general histology

1. Epithelial tissue
By dr Abdiasis Omar Mohamed
MBBS

2. Introduction
• the organs of the human body are composed of only
four basic tissue types:
• Epithelial
• Connective
• Muscular
• Nervous tissues

3. • Epithelial tissues are composed of closely aggregated
polyhedral cells adhering strongly to one another and
to a thin layer of ECM, forming cellular sheets that
line the cavities of organs and cover the body surface.
• Epithelia (Gr. epi, upon + thele, nipple) line all
external and internal surfaces of the body and all
substances that enter or leave an organ must cross this
type of tissue.

4. • The principal functions of epithelial tissues include
the following:
- Covering, lining, and protecting surfaces (eg,
epidermis)
- Absorption (eg, the intestinal lining)
- Secretion (eg, parenchymal cells of glands)

5. Characteristic features of epithelial cells
• Most epithelia are adjacent to connective tissue
containing blood vessels from which the epithelial
cells receive nutrients and O2.
• The connective tissue that underlies the epithelia
lining the organs of the digestive, respiratory, and
urinary systems is called the lamina propria.
• The area of contact between the two tissues may be
increased by small evaginations called papillae
projecting from the connective tissue into the
epithelium.

6. • Epithelial cells generally show polarity.
• The region of the cell contacting the ECM and
connective tissue is called the basal pole and the
opposite end, usually facing a space, is the apical
pole, with the two poles differing significantly in both
structure and function.

9. • Basement Membrane
• The basal surface of all epithelia rests on a thin
extracellular, felt-like sheet of macromolecules
referred to as the basement membrane , a
semipermeable filter for substances reaching
epithelial cells from below.
• Nearest the epithelial cells is the basal lamina, a thin,
electron-dense, sheet-like layer of fine fibrils, and
beneath this layer is a more diffuse and fibrous
reticular lamina.

10. • basal lamina characteristically include the following:
Type IV collagen, Laminin, integrin, Nidogen and
perlecan.
• The more diffuse meshwork of the reticular lamina
contains type III collagen and is bound to the basal
lamina by anchoring fibrils of type VII collagen, both
of which are produced by cells of the connective
tissue

11. • Intercellular Adhesion & Other Junctions
• Tight or occluding junctions form a seal between
adjacent cells.
• Adherent or anchoring junctions are sites of strong
cell adhesion.
• Gap junctions are channels for communication
between adjacent cells.

13. • Zonula occludens (Tight junctions):
• The most apical junction
• form bands completely encircling each cell
• close off the space between the cells
• The seal between the membranes is due primarily to
direct interactions between the transmembrane
protein claudin on each cell
• The number of these fusions sites is inversely
correlated with the leakiness of the epithelium

15. • Prevent the flow of material between the base and the
apex of the cell in either directions.
• It contribute to the formation of two separate
functional units , apical and basal.
• Composed of several proteins:-
- Junctional associated molecule (JAM)-
- Occludins
- Claudins

16. • Zonula adherens
• Also encircles the cell
• Immediately below the zonula occludens
• Adhesion is mediated by transmembrane
glycoproteins of each cell, the cadherins. which lose
their adhesive properties in the absence of Ca2+.
• Inside the cell, cadherins bind the protein catenin
which is linked to actin filaments

17. • Desmosome:
• does not form a belt around the cell
• Disk like structure that faces mirror image on
adjacent cell (20-30 nm gap).
• Circular plaque called attachment plaque made up of
12 proteins of anchoring proteins.
• Intermediate cytokeratin filaments are inserted to the
plaque.

19. • Hemidesmosome:
• connection between cell and BL
• Take the shape of half desmosome.
• The attachment plaque contain mainly integrin not
cadherin.

20. • gap junctions:
• Passageway between two adjacent cells
• Let small molecules move directly between
neighboring cells
• Cells are connected by hollow cylinders of protein
• Each junction is composed of 6 connexin form
hexameric complexes called connexons proteins that
span the membrane and form one half of the junction
• Permit Cell to cell exchange of low MW molecules
such as: ions, cAMP, small hormones.
• Not present in skeletal muscle.

21. Specializations of cell surface
• Microvilli:
• Non-branching fingerlike projections of cell 1 μm
high, 0.08 μm wide,at the free surface of the cell.
• Increase of cell surface
• Covered by plasma membrane
• The glycocalyx is thicker contain glycoproteins and
enzymes that allow final stage digestion
• Glycocalyx and microvilli seen under light
microscope called striated or brush border.

