Epithelial Tissue, as topic in anatomy

1. EPITHELIA TISSUES
• The epithelial tissue or epithelium is
defined as a group of closely associated
cells with scanty intercellular materials that
cover external and internal surfaces of the
body. In some places the epithelial cells
form the glands of the body.

2. Epith. Tissue…
• i) It is made up of cells, which appear in different shapes
and structure; the differences reflect the functions of the
epithelium. The cells are in close contact and united to
one another through the intercellular junctions.
• ii) Generally there is no intercellular material between the
cells and if present is very little
• iii) The cells are attached to the underlying connective
tissue known as the basement membrane.
• Epithelial cells are polarized.
– i.e. the three cell surfaces can be distinguished; these are the
basal surface, lateral surface and the apical or luminal surface.
– Each surface has its own specialization;

3. Epith. Tissue…
• iv) The epithelial tissues are not
penetrated by the blood vessels.
• The blood vessels end at the basement
membrane and therefore nutrition is
achieved through diffusion from the
connective tissue beneath.
• v) Epithelial cells form all glands including
endocrine and exocrine glands.

4. Basement Membrane
• The basement membrane is a connective tissue structure where the
epithelial cells are attached.
• It is made up of collagen fibers, reticular fibers and
polysaccharide material.
• These fibers are secreted by the connective tissue cells and the cells
associated with it.
– It has two parts; the basal lamina/lamina densa, which and it is
made up of mainly polysaccharides.
– and the reticular lamina/lamina rara, which is made up of fine
collagen fibers, and reticular fibers.
• The basal lamina is a product of the epithelial cells, while the
reticular lamina is a product of the underlying connective tissue.
• Between the lamina densa and the basal surface of the epithelial
cells is a paler area or line referred to as lamina lucida, which
contain very fine filaments that join the basal membrane of the
epithelial cells to the underlying basal lamina.

5. Functions of the basal lamina
• i) Support to the epithelium: The basement
membrane supports the epithelial cells and
holds the basal cells of the epithelium to the
underlying connective tissue.
• ii) Barrier to the epithelium: The basement
membrane serves as a barrier that prevents
contact of these cells from unwanted materials
contained in the blood and blood cells.
• It regulates the exchange of materials between
the cells and blood.

6. General functions of the
epithelial tissue
• Before going into the specific type of epithelial cells the following are
the general functions of epithelial tissue.
• i) Absorption: The epithelium lines the surfaces that absorb
nutrients into the body.
• iii) Metabolic processes: Epithelial cells make parenchymal cells of
organs such as the liver, which perform a wide range of metabolic
activities such as detoxification, and utilization and storage of
nutrients that are absorbed from the intestines.
• iv) Signal transduction: Epithelial cells form special sensory areas
located in the retina of the eye, olfactory mucosa of the nose, in the
taste buds in the oral cavity and in the membranous part of the
middle ear, where they are known as hair cells.
• These cells are capable of transducing chemical, mechanical or light
energy into electrical signals that can be transmitted to the brain by
special nerves associated with them.

7. General functions…
• v) Covering: The epithelial cells also unite
to from a sheet of cells that cover different
parts of the body.
• These include the epidermis of the skin,
mucous membrane, mesothelium that
covers body cavities and organs, and the
endothelium that covers the inside of blood
vessels and the heart.

8. Classification of epithelial tissue
• The epithelial tissue is classified based on three
factors; according to the number of cell layers,
the shape of the cells and special features that
may be present on the surface cells.
• i) Classification according to the shape:.
Cells shapes can be squamous cells, cuboidal
cells and columnar cells.
• ii) Classification based on number of layers:
According to this classification three types of
epithelia can be identified as simple, stratified
and pseudostratified epithelia.

9. Classification of Epithelium

10. Classification of epithelial tissue
• a) Simple epithelium: In simple epithelia, cells are
organized in a single layer and therefore all cells are
attached to the basement membrane.
• The cells that make simple epithelium can be squamous,
cuboidal or columnar cells.
• Therefore; simple squamous epithelium, simple cuboidal
epithelium and simple columnar epithelium.
• These names indicate the shape of the cells and the
number of cell layer which is in this case one.
• Because the shapes of the cells include possible
function, they are found in different organs depending on
their function.

