It is the topic of anatomy. It covers the epithelial cells of all the body, their classification and occurence in the body, explained with the help of diagram.

1. Tissues of body
Epithelium
By Dr Ayesha Noreen

2. Tissue:
A tissue is define as an organized aggregation of
related or similar cells specialized to carry out some
particular function. All tissue consist of cells and
extracellular matrix.
Human tissues are generally classified into four main
varieties:
The epithelial tissue/epithelium
The connective tissue
The muscle tissue
The nervous tissue
Most of the organs are comprised of two

3. Epithelium:

5. Classification of covering epithelia:
Depending on number of cell layers present,the covering
epithelia are divided inyo following three main types:
Simple epithelia:
It is also called unilaminar epithelia,consist of only a single
layer of cells.Each cells of this epithelial layer has a basal
surface which rests on the basement membrane,and free
surface which faces the cavity of an organ or a body part.
Stratified epithelia:
Also known as multilaminar epithelia,consist of two or more
layers of cells.The cellular layers are superimposed upon one
another,so that only cell of deepest layer rest on basement
membrane and only free surface of most superficial cells faces
the lumen of an organ ir external surface of the body.

6. Pseudostratified epithelium:
It is also called pseudostratified columnar
epithelium, consist of two types of cells: tall
columnar cells and short cells. Both varities of the
cells rest on basement membrane but only tall
columnar cells have a free surface which faces the
lumen of an organ which lined by pseudostratified
epithelium. The short cells end in tapering manner
between the Columnar cells and therefore do not
reach the lumen and have no free space.

7. Simple Epithelia:

8. Simple squamous epithelium:
This variety consist of a single layer of thin flat cells which
are roughly polygonal in outline.The free surface of each
flat cell faces lumen,while basal surface rest on basement
membrane.The cells have serrated or wavy margins and
centrally placed nucleus.Presence of nucleus in central
region of flat cell causes the plasma membranes above
and below to bulge slightly.On surface view,closely fitting
polygonal cell appear like a mosaic while it appears as
fusiform ( spindle shaped) in side view.
Occurence:
Internal surface of heart,bolld vessels and lymph vessels (
endothelium)
Pericardial,pleural and peritoneal cavities( mesothelium)
Type 1 alveolar cells( lung alveoli cells)

9. Because of its thinness, it allows rapid diffusion of
gases, water and water soluble molecules. However,
in many locations, it is also engaged in active
transport and vesicular transport.

10. Simple Cuboidal epithelium:
This variety also consist of a single layer of closely
packed cells.when seen in profile ,the cell gives a
roughly square outline.However, surface view reveals
that the cells are rather hexagonal or polygonal in
shape.Each cell contains a spherical nucleus which is
located in center of cell.
Occurence:
Epithelium lining the follicles Of thyroid glands
Epithelium lining the distal convoluted tubules of
kidneys

11. Simple columnar epithelium:
This variety consist of single layer of column shaped(
column- a tall upright pillar) cells .On surface
view,cell appear as a mosaic but the outline of cell is
rather smaller.In profile view,it is seen that height of
each cell is much greater than its width.The nucleus
is commonly oval shape and usually located near the
base and all nuclei roughly present at same level.
Occurence:
Internal surface of stomach, intestine,gall baladder
and uetrus.
In many locations ,it show modifications on their
apical surface which include microvilli, kinocillia and
stereocilia.

12. Stratified epithelium:
These epithelia consist of two or more layers of
cells,in which only the bottom layer rests on the
basement membrane.It is further classified into
following subtypes according to shape of cells in the
superficial layer:
Stratified squamous epithelium
Stratified cuboidal epithelium
Stratified Columnar epithelium
A special type of stratified epithelia is called
transitional epithelium.

13. Stratified squamous epithelium:
This variety of epithelium lines those surfaces which
are subjected to wear and tear.The deepest layer is
formed by low columnar or cuboidal cells which
steadily divide mitotically.Next to basal layer are
present a few layers of larger polygonal cells.As the
free surface is approached,the cells gradually become
flattened and at the surface they assume squamous
shape.It is further divide into two subvarieties:
Stratified squamous keratinized epithelium
Stratified squamous nonkeratinized epithelium

16. Stratified cuboidal epithelium:

17. Stratified Columnar epithelium:
This variety consist of Columnar surface cells which
rest on one or more layers of roughly cuboidal cells.It
occurs in conjuctiva of eye and some part of male
urethra.

18. Transitional epithelium:
This is a special type of stratified epithelium that
lines the lower urinary tract,also called urothelium,is
specially designed to withstand stretch produced by
the distension of urinary passages due to collection
and storage of urine till is voided.It is also specialized
to prevent the diffusion of urine back into tissues of
urinary tract and thence into blood stream.The
urothelium lines the renal calyces,renal
pelvis,ureters, urinary bladder and proximal parts of
male and female urethra.
In the undistended bladder, urothelium appears to
consist of 6 or more layers of cells.The basal layer
consist of cuboidal cells.Over the basal layer are

19. In distended bladder, the number of cell layers
decreases and the cells change their shape. In
distended bladder, the transitional epithelium consist
of only two to three layers. There is basal layer of
cuboidal cells over whichare present one or two
layers of large flat cells.
Under electron microscope, the apical plasmolemma
of most superficial layer is seen to contain unusually
thick and rigid areas called plaques which are about
12 nm thick.

21. Pseudostratified Columnar
epithelium:
This type of epithelium consist of tall and short
cells.All cells rest on basement membrane but all of
them do not reach the free surface of
epithelium.Only tall cells extend through the entire
thickness.Each cell has wider part that reach the free
surface and narrow part which rest on
membrane.The short cells are conical in shape having
broad base which rest on membrane and tapering
upper part which terminates between the Columnar
cells .All the nuclei are located in wider part of both
types of cells,they are present at apical part of tall
cells and basal part of short cells .Under LM,nuclei
appear to be present in two or more rows.

23. Special varieties of epithelial cells: