Epithelial tissue nature and details of the same elaborated over here

1. 1

2. SEMINAR 1
EPITHELIUM
PRESENTED BY
DR SHRIKANT SONUNE
GUIDED BY
DR ASHOK PATIL
DR SHILPA KANDALGAONKAR
DR MAYUR CHOUDHARI
DR SUYOG TUPSAKHARE
DR NIDHI GUPTA
2

3. Introduction
Classically, four basic tissues
1. Epithelium-covering , lining
2. Connective tissue- support, bone, ligament, fat
3. Muscle- movement
4. Nervous tissue- control; brain, nerve &spinal
cord
In Greek "epi" means "on or upon" and "theli"
meaning "tissue."
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4. 4

5. What is actually epithelium means?
• Tissue which(with rare exceptions) cover or
line all body surfaces, cavities & tubes.
• Continuous sheet
• Supported by basement membrane
• Avascular, abundant nerve supply
• Dependent on the diffusion of oxygen &
metabolites
5

6. 6

7. Developmentally, epithelia are derived from all
three layers of the early embryo
Ectoderm:- Epidermis,
Glandular tissue of the breast,
Cornea
The junctional zones of the buccal
cavity & anal canal.
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8. 8
•Endoderm
- the epithelial lining of
the alimentary canal
and its glands,
most of the respiratory tract
the distal parts of the urogenital tract.

9. 9
•Mesodermal derivatives
epithelia of the kidney,
mesothelium
{They line the pericardial, pleural and peritoneal
cavities.}

10. Definition of Epithelium:
 Avascular tissue made of cells that cover exterior
surfaces and line both internal closed cavities and body
tubes that communicate with the exterior. Epithelium
also forms the secretory portion of glands and
ducts.{gustafons histo notes (internet)}
 The covering of internal & external surfaces of the
body including the lining of vessels & other small
cavities. It consists of cells joined by small amount of
cementing substances.{ Dorlands medical dictionary}
10

11. Important characteristics
• Covers or lines all body surfaces.
• Polarity .
• Specific intermediate filaments called
cytokeratins.
• Attached to underlying connective via
extracellular matrix basement membrane.
• Generally Avascular.
• Gives rise to majority of glands.
• High regenerative capacity.
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12. Great diversity of function.
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Functions Example
Protective layer Epidermis
Absorption of water and solutes Intestinal epithelium
Secretion Glands (salivary, pancreas, etc)
Containment(storage) Bladder, thyroid follicles
Excretion Kidney tubules

13. Classification of epithelium
 Surface epithelium
:- Simple epithelium
:- Pseudostratified epithelium
:- Stratified epithelium
 Glandular epithelium
 Special epithelium
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14. 14

15. :- Simple epithelium
(unilayered)
-Simple Squamous epithelium
-Simple Cuboidal epithelium
-Simple columnar epithelium
{Transitional epithelium}
:- Stratified epithelium
(multilayered)
-Stratified Squamous epi
-Stratified Cuboidal epi
-Stratified Columnar epi
15

16. Stratified Squamous epithelium
• Stratified Squamous “Nonkeratinizing”
epithelium.(mucosal verity)
• Stratified Squamous “Parakeratinizing”
epithelium
• Stratified Squamous “keratinizing”
epithelium.(cutaneous)
{will discuss in depth in next session of seminar}
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17. • Glandular epithelia
Are specialized for secretion
• Special epithelia
 Myoepithelium (epithelial cells specialized for
contraction)
 Neuroepithelium (e.g., gustatory, olfactory)
 Reproductive epithelium (e.g., seminiferous
epithelium)
17

18. Simple epithelium
• One cell thick
• Every cell is in direct contact with the
underlying basement membrane.
• Selective diffusion, absorption or secretion
(The thinness)
• Little protection against mechanical forces
• Epithelium range in shape from extremely
flattened to tall columnar, depending upon
their function.
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19. Simple Squamous epithelium
• scales of fish
• pavemented epithelium.
• Fit closely together in tissues
• providing a smooth, low-friction surface over
which fluids can move easily
19

20. -Cell is spindle shaped
-Prominent oval nuclei
-Cell junctions
-Thin basement membrane
20

21. Simple Squamous Epithelium found at
• Alveolar epithelium in the lungs. (simple passive
diffusion)
• The lining of pleural, pericardial and peritoneal cavities
(permits passage of tissue fluid into and out of the
cavities.)
• Specialized Squamous epithelia lining of cavities such
as the blood vessels (endothelium)
(why specialized? - surface receptors that control the
secretion of locally acting chemical messengers.)
21

22. Endothelium
• Simple Squamous endothelial cells
• Lining of the entire cardiovascular system and
lymphatic vessels
• About 1 x 1013 cells
• Different shape in certain Locations; e.g., in the
high endothelial venules of lymphoid organs
• Major intermediate filament in endothelium is
vimentin
• Weibel-Palade bodies- unique rod-shaped
structures in cytoplasm.
22

23. Functions.
• Controlling the exchange
• Angiogenesis
• Vasomotor tone
• Regulation of immune and inflammatory
responses
• Maintenance of blood fluidity and
homeostasis
• WPBs were determined to be specialized
secretory granules(mainly von Willebrand
factor(vWF))
23

24. Simple Cuboidal epithelium
• Roughly Cuboidal in shape
• Spherical (round) nucleus
• Surface view the cells are actually polygonal
• May have excretory, secretory or absorptive functions.
• Small collecting ducts of the kidney, salivary glands,
and the (secretive) exocrine gland
• Germinal epithelium
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25. Simple columnar epithelium
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26. Simple Columnar Epithelial
• Elongated and column-shaped
• Nuclei are also elongated
• Usually located near the base of the cells.
• Lining of the stomach and intestines
• Specialized for sensory reception
• Goblet cells (unicellular glands) are found
between
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27. Pseudo stratified epithelium
27

28. Pseudo stratified epithelium
• Several nuclei appearing at different heights,
they can be confused with stratified epithelia.
Similarity to simple epithelia
• All cells are attached to their basal lamina
• Exhibit varying heights,
• Only the tallest cells reach the apical surface
of the epithelium.
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29. Similarity to stratified epithelia
• May appear to be stratified
• Due to nuclei arranged at different levels
• Thus only their nuclei appear “stratified”
• E.g.-Respiratory epithelium
-Pseudostratified ciliated columnar with goblet
cells
• Urothelium (transitional epithelium) ???
At the surface of the epithelium plasma
membrane embedded in lipid layer there are
special glycoproteins.
29

30. 30

31. 31

32. Stratified epithelium (multilayered)
• Stratified Epithelia are defined as two or more
layers of cells in which only the basal layer is
attached to the basement membrane & are
more protective in function than those of the
simple variety. The basal layer contains the stem
cells necessary for regeneration of this
renewable tissue.
32

33. Characteristic feature
• Multilayered
• To withstand mechanical or chemical insult
• Cells flatten as the layers become more apical,
though in their most basal layers the cells can
be Squamous, Cuboidal or columnar
• Do not contain junctional complexes, and have
their cells bound together only by desmosome
33

34. Stratified epithelium(multilayered)
-Stratified Squamous epithelium
-Stratified Cuboidal epithelium
-Stratified Columnar epithelium
{will focus on Stratified Squamous epithelium in
next session of seminar}
34

35. Stratified Cuboidal epithelium
Consists of two or more layers &
exhibits Cuboidal-like surface cells.
e.g.-lining the ducts of glands(salivary)
35

36. Stratified columnar epithelium
• Consists of two or more
layers and exhibits
columnar surface cells.
Examples-in portions of
the male urethra,
also in the larger ducts
of glands.
36

37. Glandular epithelia
• It consists of single cells or groups of cells
specialized for secretion.
Classification
Endocrine -secretions distributed by blood vessels.
Exocrine-secretions distributed by duct systems.
37

38. 38

39. Generally exocrine glands are classified on the
basis of
In a/c with Number of Cells
1. Unicellular glands are defined as single cells
interspersed among other epithelial cells of
different functions
e.g., mucus-secreting goblet cells.
2. Multicellular glands occur as many adjacent
secretory cells within the epithelium
e.g., surface mucous cells of stomach
39

40. In a/c with Pattern of Duct System
1. Simple glands have a simple, unbranched duct
with tubular or acinar
2. Compound glands have a branched duct system
with tubular, acinar, or tubuloacinar shaped
secretory units
A/c with shape of Secretory Portion
1. Tubular
2. Acinar/ alveolar
3. Tubuloacinar
40

41. TUBULAR ACINAR
41

42. Ib singh diagrme
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43. Types of the Secretion- Mucous glands
Serous glands
Mucous glands
• Secretions -mucins consisting of carbohydrate
rich glycoproteins.
:-mucins +water= mucus (viscous solution)
• That has important protective function.
• Lubricating functions at epithelial surfaces.
• On apical epithelial surfaces are important
components of innate immunity.
43

44. Structure of mucous secretory units
-Mostly tubular
-Composed of tall pyramidal to columnar secretory
cells.
-Staining with standard dyes (H&E) is poor
-Renders the cytoplasm quite pale in sections.
-With PAS or other mucin stains,
-Their closely-packed cytoplasmic mucigens are brightly
colored.
-Nuclei with a Squamous-like appearance
-Other organelles are characteristically flattened at the
base of cells due to abundant mucigens.
-The distinctive appearance of mucin units is in marked
contrast to that of serous glands
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45. Mucous secreting unit
45

46. 2. Serous glands
Secretions - Watery fluid,
Proteins
Other components,
- zymogens, bactericidal compounds, and ions.
Structure of serous secretory units are acinar [ grape]
• Shorter pyramidal cells arranged around a small lumen.
• Characteristically, the nucleus is basal in location, but
round in shape
• Surrounded by obvious cytoplasmic basophilia due to
the Rough Endoplasmic Reticulum.
• The apical cytoplasm typically contains numerous
secretory granules.
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47. Serous secretory unit
47

48. Mixed glands
• Contain both mucous and serous secretory units,
• Either separate or as mixed units.
• Now, clear that even single cells may be of the mixed variety.
Serous demilunes (crescents of Giannuzzi , crescents of
Heidenhain)
• Crescent shaped groups of serous cells that appear to cap the
distal ends of mucous secretory units in mixed units.
• They may largely be due to a fixation artifact, which results
from the swelling of mucins in mixed secretory units.
• These structures do provide important histological landmarks
for distinguishing the different types of glands.
48

49. Serous demilunes
49

50. Intercellular Canaliculi,
• Located between mucous cells in mixed
glands, have been described as delicate
secretory channels that connect serous cells of
demilunes with the central lumen.
• Their structure may also be largely due to the
fixation artifact
• Serous cells in mixed units to form the
demilunes.
50

51. Mechanisms of the Secretion
1. Merocrine secretion
2. Apocrine secretion
3. Holocrine secretion
1. Merocrine secretion
• mçros part, share + krinô =to separate
• Method of exocytosis
• The membrane of the secretory granule fuses
with the plasma membrane to release the
contents of the granule.
• Most exocrine glands secrete by this
mechanism, including salivary glands and
pancreas. 51

52. 2. Apocrine secretion [Gr. apo- =off, away from]
• Both the secretory product and a portion of
the apical secretory cell cytoplasm are
pinched off and released.
• Decapitation secretion
• Examples
-Apocrine sweat glands,
Mammary glands,
Ceruminous glands of the external ear
52

53. 3. Holocrine secretion
• [Gr. holos =whole,entire, complete]
apparently the result of apoptosis which is
programmed secretory cell death.
e.g. Sebaceous glands,
• (dermatologists sometimes refer to the
exfoliation of the stratum corneum of skin as
holocrine)
53

54. Specialized epithelium
Myoepithelial cells(basket cells, )
• are myoid (smooth muscle-
like) contractile cells that assist
in the secretion of certain
glands
• (e.g., sweat, salivary, lacrimal,
and mammary).
• contain a similar contractile
apparatus,
54

55. Why it is epithelium ?
Considered as true epithelial cells for the
following reasons.
1) Their major IFs are cytokeratins (5 and 14)
2) They form cadherin-mediated junctions
(desmosome and hemidesmosomes)
3) They are located between the luminal
epithelium and the basement membrane
55

56. EPITHELIAL SURFACE
SPECIALIZATIONS
Definition.
Surface specializations of epithelial cells, which
are morphologic adaptations for specific
functions, are located at, on and/or within
particular plasma membrane domains. Although
these specializations are highly developed within
epithelia, several are found in other tissues as
well and will be encountered again.
56

57. • Nomenclature of epithelial surfaces as plasma
membrane domains
1. Apical [L. apex = peak] surface specializations
occur at epithelial surfaces facing the lumen, also
known as free surface
represent the interface between epithelial cells and
their external environment.
• Apical specializations include
1. Microvilli,
2. Cilia,
3. Glycocalyx.
57

58. 2. Lateral surface specializations
Occur at the surfaces between adjacent
epithelial cells.
• Interfaces between each other cells of an
epithelial tissue.
• Lateral specializations include
1. Occluding junctions,
2. Adhering junctions,
3. Communicating junctions.
58

