Types of epithelium

1. Epithelium
Dr Vishwajit Deshmukh
MBBS, M.D (Anatomy)

2. • Epithelium is an avascular tissue composed of cells that cover the exterior
body surfaces and line internal closed cavities (including the vascular system)
and body tubes that communicate with the exterior (the alimentary,
respiratory, and genitourinary tracts).
• Epithelium also forms the secretory portion (parenchyma) of glands and their
ducts. In addition, specialized epithelial cells function as receptors for the
special senses (smell, taste, hearing, and vision)
Introduction

3. Diverse epithelial functions can be found in different organs of
the body.
• Secretion, as in the columnar epithelium of the stomach and the gastric glands;
• Absorption, as in the columnar epithelium of the intestines and proximal convoluted tubules in the
kidney;
• Transportation, as in the transport of materials or cells along the surface of an epithelium by
motile cilia or in the transport of materials across an epithelium to and from the connective tissue;
• Protection, as in the stratified squamous epithelium of the skin (epidermis) and the transitional
epithelium of the urinary bladder; and
• Receptor function to receive and transduce external stimuli, as in the taste buds of the tongue,
olfactory epithelium of the nasal mucosa, and the retina of the eye.

4. • Epithelia involved in secretion or absorption are
typically simple or, in a few cases, pseudostratified.
• The height of the cells often reflects the level of
secretory or absorptive activity. Simple squamous
epithelia are compatible with a high rate of
transepithelial transport.
• Stratification of the epithelium usually correlates
with transepithelial impermeability.
• Finally, in some pseudostratified epithelia, basal
cells are the stem cells that give rise to the mature
functional cells of the epithelium, thus balancing
cell turnover.

5. 3 Characteristic features
• They are closely apposed and adhere to one another by means of specific
cell-to-cell adhesion molecules that form specialized cell junctions
• They exhibit functional and morphologic polarity. In other words, different
functions are associated with three distinct morphologic surface domains: a
free surface or apical domain, a lateral domain, and a basal domain.
• Their basal surface is attached to an underlying basement membrane, a
noncellular, protein–polysaccharide-rich layer demonstrable at the light
microscopic level by histochemical methods

7. • Epithelioid organization is typical of most endocrine
glands; examples of such tissue include the interstitial
cells of Leydig in the testis, the lutein cells of the
ovary, the islets of Langerhans in the pancreas,
the parenchyma of the adrenal gland, and the
anterior lobe of the pituitary gland
• In some locations, cells are closely apposed to one
another but lack a free surface. Although the close
apposition of these cells and the presence of a
basement membrane would classify them as
epithelium, the absence of a free sur- face more
appropriately classifies such cell aggregates as
epithelioid tissues.
Epithelioid cells

8. Endothelium and mesothelium are the simple squamous epithelia lining the vascular system and
body cavities
1.Specific names are given to epithelium in certain locations
• Endothelium
• Endocardium
• Mesothelium

9. CLASSIFICATION
OF EPITHELIUM
The traditional classification of epithelium is descriptive and based on two
factors: the number of cell layers and the shape of the surface cells. The
terminology, therefore, reflects only structure, not function.
• Simple and stratified
• Cuboidal, columnar and squamous
• Pseudostratified
• Transitional epithelium

11. Pseudostratified epithelium and transitional epithelium are special classifications of
epithelium
•Pseudostratified epithelium appears stratified, although some of the cells do not reach the
free surface; all rest on the basement membrane. Thus, it is actually a simple epithelium.
•Transitional epithelium (urothelium) is a term applied to the epithelium lining the lower
urinary tract, extending from the minor calyces of the kidney down to the proximal part of
the urethra. Urothelium is a stratified epithelium with specific morphologic characteristics
that allow it to distend

13. • Both endothelium and endocardium, as well as
mesothelium, are almost always simple squamous
epithelia.
• An exception is found in postcapillary venules of
certain lymphatic tissues in which the endothelium
is cuboidal.
• These venules are called high endothelial
venules (HEV). Another exception is found in the
spleen in which endothelial cells of the venous
sinuses are rod-shaped and arranged like the
staves of a barrel

14. Cell Polarity
Epithelial cells exhibit distinct polarity. They have an apical
domain, a lateral domain, and a basal domain
• Microvilli, cytoplasmic processes containing a core of actin
filaments;
• Stereocilia (stereovilli), microvilli of unusual length and
• Cilia, cytoplasmic processes containing
bundle of micro-tubules.

15. Microvilli
Microvilli are finger-like cytoplasmic projections on the apical surface of most epithelial
cells
• In intestinal absorptive cells, this surface structure was originally called the striated
border;
• in the kidney tubule cells, it is called the brush border

16. • In other cell types, they are tall, closely packed, uniform projections that greatly increase the free
cell surface area.
• the number and shape of the microvilli of a given
cell type correlate with the cell’s absorptive
capacity.
• In intestinal absorptive cells, this surface structure
was originally called the striated border; in the kidney
tubule cells, it is called the brush border.
• The internal structure of microvilli contains a
core of actin filaments that are cross-linked by
several actin- bundling proteins.

