Download • Histology is the study of the tissues of the body and how these

Biology \Histology lec1
Epithelial tissue
lecturer: Farah E. AlRawi
 Histology is the study of the tissues of the body and how these tissues
are arranged to constitute organs.
 Tissue
is
an
aggregate
of cells in
an organism that
have
similar structure and function.
 The human body is composed of only four basic types of tissue:
epithelial, connective, muscular, and nervous.
 Tissues have two interacting components: cells and extracellular
matrix (ECM).
 The main characteristics of these basic types of tissue are shown in
this table
Tissue
cells
Extracellular Main Functions
matrix
Nervous
Elongated cells with Very
extremely
small Transmission
fine amount
of
nerve impulses
processes
epithelial
Aggregated
Small amount Lining of surface
polyhedral cells
or body cavities;
glandular
secretion
Muscle
Elongated
Moderate
Strong
contractile cells
amount
contraction; body
movements
connective Several
types
of Abundant
Support
fixed and wandering amount
protection
cells
tissues/
organs
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and
of
Biology \Histology lec1
Epithelial tissue
lecturer: Farah E. AlRawi
Epithelial Tissue:
[Epithelium — a single tissue; plural, epithelia; adjective, epithelial]
Epithelia are cellular sheets that line the cavities of organs and cover the
body surface.
The principal functions of epithelial tissues include the following:
1. Covering, lining, and protecting surfaces. Example: epithelial cells
of the skin protect underlying tissue from mechanical injury,
harmful chemicals, invading bacteria and from excessive loss of
water
2. Absorption: certain epithelial cells lining the small intestine absorb
nutrients from the digestion of food.
3. Secretion: in glands, epithelial tissue is specialized to secrete
specific chemical substances such as enzymes, hormones and
lubricating fluids
4. Contractility: myoepithelial cells.
5. Sensation: Specialized epithelial tissue containing sensory nerve
endings is found in the skin, eyes, ears, nose and the tongue.
6. Cellular transport: Transport of molecules across epithelial layers.
Characteristics features of epithelial tissue:
 Epithelium lines the surfaces of the body and is mainly located on the
borders between the external and internal environments. Epithelium also
lines all the internal body spaces that have a connection with the external
environment at some stage.
 Epithelium plays an important role in homeostasis of the body and in
maintaining the physiological parameters of the internal environment
different from those outside the body.
 Epithelium is an avascular tissue and has no integral blood supply.
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Biology \Histology lec1
Epithelial tissue
lecturer: Farah E. AlRawi
 Epithelium develops in the embryo from all the three germ layers
(Ectoderm, Mesoderm, and Endoderm). For example, the epidermis of
the skin is derived from ectoderm, the epithelium lining the serous
cavities (peritoneum, pleura, pericardium) is derived from mesoderm (and
is often referred to as mesothelium), whereas the epithelium lining most
of the intestinal tract is derived from endoderm.
 Most epithelia rest on connective tissue that contains the
microvasculature bringing nutrients and oxygen to both tissues.
The connective tissue that underlies the epithelia lining the organs of the
digestive, respiratory, and urinary systems is called the lamina propria.
The area of contact between the epithelium and connective tissue may be
increased by irregularities at the interface in the form of small
evaginations called papillae. Papillae occur most frequently in epithelial
tissues subject to friction, such as the covering of the skin or tongue.
 Polarity– all epithelia have an apical pole (Apical domain, directed
towards the exterior surface) and a lower attached basal pole (Basal
domain, rest on the basal lamina anchoring the cell to underlying
connective tissue) that differ in structure and function.
Regions of cells that adjoin the neighboring cells are the lateral surfaces.
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Biology \Histology lec1
Epithelial tissue
lecturer: Farah E. AlRawi
 Epithelial cells have specific structural modifications on the apical
surface (the free surface facing the lumen or external environment)
include: microvilli, stereocilia, cilia or flagella.
 Epithelial cells adhere strongly to neighboring cells and basal laminae
with no intercellular connective tissue, particularly in epithelia subject
to friction or other mechanical forces. Lateral surfaces of epithelial
cells exhibit several specialized intercellular junctions, which play
roles in maintaining the integrity of the tissue.
