Types of epithelia
By
Dr. mohamed sabaa ch.
M.BCH.B M.Sc.Pathology
- Epithelia can be divided
into 2 main groups
according to their structure
and function:
1- covering(or lining).
2- glandular .
Covering or lining epithelia:
1- the cells are organized in layers that
cover the external surface or line the
cavities of the body.
2- they are classified according to the
……………..and ……………..of the cells in
surface layer.
3- they classified according to no. of cells layers
into :
A- Simple (only one layer).
B- Stratified(2 or more layer).
4- Based on the shape they classified into :
A-Cuboidal.
B- Squamous.
C- Columnar.
D- Transitional.
Simple squamous epithelia
1-cells of single layer are flat and usually very thin, with only
the thicker cell nucleus appearing as a bulge to denote the
cell.
2- specialized as lining of vessels and cavities .
3- also regulate substances which can enter underlying
tissue from the vessels or cavity.
4- often exhibit transcytosis.
5- example : mesothelium , endothelium lining the inner
surface of cornea.
Simple cuboidal epithelium
1- the cells vary in their height but are roughly
as tall as they are wide.
2- their greater thickness often includes
cytoplasm rich in mitochondria. What is the
benefit.
3- example : renal collecting tubule, pancreatic
duct.
Simple columnar epithelium
1- the cells are taller than they are wide .
2- such cells are usually highly specialized for
absorption, with microvilli .
3- they always have tight and adherent junctional
complexes at their apical ends , but are often
loosely associated in more basolateral areas.
4- this allow for rapid transfer of absorbed
material.
5- the additional cytoplasm in columnar cells allows
additional mitochondria and other organelles
needed for absorption .
Stratified squamous epithelium
1- the very thin surface cells of stratified
squamous epithelia can be :
A- Keratinized
(rich in keratin intermediate filaments).
B- Nonkeratinized .
A- The stratified squamous keratinized epithelium:
1- found mainly in the epidermis of skin.
2- its cells form many layers.
3- the cells closer to the underlying connective
tissue are cuboidal or low columnar .
4- the cells become irregular in in shape and
flatten ? Why?
5- this surface layer of cells helps protect against
water loss across this epithelium , also protect
against easy invasion by microorganisms.
B- The stratified squamous non keratinized
epithelium:
1- lines wet cavities
(………… , …………..)
2- the surface layer are living cells containing
much less keratin and retaining their nuclei.
Stratified cuboidal and stratified columnar :
1- rare.
2- stratified columnar can be found in the
conjunctiva lining the eyelids , where it is both
protective and mucus secreting .
3- stratified cuboidal epithelium is restricted to
large excretory ducts of sweat and salivary
glands.
Transitional Epithelium or Urothelium
1- lines only urinary bladder , ureter ,and upper
part of urethra.
2- k.k by superficial layer of domelike cells that
are neither squamous nor columnar, they also
called umbrella cells.
They are protective against hypertonic and
cytotoxic effects of urine.
Transitional epithelium
Psedostratified columnar epithelium
Glandular Epithelia
1- formed by cells specialized to secrete.
2- the molecules to be secreted are generally stored
in the cells in small membrane bound vesicles called
secretory granules.
3- they may synthesize , store , and secrete
-proteins(eg , in the pancreas)
-lipids(eg , adrenal , sebaceous gland)
-complexes of carbohydrate and proteins(salivary
gland).
- The ……….......… gland secrete all of them
- Some have low synthetic activity like……………….
4- we have :
-unicellular glands which consist of large isolated
secretory cells .(goblet cell )
- multicellular glands which have clusters of cell.
Gland formation
A- Exocrine glands:
1- retain their connection with the surface
epithelium.
2- the connection taking the form of tubular ducts
lined with epithelial cells through which the
secretions pass to the surface.
3- they have secretory portion which contain
the cells specialized for secretion , and ducts
transport the secretion out of gland.
Exocrine glands classification
Structural classes
1- the ducts can be simple (unbranched) or
compound(with 2 or more branches).
2- secretory portion can be tubular(either short or
long and coiled) or acinar (round or globular).
3- either type of secretory portion may be
branched.
Functional classification
Merocrine Glands:
1- they secrete products , usually
containing proteins ,by means of
exocytosis at the apical end of the
secretory cells.
2- most exocrine glands are merocrine.
3- they are further classified according to
the nature of the protein or glycoproteins
secreted and the resulting staining
properties of the secretory cells into :
a- serous (example , acinars cells of
pancreas and parotid salivary gland).
b- mucous (goblet cells)
Holocrine glands:
1- their secretion produced
by the disintegration of the
secretory cells themselves
as they complete
differentiation which involve
becoming filled with
product.
2- sebaceous glands of hair
follicles are the best
example of holocrine
glands.
Apocrine glands
-the secretory product is typically a large lipid droplet
and is discharged together with some of the apical
cytoplasm .
Endocrine glands
1- they lost their connection to the surface from which
they originated during development.
2- they are producers of hormones.
3- hormones diffuse into blood and bind specific
receptors in the body.
4- define (paracrine , autocrine)