISTICS OF EPITHELIA
B. Basal lamina consists of the,
* lamina densa - a fibrous, darkly staining layer into which fibers from
hemidesmosomes, as well as adjacent extracellular matrix, insert
* lamina rara (lamina lucida, reticular lamina) - a clear, non-staining layer
(may be on one or both sides of the lamina densa).
http://www.med.uiuc.edu/histo/large/atlas/image/tem38a/48750a1.htm
GENERAL CHARACTERISTICS OF EPITHELIA
II. Cohesion - like sticks to like
A. Epithelial cells cohere strongly to each other. In part due to binding action of
surface molecules such as glycoproteins.
B. Calcium is important to the maintenance of this cohesive effect. Evidenced by fact
that cells tend to disassociate in calcium free media.
GENERAL CHARACTERISTICS OF EPITHELIA
Cohesion
C. In addition, there are structures called desmosomes that also attach cells to one and
other. These can only be seen with the electron microscope.
1. Desmosomes (macula adherens) are
disk-shaped structures on cells that form
adjacent electron dense areas seen in
ultrastructural preps. Intermediate
filaments called tonofilaments (keratin)
insert from the cytoplasm into the electron
dense regions.
2. The electron dense region on the
cytoplasm side of the plasmalemma is an
attachment placque composed of
proteins called desmoplakin, plakoglobin
and plakofilin.
3. Cadherin proteins called desmogleins
and desmocollins extend between the
disk-like electron dense areas of each
adjacent cell.
4. These structures bind cells together.
They form bonds between cell
membranes below apical ends of cells.
http://celljunctions.med.nyu.edu/desmosomes/des-stokes.html
d. There are other types of
desmosomes
* septate demosome - have
very apparent fibrillar
structures in intercellular
space.
* hemidesmosome - half a
desmosome. Electron
dense substance ,and
tonofilaments are only
found on one side.
Example - binding points of
epithelia to basal lamina integrins bind the electron
dense region to the basal
lamina.
http://www.ru.ac.za/administrative/emu//gr10p7.htm
http://occawlonline.pearsoned.com/bookbind/pubbooks/be
cker_awl/medialib/art-ch11/1116.html
GENERAL CHARACTERISTICS OF EPITHELIA
Cohesion
4. Other junctional structures
Junctional complex at apical end
of cells. This was originally
observed with the light
microscope, but at the electron
microscopic level, it consists of
CONSISTS OF:
a. an apical tight junction (nothing can
get through it) that is called the zonula
occludens. The zonula occludens
restricts direct passage of solids and
fluids from outside of epithelia into
regions between the epithelial cells.
A number of proteins are involved in
the binding of adjacent plasmalemmas
in the zonula occludens region. The
major proteins are claudins and
occludins.
b. zonula adherens - some
similarity to desmosome, acts
as anchoring point for terminal
web of apical actin filaments
found in tall columnar epithelial
cells such as those lining the
intestine.
c. Also may be interdigitating
membrane below the zonula
adherens. This increases
area of contact between
adjacent cells and binding of
glycoproteins that hold cells
together.
http://www.siumed.edu/~dking2/erg/GI125b.htm
GENERAL CHARACTERISTICS OF EPITHELIA
III. Surface specializations of epithelia
A. microvilli - finger-like folds in the cell membrane of the free surface
of an epithelium. These structures increase surface area. Some times
they are branched and called stereocilia (however, branched microvilli
are not actually cilia). There is a core of actin microfilaments within the
microvillus that connects to the terminal web.
Stereocilia link
brush border
http://www.uni-mainz.de/FB/Medizin/Anatomie/workshop/EM/EMStereociliaE.html
http://education.vetmed.vt.edu/Curriculum/VM8054/Labs/Lab3/Examples/exmicvil.htm
III. Surface specializations of epithelia
B. cilia, flagella - same basic structure, differ in length (cilia short,
flagella long). Cause movement of materials over epithelia. Can
remove debri- e.g. tracheal cilia that direct mucus carrying
particulates out of trachea.
•These cilia are probably working hard at this time of
year.
•Smoking destroys tracheal cilia - this is one reason why
colds may be more severe for smokers than for nonsmokers. Also accounts for smoker’s cough.
http://cal.vet.upenn.edu/histo/epithelium/pseudostrat.html
SPECIALIZATIONS OF EPITHELIA
Glandular epithelia - secretory in function - glands are specializations of
epithelia. May remain connected to the originating epithelium or may separate
from it.
Major types of glands:
1. endocrine - secretion released into surrounding capillaries, no ducts.
Secretory cells often described as epithelioid.
2. exocrine - secretion usually released through ducts. Secretory cells
form a true epithelium.
* Both types (endocrine and exocrine) exist in the pancreas.
•exocrine - secreted digestive enzymes are carried from
pancreas through duct to duodenum.
•endocrine - Islets of Langerhans, insulin secreted directly
into capillary blood. Pituitary gland.