17. Stereocilia

18. • Stereocilia are not widely distributed among epithelia. They are, in fact,
limited to the epididymis, the proximal part of the ductus deferens of
the male reproductive system, and the sensory (hair) cells of the inner
ear.
• Stereocilia of the genital ducts are extremely long processes that extend
from the apical surface of the cell and facilitate absorption.
• Stereocilia develop from microvilli by the lateral addition of actin
filaments to the actin bundle as well as elongation of the actin
filaments.
• A striking difference between microvilli and stereocilia, other than size
and the presence of ezrin, is the absence of villin from the tip of the
stereocilium.
• Stereocilia of the sensory epithelium of the ear also derive
from microvilli. They are exquisitely sensitive to mechanical vibration
and serve as sensory mechanoreceptors rather than absorptive
structures.

19. Cilia

20. Motile cilia have been historically the most studied of all cilia. They are found in
large numbers on the apical domain of many epithelial cells. Motile cilia and their
counterparts, flagella, possess a typical 9 +2 axonemal organization with microtubule-associated
motor proteins that are necessary for the generation of forces needed to induce motility.
Primary cilia (monocilia) are solitary projections found on almost all
eukaryotic cells. The term monocilia implies that only a single cilium per cell is usually
present. Primary cilia are immotile. They function as chemosensors, and
mechanosensors, and they mediate light sensation, odorant, and sound perception in multiple
organs in the body.
Nodal cilia are found in the embryo on the bilaminar embryonic disc at the time of gastrulation.
They are
concentrated in the area that surrounds the primitive node, hence their name nodal cilia. They have a
similar ax- onemal internal architecture as primary cilia; however, they are distinct in their ability to perform
rotational movement. They play an important role in early embryonic
development.

21. Basal Body

23. •Kartagener’s
Syndrome

24. Lateral domain
Intercellular cement

25. There are three types of
junctional complexes
•Occluding junctions
•Anchoring
•Communicating

27. Occluding Junctions
• Occluding junctions are impermeable and allow epithelial cells to function as a barrier. Also called
tight junctions, occluding junctions form the primary intercellular diffusion barrier
between adjacent cells.
• By limiting the movement of water and other molecules through the intercellular space, they
maintain physicochemical separation of tissue compartments.
• Examination of the zonula occludens or tight junction with the transmission electron
microscope (TEM) reveals a narrow region in which the plasma membranes of
adjoining cells come in close contact to seal off the intercellular space.
• At high resolution, the zonula occludens appears not as a continuous seal
but as a series of focal fusions between the
cells.
• Several proteins are involved in the formation of zonula
occludens strands
• Occludin
• Claudin
• Junctional adhesion molecules

29. • Anchoring junctions provide mechanical stability to epithelial cells by linking the cytoskeleton of one
cell to the cytoskeleton of an adjacent cell.
• These junctions are important in creating and maintaining the structural unity of the epithelium.
• Anchoring junctions interact with both actin and intermediate filaments and can be
found not only on the lateral cell surface but also on the basal domain of the epithelial cell.
• Through their signal transduction capability, anchoring junctions also play important roles in cell-to-cell
recognition, morphogenesis, and differentiation.
Anchoring Junctions
• Zonula Adherens which inter- acts with the network of actin filaments inside the cell
• Macula Adherens or desmosome, which interacts with intermediate filaments
In addition, two other types of anchoring junctions can be found where epithelial cells rest on the connective
tissue matrix. These focal adhesions (focal contacts) and hemidesmosomes

33. Communicating Junctions/ Gap junctions or Nexuses
• The only known cellular structures that permit the direct passage of signaling
molecules from one cell to another.
• Gap junctions are important in tissues in which activity of adjacent cells must be coordinated, such as epithelia
engaged in fluid and electrolyte transport, vascular and intestinal smooth muscle, and heart muscle.
• Various procedures have been used to study gap junctions,
including the injection of dyes and fluorescent or radiolabeled
compounds.
• High-resolution imaging techniques such as cryo-electron
microscopy have been used to examine the structure of gap
junctions. These studies reveal groups of tightly packed channels,
each formed by two half- channels called connexons
embedded in the facing mem- branes.
• Each connexon contains six symmetrical subunits of an integral membrane protein called
connexin (Cx) that is paired with a similar structure from the adjacent membrane.

35. Atomic force microscopic image of the gap junctions

36. Basal Domain
• The basement membrane is a specialized
structure located next to the basal domain of
epithelial cells and the underlying connective
tissue stroma.
• Cell-to-extracellular matrix junctions anchor the
cell to the extracellular matrix; they are
represented by focal adhesions and
hemidesmosomes.
• Basal cell membrane infoldings increase the cell
surface area and facilitate morphologic
interactions between adjacent cells and
extracellular matrix proteins.

37. This section was stained by the
PAS method

39. Electron micrograph
of epithelial cells

40. Thanks
VDsnaps