Download epithelium - Shabeer Dawar

EPITHELIUM
TISSUE
Groups of cells which are similar
in structure and which perform
common or related functions.
There are (4) types of tissue:
1. Epithelial
2. Connective
3. Muscle
4. Nervous
TERMS TO KNOW
Terms referring to the cell shapes
• Squamous = flat
•
Cuboidal = cube
•
Columnar = rectangular (column)
• Transitional = ability to change shape
Terms referring to the layers
• Simple = one layer
• Stratified = more than one layer
• Pseudostratified = false layered (appears to
be more than one layer, but has only one
layer)
EPITHELIUM TISSUE
A collection of closely apposed cells that lines the hollow
organs and glands is called epithelium.
• Epithelium is avascular and is nourished by diffusion.
• All epithelia are supported by loose connective tissue.
• Between epithelium and connective tissue there is always
a basement membrane.
• Epithelial cells cover the internal and external surfaces of
the body and also form parenchyma of various glands on
the basis of which they are divided into two main groups
1: glandular epithelia
2: covering epithelia (surface epithelia)
EPITHELIAL CLASSIFICATIONS
Classified on the basis of shape of cell and number of cell
layers
 Shape of cells
• squamous = thin, flat cells
• cuboidal = cube-shaped cells
• columnar = tall, elongated cells
 Number of cell layers
• simple = single layer
• stratified = 2 or more layers
Apical
surface
Basal surface
Simple
Apical
surfac
e
Squamous
Cuboidal
Columnar
Classification based on cell shape.
Glandular epithelium
Glandular epithelium is the membranous tissue made up of
cells that covers all the glands in the body and secret fluids
into ducts or fluids of the body.
• A gland consists of one cell or a group of cells
• Endocrine Glands – secretions enter the interstitial fluid
• They secret Hormones
• Exocrine Glands – secrete their products into ducts (tubes)
that empty at the surface of the covering/lining epithelium
• They secrete Mucus, oil, earwax, digestive enzymes
COVERING EPITHELIA
• The cells of the covering epithelia are arranged into one or
more layers which coat the body surface that is why it is also
called surface epithelia.
Example: epidermis of the skin that covers the outer surface of
the body is the best example
• Spaces and passages that open to outside are also lined by
covering epithelia(respiratory, digestive, and urogenital
passages)
• It also cover the internal(luminal surfaces) of heart, blood
vessels and lymph vessels
Body cavities except joint cavities are also lined by surface
epithelia
CLASSIFICATION OF COVERING EPITHELIA
Depending on the number of cell layers present covering
epithelia are divided into following main types
1. Simple epithelia(unilaminar epithelia)
It has only single layer of cells.
2. Stratified epithelia(multilaminar epithelia)
Consist of two or more layers of epithelial cells.
SIMPLE EPITHELIA
• It has only one layer of cells
• Each cell has a free surface and a basal surface resting on basement
membrane.
• According to the shape of the component cells simple epithelia are further
classified as under
1. simple squamous epithelium
This type has flat cells
2. simple cuboidal epithelium
It consists of cube like cuboidal cells
3. simple columnar epithelium
It consists of column shaped cells
4. simple pseudo stratified epithelium
Which really have one layer of cells but give the appearance of being
stratified(having many layer of cells)
Simple squamous epithelium
Description:
• Single layer of flattened cells
• the simplest of the epithelia.
• roughly polygonal
• Cells are having serrated or wavy margins and from a
sheet
• Has ovoid or flattened nuclei in located in center
Function:
• Allows passage of materials by diffusion and filtration
• secretes lubricating substances in serosae.
Location:
Kidney glomeruli; air sacs of lungs; lining of heart, blood
vessels, and lymphatic vessels; lining body cavity
(serosae).
b) Simple cuboidal epithelium
Description:
• single layer of cube like cells with large, spherical central
nuclei.
• the height and width of each cell is equal
• the cells form a mosaic
Function:
Secretion and absorption.
Location:
Kidney tubules; ducts and secretory portions of small
glands; ovary surface.
(c) Simple columnar epithelium
Description:
• Single layer of tall cells with round to oval nuclei located in the base
of the cell
• some cells bear cilia
• layer may contain mucus- secreting unicellular glands (goblet cells).
• On surface view appear like a mosaic as like simple cuboidal cells
Function:
Absorption and secretion of mucus, enzymes, and other substances;
ciliated type propels mucus (or reproductive cells) by ciliary action.
Location:
• Nonciliated type lines most of the digestive tract (stomach to anal
canal), gallbladder, and excretory ducts of some glands;
• ciliated variety lines small bronchi, uterine tubes, and some regions
of the uterus.
SIMPLE EPITHELIUM
(d) Pseudo stratified (columnar )epithelium
Description:
• this is a modification of simple columnar epithelium in
•
•
•
•
which the nuclei are at different levels giving the false
appearance of being stratified
Single layer of cells of differing heights, some not reaching
the free surface
may contain mucus secreting cells and bear cilia.
All cells rest on the basement membrane but all do not
extend to the free surface
Those that reach the free surface are tall columnar cells and
are widest near the free surface but basal part of each cell
narrows to form a slender process
(d) Pseudo stratified (columnar )epithelium
• A second kind of cells called basal cells are present
between the columnar cells, they are conical in shape
with a broad base resting on the basal lamina
• The basal cells taper early and do not reach to the
surface
Function:
Secretion, particularly of mucus; propulsion of mucus by
ciliary action.