11. Classification of epithelial tissue
• i) Simple squamous epithelium
• Simple squamous epithelium forms a thin membrane
and therefore it is found in places where exchange of
material from one compartment to the other occurs.
Such areas include alveoli of the lung, Bowman’s
capsule of the nephron and the capillaries (Fig. 44). It
also forms the endothelium that lines the inside of blood
vessels and the mesothelium lining different organs. The
mesothelium is a serous membrane that covers organs
and body cavities and it includes peritoneal membrane,
pericardium and pleural membrane. Simple squamous
epithelium also forms the blood capillaries.

12. Simple Squamous Epith

13. Classification of epithelial tissue
• ii) Simple cuboidal epithelium: Cuboidal cells are larger
compared to squamous cells and therefore have a larger
cytoplasm for carrying synthetic activities and storage of secretory
vesicles. Simple cuboidal epithelium forms secretory cells, lining of
the small ducts of the glands and it covers the outer surfaces of
organs such as the ovaries
• iii) Simple Columnar epithelium: Columnar cells carrying out
metabolic activities and storage.
• Simple columnar cells are found in the glands where they form
glandular cells and the walls of the large ducts of the glands.
• They also line the absorptive surfaces of the gastrointestinal
tract, proximal convoluted tubules in the kidneys and the
gallbladder.
• In some organs the simple columnar epithelium contains cilia;
when cilia are present the epithelium is named simple ciliated
columnar epithelium.
• Ciliated cells line the uterine cavity, fallopian tubes and central
canal of the spinal cord.

14. iv) Pseudostratified epithelium
• :Pseudostratified epithelium is a simple
epithelium with one layer of cell but when viewed
by the microscope it appears to have more than
one layer of cells.
• Some cells are taller while others are shorter but
all cells are attached to the basement membrane.
• The other feature is that the nuclei of the cells are
seen at different levels.
• When this is viewed under the microscope, it
gives a false picture of stratification, thus the
name pseudostratification or false stratification.

15. Pseudostratified epithelium
• The shorter cells consist of
– mucus secreting cells, known as goblet cells and
mitotic cells that divide to replace other cells.
– This epithelium is found lining the respiratory tract,
normally the nasal cavity, nasopharynx, trachea, and
bronchi.
• The presence of mucus and cilia in this
epithelium helps to remove the particles that may
be inhaled.
• The mucus trap particulate material in the
inspired air and the cilia beats towards the
pharynx where they may be spitted, swallowed,
or expectorated during sneezing.

16. Stratified epithelium
• In stratified epithelium there is more than one layer of cells that
makes up the epithelium.
• The epithelium has basal cells that are attached to the basement
membrane and superficial cells that do not reach the membrane.
• Classification of stratified epithelium is based on the shape of
the surface cells and the presence or absence of keratin.
• i) Stratified squamous epithelium: It is the main protective
epithelium of the body external surface.
• e.g. in the skin it forms the epidermis that may contain up to 13
layers of cells.
– the cells are arranged into superficial, intermediate and basal
(deep) layers.
– The basal cells are oval round in shape and they show a high
rate of mitosis.
– Newly produced cells move to the upper superficial layers.

17. Stratified squamous epithelium
• The intermediate layers have got polyhedral cells, which
flatten towards the surface so that the most superficial
cells are squamous in shape. Two varieties of this type
of epithelium are recognized, that is non-keratinized and
keratinized.
• Stratified squamous non-keratinized epithelium: The
stratified squamous non-keratinized type is found on wet
mucous surfaces that are subjected to wear and tear due
to constant friction such as the oral cavity, oral pharynx,
oesophagus, distal half of the anal canal, the vagina, and
conjunctiva (Fig. 45 d). All cells in this epithelium are
living and therefore even the surface cells are nucleated.

18. Stratified squamous epithelium
• Stratified squamous keratinized epithelium: Stratified
squamous keratinized epithelium forms the epidermis of
the skin.
• In this epithelium the cells start producing and
accumulating keratin as they move from the deeper
layers.
• Keratin gradually replaces most of the cytoplasmic
organelles and the nucleus and as a result the
superficial cells die.
• Keratin is a tough fibrous protein that is impermeable to
bacteria and water, it therefore form a protective shield to
the epidermis
• As the cells ascend to the superficial layers they also
change in shape becoming more flattened as they
ascend to the surface, dead cells desquamate from the
skin.

