Epithelia
Epithelia are tissues consisting of closely
apposed cells with very little intercellular
substances.
 They Epithelia are avascular but all
epithelia grow on vascular connective
tissue.
 Epithelia and underlying connective tissue
are seperated by basement membrane
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Functions
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Covering (eg: skin)
Lining of all the body cavities (eg :
intestines)
Secretory(eg: glands)
Epithelia covers all free surface of the
body, where it is called mesothelium and
The internal surfaces of blood and lymph
vessels are lined by epithelium, here they
are called endothelium
The cells on the free surface are called
apical cells
Classification of epithelia
Covering epithelia
 Glandular epithelia(with secretory
function)
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Classification of covering
epithelium
Epithelial tissue is classified based on the number of cell
layers and the shape of cells in apical layer
Based on number of
cells:
1. Simple epithelium:
there is one layer of
cells
2. Stratified
epithelium:two or
more layers of cells,
it is termed stratified

Based on the shape of
cells in the surface
layer:
1. Squamous :
flattened scale or
plate like cells
2. Cuboidal : Cube
shaped cells
3. Columnar : cells are
taller than wide
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Nucleus of the cells corresponds to the
shape of the cells and also the long axis of
the nucleus is always parallel to the main
axis of the cell.
Simple Squamous Epithelium
One layer of flattened, scale or plate like
cells.
 It lines all serous lining of body cavities;
pericardium, pleura, peritonium ; blood
vessels and lymph vessels(endothelium)
 The nuclei are often flattened or ovoid, ie
egg shaped, and they are located close to
the centre of cells
 Diffusion and filtration takes place across
the single layer of cells
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Simple Cuboidal Cells
One layer of cube shaped cells
 It occurs in,
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 small excretory ducts of many glands(eg salivary
glands),
 follicles of thyroid gland,
 tubules of kidney and
 on the surface of ovaries.
Nuclei are spherical and are at the centre
 Helps in secretion and absorption
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Simple Columnar Epithelium
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One layer of tall,rectangular shaped
cells;cells are taller than they are wide
Lines the internal surface of GI tract from
cardia of the stomach to the rectum
Lining of gall bladder
The nucleus is located at the same height
within the cells,often close to the base of
the cells
Helps in secretion and absorption
Pseudo-stratified Columnar
epithelium
One layer of columnar or irregularly
shaped cells. Some kinds have
cilia(pseudo stratified ciliated columnar
epithelium) and goblet cells.
 Lines trachea,bronchi and nasal cavity
 Helps in protection and secretion
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Stratified Squamous Epithelia
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Basal cells are present in the basal layer
which are in contact with basement
membrane
Mitotically active and renew the cells
These basal cells are either columnar or
cuboidal in shape.
The apical layer cells of the epithelium
become more flattened.
Present in the places subjected to wear
and tear like skin, mouth, esophagus and
vagina
Classification of stratified
squamous epithelium
Keratinized: dry
surfaces such as the
epidermis of the skin
 Contains keratin,
most superficial layer
consisting of dead
cells
 Functions in
protection
Non-Keratinized:
most superficial cells
are alive kept moist
 Linings of nasal
cavity, oral
cavity,pharynx,
vagina and the lower
anal canal.
 Functions in
protection
Stratified Cuboidal and
Columnar
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Stratified cuboidal
epithelium, for
example is seen in the
ducts of the sweat
glands.
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For example found in
the excretory ducts of
the mammary glands
and the main
excretory ducts of the
large salivary glands
Transitional epithelium
Dome shaped cells which are neither
squamous nor columnar.
 Some cells are binucleated
 Found in ureter,urinary bladder and upper
part of urethra
 Withstands distention and relaxing of
bladder.
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Glandular Epithelium
All glands of the body arise from covering epithelium by means of
proliferation and invasion of the epithelial cells. Glands further
differentiate into exocrine and endocrine glands
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Exocrine glands retain
their connection with the
surface epithelium from
which they originated.
Their connection is
transformed into tubular
ducts lined with epithelial
cells through which the
glandular secretions pass
to reach the surface
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Endocrine glands are the
ones which lose their
connection during
development process
Releases their secretory
products (typically
hormones) into spaces
between secretory cells
from which it enters the
bloodstream
Exocrine glands
According to the # of cells the glands are
classified into unicellular and multicellular
glands
 Multicellular glands are further classified
into merocrine, apocrine, holocrine glands
based on the secretory mechanisms
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Unicellular Glands
Goblet cells are the only unicellular glands
secreting mucus
 Respiratory and Digestive system and
other mucus membranses
 Goblet cells secretes glycoprotin mucin
which after combination with water is
secreted as mucus
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Merocrine glands
Merocrine secretion: corresponds to the
process of exocytosis.
 Vesicles open onto the surface of the cell
and the secretory product is discharged
from the cell without any further loss of
cell substance.
 Secretes a watery fluid through cell
membrane
 Eg:salivary glands
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Apocrine glands
Mechanism in which part of apical
cytoplasm of the cells is lost together with
the secretory product.
 Apocrine sweat glands, the mammary
glands, and the prostate.
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Halocrine glands
Designates the break down and discharge
of the entire secretory cell
 Entire cell is released and it ruptures and
dies
 Eg:sebaceous glands
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Multicellular exocrine glands
Simple columnar
B. Simple columnar
epithelium with cilia
C. Stratified squamous
D. Simple squamous
E. Transitional
F. Pseudostratified
G. Cuboidal epithelium
H. Choanocytes
I. Stratified columnar
epithelium with cilia
A.
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Simple columnar epithelium
Simple columnar epithelium with cilia
Stratified squamous epithelium
Simple squamous epithelium
Transitional epithelium
Pseudostratified epithelium
Cuboidal epithelium
Choanocytes
Stratified columnar epithelium with cilia