59. 3. Basal surface specializations
Occur at the base/bottom of epithelia.
Interfaces between an epithelium and its
underlying connective tissue.
• Basal specializations include
1. Basement membranes,
2. Hemidesmosomes
3. Redundant surface membrane increases
infoldings and outfoldings
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60. APICAL SURFACE SPECIALIZATION
1. Microvilli
• Villus =tuft of hair
• Plasma membrane evaginations of the apical
surface that greatly increase the area available
for absorption (or secretion).
• They contain a core of actin microfilaments and
are covered by a coat of surface bound
carbohydrates, called the glycocalyx
• Gives a PAS-positive appearance under the light
microscope.
60

61. a. Striated border of the intestines consists of
densely packed microvilli that assist and
enhance nutrient absorption.
b. Brush border of the kidney proximal tubules
also consists of thousands of densely packed
microvilli, which facilitate the reabsorption
that takes place in this location.
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62. c. Stereocilia are elongated microvilli found at the
apices of cells lining the ductus epididymidis and
ductus deferens of the male genital tract.
Stereocilia also occur on cochlear and vestibular
hair cells of the internal ear.
d. Other locations. Microvilli are found on many
other cells including mesothelium. In general,
their purpose is to increase membrane surface
area for absorption or secretion. Typically, they
are shorter and less numerous in cells that are
not highly specialized for absorption or transport.
62

63. 2. Cilia
• Are plasma membrane and cytoplasmic
projections from the apical surface.
• They are specializations for- motility and
contain a core of microtubules arranged in a
specific configuration called the axoneme.
• Cilia are longer and thicker than microvilli, and
their core of microtubules are extensions of
centriole-like basal bodies that are located in
the terminal web.
• These motors are powered by ATP and are
responsible for ciliary/flagellar motion. 63

64. Basal bodies, which can arise from centrioles
and have a similar structure, are responsible
for giving rise to cilia.
Motility.
• Cilia move back and forth to propel fluid and
particles unidirectional.
• Movement is brought about by differential
sliding of adjacent doublets past each other,
• Which is facilitated by the energy-dependent
movement motor proteins along adjacent
microtubules.
64

65. 3. Flagella, which have the same microstructure
as cilia, occur singly and are much longer in
length.
• They provide motility for spermatozoa.
4. Glycocalyx
• Gr. glykys =sweet + kalyx =husk, shell
• Refers to a carbohydrate-rich, PAS-positive
covering on the apical surface of certain
epithelial cells, which can be seen with the LM
due to the high density of carbohydrate
residues.
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66. Lateral Surface Specializations
junctional complex
66

67. Junctional complexes in epithelia.
1. Zonula occludens
2. Zonula adherens
3. Macula communicans
1) Tight junction.
Zonula occludens (occluding junction, tight
junction) occurs on the lateral cell surfaces
just beneath the apical poles.
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68. Structure of Tight junction
Formed by interactions of
special trans membrane
proteins (claudins,
occludin) in the plasma
membranes of adjacent
cells, these junctions form
a network that extends
completely around the cell
perimeter and represent
the closest contacts
between cells.
68

69. Functions.
Restrict Paracellular Flow
Occluding junctions serve as permeability seals to
isolate the underlying intercellular clefts (paracellular
compartment) from the outside environment,
therefore restricting intercellular movement of
materials.
Restrict Membrane Flow
In addition, they separate the apical and basolateral
domains in cell membranes, which insures that
specific proteins will remain in specific domains.
69

70. 2. Zonula adherens
It is also a band-like junction that extends around the
perimeter of cells; it serves in the attachment of
adjacent epithelial cells
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71. 3. Macula communicans
• also known as
-communicating junction
-macula communicans,
-maculae communicantes
• This is a junctional area of between adjacent cells
that facilitates intercellular communication by
allowing the passage of small molecules and ions
across the narrow intercellular gap through a
multitude of junctional pores.
71

72. Structure of Macula communicans
The junction consists of a
hexagonal lattice of
connexin protein subunits
called connexons, which
form intramembrane
hydrophilic channels
connecting the cytoplasm
of adjacent cells.
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73. 73
Function.
• Permit intercellular signaling and electrical coupling
by allowing the regulated passage of ions and small
molecules between cells.
• Important cellular strategy for approaching the
efficiency of a syncytium.
Distribution.
• Gap junctions are not only found between epithelial
cells, but also in all primary tissues. They occur
between cells of other tissues that require direct
communication,
e.g., osteocytes in bone matrix. In addition, impulses
for cardiac and smooth muscle contraction pass
from one cell to the next through gap junctions.