19. ii) Stratified cuboidal epithelium
• stratified cuboidal epithelium consists of
two to three layers of cells of which the
superficial ones are cuboidal the rest
polyhedral in shape. It is a rare type of
epithelium and is found in ducts of sweat
glands, ducts of exocrine glands and in
ovarian follicles.

20. Strat. Columnar, Transitional
• iii) Stratified columnar epithelium: The stratified columnar
epithelium is a very rare type of epithelium. The deeper layers
contain cuboidal cells while superficial cells are columnar. It is found
in large ducts of exocrine glands and in the seminiferous tubules of
the testis.
• iv) Transitional epithelium: The transitional epithelium is a form of
stratified epithelium having the property of being considerably
stretchable.
• In the unstretched condition it may consist between 5 and 8 layers
thick.
– The basal cells are columnar
– the intermediate layers contain pear shaped cells and
– the superficial cells are umbrella-like and sometimes binucleated.
• In maximal distension the numbers of layers are reduced up to 3-4
layers with surface

21. Transitional Epith

22. Surface specializations of
epithelia
• Free Surface of Epithelial Cells may show three types of
specialization, including, cilia, microvilli and stereocilia.
• Microvilli: Microvilli are cell membrane extensions from
the free surface of epithelia; they are more prominent in
absorptive and actively secreting cells.
• Presence of microvili is believed to increase the surface
area for absorption or in exocytosis in secretory
cells.
• In the intestines and the proximal convoluted tubules
they are numerous, regularly arranged and uniform in
height.
• They form the striated border and in the proximal renal
tubules they form the brush border.

23. Cilia
• These are hair-like processes, which protrude from the
free surface of a cell and may number up to 270.
• Each cilium is 5-10 m in length and they grow from
within the cells from a structure called basal body.
• Cilia are motile and their function is to propel liquid,
mucus or foreign bodies.
• Cilia contain microtubules, an internal characteristic
feature, which makes them motile.
• Cilia are commonly found in the cells that line the
respiratory tract, the uterus and the fallopian tubes.
• In the respiratory tract, in which direction do the cilia
beat? Why
• In the fallopian tubes and uterus in which direction do the
cilia beat? Why

24. Stereocilia
• Stereocilia are also known as non-motile cilia.
These are long hair-like processes projecting
from the free surface of a cell.
• Stereocilia lacks microtubuleas and have been
suggested to be very long microvilli.
• Stereocilia are commonly found in the
epididymis, where they are believed to be
absorptive in function.
• Stereocilia is also found in the hair cells, which
are sensory cells of middle ear and they are
considered to increase the area for the sensory
receptors.

25. Glands
• In glands epithelial cells are modified to synthesize and
secrete the secretory materials.
• This transformation from the epithelial cells to glandular
cells occurs during development, when the glands are
developing.
• The epithelial cells proliferate to form cellular cords, which
evaginate into the underlying tissue to form clusters of
cells, forming the secretory end pieces of exocrine glands.
• Some of the cords persist to form the ducts of exocrine
glands.
• However all cords may disintegrate so that the clusters
are disconnected from the surface, thus forming ductless
glands or endocrine glands.

26. Classification of exocrine Glands
• Exocrine glands are classified under
morphological characteristics of their secretory
units, mode of secretion and the type of
secretory material produced by the gland.
• Morphological classification: In this type
secretory end-pieces are commonly either
tubular or rounded in shape; the rounded end-
pieces are commonly referred to as acini or
alveoli endpieces.
• The tubular and rounded endpieces may occur
together in one gland, the gland is named
tuboalveolar gland.

27. exocrine Glands
• Exocrine glands can therefore be
– simple alveolar glands,
– simple tubular glands and
– compound tubuloalveolar glands.
• The serous secretory glands, of the
exocrine pancreas are examples of
tubuloalveolar glands.

28. Classification according to mode
of secretion
• There are three types of secretory mechanisms, from the glandular
cells.
– exocytosis,
– apocrine secretion and
– holocrine secretion.
• Some glands show exocytosis, others show predominantly apocrine
but sometimes exocytosis occurs with apocrine.
• Exocytosis: In this type secretory vesicles are released without loss
of cytoplasm.
• It constitutes most glands of the body including endocrine glands.
• The material to be secreted is normally synthesized in the rough
endoplasmic reticulum, which pack in membrane bound vesicles
called transport vesicles; which are conveyed to the Golgi apparatus
• In the GA, they are modified and packed into membrane vesicles
called secretory vesicles.