Location:
• Nonciliated type in male’s sperm-carrying ducts and
ducts of large glands
• ciliated variety lines the trachea, most of the upper
respiratory tract.
(d) Pseudo stratified (columnar )epithelium
STRATIFIES EPITHELIUM
Epithelia that consists of two or more layers is called
stratified epithelium.
Further classified on the basis of shape of cells in the
superficial layer as under
1. Stratified squamous
2. Stratified cuboidal
3. Stratified columnar
(e) Stratified squamous epithelium
Description:
• Thick membrane composed of several cell layers; basal
cells are cuboidal or columnar and metabolically active;
• surface cells are flattened (squamous);
• basal cells are active in mitosis and produce the cells of the
more superficial layers.
• In places where the surafce cells are placed to more
attrition and drying like skin epidermis the cells are nonnucleated and the cells cytoplasm has protein keratin and
are called keratinized cells.
(e) Stratified squamous epithelium
• In wet areas the cells are nucleated and contain little or
no keratin.
• In both sub-types of stratified squanous epithelium the
most superficial layer constantly flake off from the
surface.
• The basal layer produce new cells which move toward
the surface and flake off.
Function: Protects underlying tissues in areas subjected
to abrasion.
Location:
• Nonkeratinized type forms the moist linings of the
esophagus, mouth, and vagina
• keratinized variety forms the epidermis of the skin.
STRATIFIED CUBOIDAL EPITHEILIUM
• Consists of two layers of cuboidal cells
• Location: ducts of sweat glands
STRATIFIED COLUMNAR EPITHELIUM
Consists of columnar cells resting on one or more layers of
roughly cuboidal cells.
Location:
• lines ducts of main salivary glands and pancreas
• Lines ductus deferens and male urethra
(f) Transitional epithelium
Description:
• Resembles both stratified squamous and stratified cuboidal
epithelium.
• basal cells are cuboidal or columnar
• surface cells are dome shaped or squamous like, depending
on degree of organ stretch.
Function: Stretches readily and permits distension of urinary
organ by contained urine.
Location:
Lines the ureters, urinary bladder, and part of the urethra.
TWO SPECIAL VARIETIES OF EPITHELIAL CELLS
1: NEURO-EPITHELIAL CELLS:
• These are columnar cells and have cilia or sterocellia on
their free surface.
• Found in special sense organs like taste buds and
vestibular receptor system.
• These cells are specialized as sensory receptors for the
reception of external stimuli.
• Afferent nerve fibers terminate at neuro-epithelial cells
and form synapse with them.
TWO SPECIAL VARIETIES OF
EPITHELIAL CELLS
2: MYO-EPITHELIAL CELLS:
• Also called basket cells.
• Their cytoplasm contain contractile filament(actin,myosin)
• These cells contract when stimulated by nervous or neurohormonal signals
• Found around secretory acini of mammary ,salivary, lacrimal
and sweat glands.
Function:
• Their contraction assist in the flow of secretion into the larger
channels.
Microvilli
• Small slender, finger like projections found on surface of epithelial
cells
• E/M reveals that they are cytoplasmic processes that extend from
cell surfaces
• E/M also shows that each microvillus contains a central bundle of 20
to 30 actin filaments.
• they Insert into terminal web
LOCATION:
• Columnar cells lining the luminal surface of small intestine has
numerous microvilli
• Also located in proximal convoluted tubules of kidney where they
are longer and appear as a brush border on L/M
FUNCTION:
• Increase area for absorption as in small intestine
• Facilitate the absorptive process
CILIUM
• Hair like structure found on the free surface of those
epithelial cells which are specialized for transport of fluid
or mucus .
• They are Motile processes of microtubules that move
synchronously
• Visible under L/M measuring 5-10 micro-meter in length
and 0.2 micrometer in diameter.
• They can move to and fro and are also called kinocillia.
• Distinguished from sterocillia which are nonmotile.
CILLIUM………….
• 9+2 microtubule arrangement, two central microtubules
surrounded by 9 circularly arranged doublet peripheral
microtubules
• The microtubules extend from the tip of the cilium to the
base
FUNCTION:
Helps in
• propulsion or
movement of
 fluid
mucus
particulate
matter
ovum
STEREOCILIA
• Under L/M appear as thin, hair like structure which are in contact with
each other and form small tufts.
• Under light microscope they resemble cilia but they are non-motile and
that is why they are called sterocillia (stero=solid or non-motile)
• Long microvilli (NOT CILIA)
• Non-motile
• Average length of a sterocillium is 30 micrometer
LOCATION:
• Located in the cells lining the ducts of epididymis
• In neuroepithelium
• In taste buds
FUNCTION:
Increase the surface area in epididymis to facilitate reabsorption.
In taste buds they respond to electrical signals
FLAGELLUM
• A long whip like motile projection from a cell is called
flagellum.
• It has two central microtubules and 9 peripheral doublets
like cilia but a flagellum has a greater length.
• Its movement is different from that of cilia
FUNCTION:
• Help in cell movements
LOCATION:
Spermatozoon(sperm cells)