74. Macula adherens (desmosome)
• Desmosomes are adhesive intercellular junctions,
which are found in tissues subjected to
mechanical strain.
• Widespread in epithelia,
• Particularly in stratified squamous varieties, in
cardiac muscle.
• Intermediate (keratin) filament cytoskeletons
across cells.
• These junctions are “spot welds” between
adjacent cells,
• Which are formed by the juxtaposition and
attachment of two symmetrical disk-shaped
structures provided by each cell.
74

75. a. Structure: light microscopic.
At the Light Microscopic level, only observed in
Stratified Squamous Epithelium due to their
high density between cells.
• They appear as small punctate bodies,
• Hence onces they were considered as
“cytoplasmic bridges”
75

76. Structure: electron microscopic.
Their ultra structure
revealed that
desmosomes are
bipartite junctions,
which consist of
symmetrical, mirror-
image-like components
provided by each
adjacent cell.
76

77. • Intracellular components
Inside each cell, just beneath their lateral plasma
membranes, electron dense structures called
attachment plaques were found to connect the
cell membrane on one side with intermediate
filaments of the keratin cytoskeleton on the
cytoplasmic side.
• Extracellular components
Between each cell in the middle of the
intercellular space, an intermediate dense
midline was observed, which appeared to be a
region of attachment between adjacent cells.
77

78. c. Structure: submicroscopic.
At the molecular level,
 specialized adhesive proteins.
 major components cadherin superfamily of adhesion
molecules.
 two major desmosomal cadherins have been described
and a schema for their nomenclature proposed
desmocollins and desmogleins.
 Region-specific expression for various isoforms of these
proteins has also been shown to occur in SSE
 e.g., Dsg1 is normally located in upper layers of oral
mucosa but throughout entire epidermis (especially in
upper layers),
 Dsg3 is located in basal layers of epidermis but
throughout oral mucosa. This localization has proved to
be important for understanding certain diseases.
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79. Function.
• Most abundant in lining membranes subject to
wear and tear,
• present in all epithelia,
• desmosomes are important cellular spot welds
that hold cells together by a calcium dependent
adhesion mechanism.
• They represent an important means of resistance
to lateral shearing forces between cells by
coupling external attachment of adjacent cells to
internal linkage of their keratin cytoskeletons
across the epithelium.
79

80. Basal Surface Specializations
Basement membrane is a sheet-like layer that
underlies virtually all epithelia, and isolates
them from the subjacent Connective Tissue.
(Exceptions are found in lymphatic capillaries and
certain blood capillaries where the basement
membrane may be poorly developed or
discontinuous.)
At LM level,
• visualization usually requires special stains such
as PAS reaction, silver,
• except in some locations such as the trachea
where it is exceptionally thick.
80

81. • It consists of a basal lamina and a variably-present
reticular lamina
Basal lamina is a product of the epithelium and
consists of an electron-lucent zone called the lamina
lucida , located next to the basal plasma lemma, and
an electron-dense layer called the lamina densa,
located next to the CT.
Composition is complex and may vary in different
locations, but consists primarily of type IV collagen,
glycoproteins [mainly laminin; also nidogen ], and
the large heparin sulfate proteoglycan called
perlecan.
.
81

82. • External lamina
-It is the basal lamina that surrounds muscle cells,
peripheral glial cells (e.g., Schwann cells), and
adipocytes.
-It serves both anchoring and separating functions in
relation to the surrounding CT compartment. --
• Reticular lamina
It is a product of the CT and is composed of delicate
reticular (type III collagen) and type I collagen fibers.
• Anchoring fibrils, which are composed of type VII
collagen, form an important linkage between the basal
and reticular laminae.
82

83. 2. Hemidesmosomes have the appearance of
half-desmosomes and are found at sites
where epithelial cells are attached to
underlying extracellular matrix.
• However, in hemidesmosomes, integrins are
the integral membrane proteins and not
cadherins as with desmosomes
83

84. Basolateral infoldings of plasma membrane.
• As microvilli provide increases in surface area
on the apical surface, these infoldings increase
the surface area of the basolateral surfaces.
• They are found in transporting epithelia and
are associated with numerous mitochondria,
which provide energy for the ionic pumps.
• Important examples include the proximal and
distal kidney tubules, and the striated ducts of
salivary glands
84

85. Basal outfoldings of plasma membrane.
• Another specialization of the plasma
membrane is seen as elaborate basal
protrusions in certain epithelia.
• These outfoldings occur along the basement
membrane and serve the purpose of
increasing surface area for attachment of the
epithelial cells to the underlying Connective
Tissue.
• An important example of this specialization is
found at the dermal epidermal junction in the
skin.
85

86. 86