29. Exocytosis
• Exocytosis is the most commonly occurring type of
secretion and most apocrine releasing cells also secrete
via exocytosis.
• Goblet cells and glandular cells of the prostate,
bulbourethral glands and seminal vesicles show both
exocytosis and apocrine secretory processes.
• During exocytosis the exocytotic vesicles move and
make contact with the plasma membrane; this induces
membrane fusion and release of secretory materials into
the extracellular space.
• Molecules such N-ethylmaleimidine sensitive factor
(NSF), soluble NSF attachment proteins (SNAPs),
synaptophysin, synaptotagmin and synaptobrevin trigger
exocytosis and pore formation.

30. Role of Intracellular calcium in
exocytosis
• Intracellular calcium ions also play important role
during exocytosis.
• Observations that have been made in exocrine
and endocrine glands have showed there are 2
types of exocytosis:
– constituve and
– regulated secretion.
• In constitutive secretion occurs without
stimulation and may occur at lower levels of
calcium ions.
• In regulated secretion occurs when the
glandular cell is stimulated, in presence of high
concentration of calcium ions.

31. Apocrine
• In apocrine secretion there is loss of part of cytoplasm,
when secretory process occurs.
• It is seen currently in the mammary gland, Hardenian
gland, sex glands and apocrine glands of the skin.
• Most glands that show apocrine secretion also secrete
via exocytosis.
• Apocrine secretion is commonly associated with the
appearance of apical protrusions, but it can also occur in
the absence of apical protrusion.
• In non-apical protrusion forming apocrine secretion there
is massive release of the secretory vesicles that become
associated with loss of cytoplasm.

32. Apocrine
• The protrusions that are seen during
apocrine secretion contain the secretory
materials and part of the cytoplasm.
• During secretion the secretory vesicles
and the cytoplasm are released to the
extracellular compartment.
• Apical protrusions are released via three
processes; these are pinching-off,
decapitation and pore formation.

33. Pinching off, Decapitation
• Pinching-off: Pinching-off has been described in the
prostate, breast and the Harderian gland.
• Morphologically the protrusion is connected to the
plasma membrane by a stalk.
• Progressive thinning of the connecting stalk causes
pinching-off at the base of the stalk and release of the
protrusion.
• Decapitation: Decapitation release process has been
described in the apocrine glands of human axilla and the
Harderian gland.
• Decapitation occurs via two mechanisms:
– The first one involves formation of dermacating vesicles at the
base of the protrusion. This is the most observed process.
– In this mechanism the dermacating vesicles appear at the base of
the protrusion separate the vesicle from the plasma membrane.
– Fusion of the vesicles with each other and with the lateral cell
membrane results into decapitation and release of the protrusion.

34. Decapitation
– The demarcating vesicles can be the secretory vesicles or
vesicles other than the secretory ones.
• The second mechanism is when there is seen in glands
in which the protrusion forms over the entire apical
membrane of a cell.
– It is characterized by division of the plasma membrane; one
beneath the protrusion and the other on the apical part of the
remaining cell.
– Both membranes terminate at the lateral cell margin.
– Formation of the dividing membrane is followed by the
appearance of tubules above and parallel to the dividing
membrane.
– Fusion of the tubules causes detachment of the decapitation.
– Decapitation via formation of the dividing membrane is observed
in the apocrine glands of the skin.

35. Pore formation
• Pore formation: In pore formation the
appearance of apical protrusion is followed
by the formation of small defective areas
(sieve-like pores), through which
cytoplasmic secretory materials escape to
the exterior.
– Pore formation occurs in human axillary
apocrine glands.
– The cytoplasm that escapes through the
pores contains the materials to be secreted

36. Holocrine
• This is a very rare type where the entire cell is
lost during secretion.
• Holocrine secretion is found in the sebaceous
gland and has also been described in the
immune cells and cells of the bovine pituitary
gland.
• Sebaceous glands secrete oily materials called
sebum and are located in hair covered areas of
the body and at the mucosal margins of the
lips, eyelids, penis, labia minora and the
nipples.

37. Classification according to
secretory materials
• Mucous secretory cells
• Serous secretory cells