Junctional complexes of epithelial cells:
1. Tight junctions (zonulae occludens) form a seal between adjacent
cells prevent passive flow of chemical materials or waste products
between the cells and into the blood stream but are not very strong
junction .eg: the lining epithelium of the urinary bladder.
2. Adherent or anchoring junctions (zonulae adherence) are sites
of string cell adhesion serve to stabilize and strengthen the circular
occluding bands and help hold the cells together.
3. Desmosome or macula adherence form very strong attachment
points and play a major role to maintain the integrity of an
epithelium. most commonly seen in the epithelium of the skin and
in cardiac muscle fibers
4. Gap junctions have little strength but serve as intercellular
channels for flow of molecules and communication between
adjacent cells.
 Hemidesmosomes bind epithelial cells to the underlying basal
lamina.
Basal laminae and basement membranes
 All epithelial cells have at their basal surface a sheet like extracellular
structure called the basal lamina, separating them from the
underlying connective tissue (lamina propria). The basal laminae, are
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Biology \Histology lec1
Epithelial tissue
lecturer: Farah E. AlRawi
visible only by transmission electron microscopy, where are formed
from an electron-dense layer (20-100 nm thick) composed of:
1. lamina lucidae: which appear to be transparent.
2. lamina densa: a delicate network of fine fibrils.
Basal laminae are composed mainly of type IV collagen and a
glycoprotein, called laminin.
Basal laminae are not exclusive features of epithelia, but are also found
associated with some other cell types.
Basal lamina is sometimes attached to the underlying connective tissue by
anchoring fibrils of unknown composition.
In some instance, reticular fibers are closely associated with the basal
lamina forming a layer termed the reticular lamina, the reticular fibers
are produced by connective tissue cells and it is responsible for affixing
the lamina densa to the underlying connective tissue thus the epithelial
sheath is bound to the underlying connective tissue.
 Basement membrane:
Is formed by the combination of a basal lamina and a reticular lamina and
therefore, it is thicker. It is stained well by Periodic acid Schiff reagent
and so it is seen by the light microscope.
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Biology \Histology lec1
Epithelial tissue
lecturer: Farah E. AlRawi
Functions of basal lamina:
1. It is considered as a molecular filter and as a flexible, firm support for
the overlying epithelium.
2. Provide a selective barrier between connective tissue and other cells.
3. The presence of the basal lamina around a muscle cell is necessary for
the establishment of new neuromuscular junctions.
4. The ability to influence cell polarity.
5. Regulate cell proliferation and differentiation by binding with growth
factors.
6. Influence cell metabolism.
Classification of Epithelia
 Epithelia are divided into main groups according to their structure
and function:
1. Covering epithelia or lining epithelia.
2. Glandular (secretory) epithelia.
Covering epithelia
 Covering epithelia are tissues in which the cells are organized in
layer that cover the external surface or line the cavities of the body.
 Covering epithelia are classified according to the number of cell
layers to Simple epithelia contain one cell layer and stratified
epithelia contain two or more layers.
 Covering epithelia are classified according to the cell morphology
in the surface layer Based on cell shape, as squamous (thin cells),
cuboidal (cell width and thickness roughly similar) or columnar
(cells taller than they are wide).
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Biology \Histology lec1
Epithelial tissue
lecturer: Farah E. AlRawi
Simple epithelia
1. Simple squamous epithelium:
Squamous cells have the appearance of thin, flat
plates. Squamous cells tend to have horizontal
flattened nuclei because of the thin flattened form
of the cell. They form the lining of cavities such as
blood vessels (endothelium), and parietal wall of
Bowmanns capsule in kidney glomeruli, also line the serous cavities of
the body (peritoneum, pleura, pericardium) which known as Mesothelia.
The main functions of simple squamous epithelium are filtration,
diffusion, transport, secretion, and reduction of friction.
2. Simple Cuboidal Epithelium:
Cuboidal cells are roughly square or cuboidal in shape. Each
cell has a spherical nucleus in the centre. Cuboidal
epithelium line many small ducts in the body. Examples
include the urinary ducts of the kidney, the bile ductules of
the liver, or the cells lining thyroid follicles.
The main functions of simple cuboidal epithelium are:
Protects ducts; transports and absorbs filtered material in kidney tubules.