22. • It’s a cluster of actin filaments that crossed linked to
each other proteins such as fimbrin and villin and to
plasma membran by proteins such as myosin I.
• Hundred of microvilli can be seen in absorptive cells.

24. • Stereocilia
• Branched non – motile microvilli of cells of
epididymis and ductus deference
• Its function to increase the surface area
• Cilia
• Non-branching motile projections
• Transport of extracellular mass
• May be numerous
• 270 per cell in trachea

25. • 5-10 μm tall ,0.2 μm wide
• Attached to basal bodies ( analogus to the centrioles)
at the apical area
• Flagella
• very long cillia 100 μm , only in spermatozoa

26. Type of Epithelia
• Covering epithelia
 Number of cell layer
• Simple
• Stratified
 Shape of the cell at the surface layer
• Squamous
• Cuboidal
• Columnar
• Glandular epithelia

27. • Simple Squamous Epithelium
• single layer of flat cells with disc-shaped nuclei
• Special types
- Endothelium (inner covering) slick lining of hollow
organs
- Mesothelium (middle covering)Lines peritoneal,
pleural, and pericardial cavities
• Covers visceral organs of those cavities

29. • Simple Cuboidal Epithelium
• single layer of cube-like cells with large, spherical
central nuclei
• Function
- secretion and absorption
• Location
- kidney tubules
- secretory portions of small glands, ovary & thyroid
follicles

31. • Simple Columnar Epithelium
• Single layer of column-shaped (rectangular) cells
with oval nuclei
• Some bear cilia at their apical surface
• May contain goblet cells
• Function: Absorption; secretion of mucus, enzymes,
and other substances
• Ciliated type propels mucus or reproductive cells by
ciliary action

33. • Non-ciliated form
- Lines digestive tract, gallbladder, ducts of some
glands
• Ciliated form
- Lines small bronchi, uterine tubes, uterus

34. • Pseudostratified Columnar Epithelium
• All cells originate at basement membrane
• Only tall cells reach the apical surface
• May contain goblet cells and bear cilia
• Nuclei lie at varying heights within cells
• Gives false impression of stratification
• Function: secretion of mucus; propulsion of mucus by
cilia

36. • Non-ciliated type
- Ducts of male reproductive tubes
- Ducts of large glands
• Ciliated variety
- Lines trachea and most of upper respiratory tract

37. • Stratified Squamous Epithelium
• Many layers of cells – squamous in shape
• Deeper layers of cells appear cuboidal or columnar
• Thickest epithelial tissue – adapted for protection
• Specific types
 Keratinized – contain the protective protein keratin
• Surface cells are dead and full of keratin

39.  Non-keratinized – forms moist lining of body
openings
• Function: Protects underlying tissues in areas subject
to abrasion
• Location
- Keratinized – forms epidermis
- Non-keratinized – forms lining of esophagus, mouth,
and vagina

40. • Stratified Cuboidal Epithelium
• generally two layers of cube-shaped cells
• Location:
- Forms largest ducts of sweat glands
- Forms ducts of mammary glands and salivary
glands

41. • Stratified Columnar Epithelium
• several layers; basal cells usually cuboidal;
superficial cells elongated
• Function: protection and secretion
• Location:
- Rare tissue type
- Found in male urethra and vas deferens, largest
ducts of salivary glands, nasopharynx

42. • Transitional Epithelium
• Basal cells usually cuboidal or columnar
• Superficial cells dome-shaped or squamous
• Function: stretches and permits distension of urinary
bladder
• Location: Lines ureters, urinary bladder and part of
urethra

44. • Secretory Epithelia & Glands
• Epithelial cells that function mainly to produce and
secrete various macromolecules may occur in
epithelia with other major functions or comprise
specialized organs called glands.
• Secretory cells may synthesize, store, and release
proteins (eg, in the pancreas), lipids (eg, adrenal,
sebaceous glands), or complexes of carbohydrates
and proteins (eg, salivary glands). Epithelia of
mammary glands secrete all three substances.

45. • Exocrine glands remain connected with the surface
epithelium, the connection forming the tubular ducts
lined with epithelium that deliver the secreted
material where it is used.
• Endocrine glands lose the connection to their original
epithelium and therefore lack ducts.

46. • glands can be simple (ducts not branched) or
compound (ducts with two or more branches).
• Secretory portions can be tubular (either short or long
and coiled) or acinar (rounded and saclike); either
type of secretory unit may be branched, even if the
duct is not branched.
• Compound glands can have branching ducts and can
have multiple tubular, acinar, or tubuloacinar
secretory portions.

50. THE END