Types of exocrine secretion
a. merocrine - product released by exocytosis from cell
cytoplasm, e.g. acinar cells of pancreas
b. holocrine - whole cell is shed as secretion, e.g. sebaceous
glands of skin. Stem cells very important in this type of
secretion.
c. apocrine - apical portion of cell is shed as secretion, e.g.
mammary gland secretory cells, goblet cells
GLANDULAR EPITHELIA
Classification of exocrine glands (based on duct structure)
•Unicellular gland - no duct, individual secretory cells embedded in an
epithelium that lines a structure, e.g. goblet cells in epithelium of small
intestine, enteroendocrine cells.
•Simple gland - has a single non-branched (non-ramified)
duct
In figures below, duct is in green and the glandular (secretory) tissue in
red. The duct is what carries secreted material to the epithelial surface.
•coiled
Several bags or tubes sharing
the same duct
Classification of exocrine glands (based on duct structure)
•compound gland - has a branched (ramified) duct
Classification of exocrine glands (based on secretory component structure)
•tubular gland - secretory parts consist of tubes
Classification of exocrine glands (based on secretory component structure)
•coiled gland - type
of tubular gland
secretory portion
consists of a coiled tube
•acinar gland (grape-like)
•alveolar gland (tub-like)
essentially the same, secretory
parts, consist of bag-like structures
Classification of exocrine glands (based on secretory component structure)
•tubuloacinar gland
•tubuloalveolar gland
essentially the same, secretory parts
consist of a combination of bags and
tubes
BIOLOGY OF EPITHELIAL CELLS
A. There are no blood vessels in epithelial tissues and epithelial cells
usually do not contact capillaries (but some do, i.e. in kidney, pancreatic
islets) so nutrition usually depends on diffusion through tissues to
epithelial cells. Similarly, excretion of wastes usually is dependent on
diffusion. Excretion is sometimes accomplished by exocytosis.
B. Epithelia are often subject to high wear and tear. In such cases the
cells are continuously renewed by the mitotic activity of stem cells in the
epithelium, e.g. skin.
C. Epithelia may be subject to action of hormones - glandular epithelia
often increase or decrease secretion of product in response to
hormonal control.
BIOLOGY OF EPITHELIAL CELLS
D. Often have active transport systems that affect ionic balance within the cells and
underlying tissues
e.g. Renal tubules
•Tight junctions prevent direct flow of ions between cells of the epithelial
lining, i.e. ions must flow from lumen into epithelial cells.
•Thus, cell membranes of these epithelia contain various ion pumps that
can allow ions to move in either direction - active transport systems that
require ATP to function.
Kidney - glomerulus and convoluted tubules
http://www.homepage.montana.edu/~awmsg/510/histoex.html
•Since cells of the renal tubule epithelia are very active in the transport
of ions, lots of ATP is needed, i.e. will have lots of mitochondria.
•In some cases, umerous microvilli at lumenal surface of cells increase
the surface area of the plasmalemma across which ions may be
transported.
Figure 11.
A - nucleus of capillary
endothelial cell
B - basement membrane
C - lumen of renal tubule
D - intercellular space
(shrinkage artifact)
E - microvillar brush border
F - intertubular space
(connective tissue or
extracellular matrix)
G - lumen of capillary
http://www.homepage.montana.edu/~awmsg/510/histoex.html
BIOLOGY OF EPITHELIAL CELLS
E. Since cells of these sorts of epithelia have high rates of active
transport they are very metabolically active,
•Enzymes necessary for oxadative phosphorylation in
mitochondria, as well as other enzymes in the Krebs cycle and
glycolysis can be easily demonstrated by histochemical
methods.
RELATION OF STRUCTURE AND FUNCTION
A. See notes on renal tubules above.
B. Epithelial cells may be involved in controlling movement of large molecules into or out
of a particular part of the body. Thus, where diffusion is insufficient, you may find
exocytotic and/or pinocytotic vesicles, e.g.
1. Simple squamous epithelium (endothelium) of blood vessels - may transport
molecules across (into and out of) the cells via pinocytosis.
http://medocs.ucdavis.edu/cha/402/labsyl/99/05/em10.htm
RELATION OF STRUCTURE AND FUNCTION
2. Merocrine protein (enzymes)
secreting cells such as those
found in epithelium lining the
exocrine portion of pancreas
(pancreatic acinar cells) or
embedded in an epithelium
such as that lining glands of
the intestine. These cells have
membrane-bound vesicles
containing secretory product in
their cytoplasm. Exocytosis of
the contents of these vesicles
usually occurs at the apical end
("free surface") of the epithelial
cells.
http://www.udel.edu/Biology/Wags/histopage/empage/ep/ep.htm
RELATION OF STRUCTURE AND FUNCTION
a. Protein secretory cells have a
number of structural components
in common.
* lots of rough ER
* active golgi bodies, that are
usually perinuclear
* fairly large number of
mitochondria
* secretory granules for
exocytosis
* microvilli on apical surface
http://www.udel.edu/Biology/Wags/histopage/empage/ep/ep.htm