3. Simple Columnar Epithelium:
Columnar epithelial cells are elongated and
rectangular-shaped. The nuclei are elongated and
are usually located near the base of the cells. It is
either ciliated or non ciliated. Non ciliated
columnar epithelium forms the lining of the stomach and gall bladder, the
larger ducts near the urinary papilla of the renal pyramids.
Ciliated epithelium (or called simple columnar
with striated border or with brush border) is
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Biology \Histology lec1
Epithelial tissue
lecturer: Farah E. AlRawi
usually found in the small intestine (jujenum), there are goblet cells
between the columnar cells in the jujenum and the cilia is called brush
border.
The main functions of simple columnar epithelium are: Secretes
protective mucus for stomach lining, absorption of nutrients in small
intestine, protection and lubrication.
4. Pseudo stratified epithelium:
Pseudo stratified epithelium is consists of a
single layer of cells in which all cells attach to
the basement membrane but not all cells reach
the surface. It consist of different cell shapes
(fusiform, columnar, and basal cells), the nuclei of these cells located at
different levels so giving the tissue this pseudo stratified pattern.
It either be ciliated or non ciliated. It is lining the passage of the
respiratory system (e.g. trachea) which is the ciliated form. The non
ciliated form is present in ducts of parotid glands.
The main functions of pseudostratified epithelium are:
Protection, secretion; cilia-mediated transport of particles trapped in
mucus out of the air passages
Stratified epithelia
The stratified epithelial tissue is made of multiple layers of cells and the
superficial cell layer determining the epithelial type
1. Stratified squamous epithelial tissue:
Composed of many layers of cells, the basal layer composed of columnar
or cuboidal cells, the middle cell layers composed of polygonal cells, and
the superficial layer is squamous cells.
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Biology \Histology lec1
Epithelial tissue
lecturer: Farah E. AlRawi
There are two types of stratified squamous epithelium:
a) Stratified squamous non- keratinized type (moist):
These exhibit live superficial cells contain nuclei.
It lines the moist cavities of the mouth, esophagus, vagina and anal canal.
The main functions of Nonkeratinized squamous are: protection,
secretion and prevent water loss.
b) stratified squamous keratinized type(dry):
It lines the dry areas like the skin and contains non- living cells rich with
keratin intermediate filament and containing no nucleus.
The main functions of keratinized epithelium are protection against
abrasion, bacterial invasion, and desiccation.
2. stratified cuboidal epithelium:
It is Consist of two layers of cuboidal cells, the nuclei are large and
central, is found in large excretory ducts of sweat and salivary glands.
The main function of Cuboidal epithelium is providing protection for the
ducts.
3. stratified columnar epithelium:
It is present only in small areas in the human body such as the
conjunctiva of the eye. The superficial cells are columnar and below there
are polygonal cells and the basal cells either cuboidal or columnar. And
the main function of stratified columnar epithelium is protection
4. Transitional epithelium:
Transitional epithelium is found exclusively in renal calyces, renal pelvis,
ureters, and bladder
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Biology \Histology lec1
Epithelial tissue
lecturer: Farah E. AlRawi
Transitional epithelium is characterized by a superficial layer of large,
dome-like cells sometimes called umbrella cells. These cells are
specialized to protect underlying tissues from the hypertonic and
potentially cytotoxic effects of urine (forms a protective osmotic barrier
between urine in the bladder and the underlying connective tissue).
The epithelium changes its cell shape in response to either stretching, as a
result of fluid accumulation, or contraction during voiding of urine.
In a relaxed, unstretched condition, the surfaces cells are usually cuboidal
or appear dome-shaped. Frequently, binucleate (two nuclei) cells are
visible in the superficial layers or surface cells of the bladder. While in
stretched conditions, the surface epithelium appears squamous.
Proliferation and regeneration
Epithelia are present in vulnerable sites of the body, where they are
continually exposed to the hazards of the external environment. Epithelial
cells are constantly subjected to mechanical damage, destroyed, or
sloughed off.
Epithelium have a high regenerative capacity and can reproduce rapidly
as long as they receive adequate nutrition and typically show many
dividing cells (mitoses) in order to replace the cells lost and maintain the
integrity of the tissue.
In cases of trauma or wounds, the epithelia need to cover the lesion as
rapidly as possible, repair the lining tissue and prevent damage to the
underlying tissues.
 Most of the cancers of the body are the result of uncontrolled
proliferation of epithelial cells.
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