2. CONTENTS
Definition
Development
Characteristics
Classification
Functions
Cell polarity
Membrane specialization of epithelia
Glands
Epithelial cell renewal
Structure of the oral epithelium
3. DEFINITION
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).
4. DEVELOPMENT
EPITHELIUM IS DERIVED FROM 3 GEREM LAYERS
ECTODERM
MESODERM
ENDODERM
ALTHOUGH MOST OF THE EPITHELIAARE DERIVED
FROM ECTODERM AND ENDODEREM
5.
6. ECTODERM: Oral and nasal mucosa, cornea, epidermis
of the skin & glands of the skin & the mammary glands.
ENDODERM: The liver, the pancreas & the lining of the
respiratory and GIT.
MESODERM: Uriniferous tubules of the kidney, the lining
of the male and female reproductive systems, the
endothelial lining of the circulatory system and the
mesothelium of the body cavity.
7.
8. CHARACTERISTICS
They are closely apposed and adhere to one another by
specialized cell junctions.
They exhibit functional and morphologic polarity through
a free surface or apical domain, a lateral domain, and a
basal domain.
Their basal surface is attached to an underlying basement
membrane.
9.
10.
11. EPITHELIAL CELLS that lack a free surface are seen
in:
interstitial cells of Leydig in the testis
the lutein cells of the ovary
the parenchyma of the adrenal gland
anterior lobe of the pituitary gland
Epithelioreticular cells of the thymus
certain types of injury, Infections and tumors
12. EPITHELIUM creates a selective barrier between the
external environment and the underlying connective
tissue seen in
Blood
lymph
14. SIMPLE- When it is one cell layer thick
STRATIFIED- When it has two or more cell layer.
SQUAMOUS- When the width of the cell is greater than
its height.
CUBOIDAL- When the width, depth & height are
approximately the same.
COLUMNAR- When the height of the cell
approximately exceeds the width.
15.
16. SIMPLE SQUAMOUS EPITHELIUM
Composed of flattened, irregularly shaped cells
forming a continuous surface which may be reffered
to as pavemented epithelium.
Term ‘squamous’ derives from the comparison of the
cells to the scales of a fish.
Supported by an underlying delicate membrane.
Involved in passive transport of either gases or fluids.
17. PRESENT IN:
Lining- pulmonary alveoli, loop of Henle, parietal layer of
Bawman capsule, inner and middle ear, blood and
lymphatic vessels, pleural and peritoneal cavities.
FUNCTION:
Limiting membrane
Fluid transport
Gaseous exchange
Lubrication
Reducing friction
Lining membrane
19. SIMPLE CUBOIDAL EPITHELIUM
Intermediate form between simple squamous and
simple columnar epithelium.
Nucleus is round and located in the centre of the cell.
Present in:
Ducts of exocrine glands
Surface of ovary
Kidney tubules
Thyroid follicles
22. SIMPLE COLUMNAR EPITHELIUM
Cells are taller and appear columnar in sections at right
angles to the basement membrane.
Nuclei are elongated and may be located towards the
base, the centre or occasionally the apex of the
cytoplasm, this is known as polarity.
PRESENT IN:
Small intestine & colon
Stomach lining and gastric glands
Gall bladder
25. SIMPLE COLUMNAR CILIATED EPITHELIUM
Described as a special entity because of the presence of
surface specialisation called cilia.
Each cilia consists of a finger like projection of the
plasma membrane.
Not common in humans except in the female
reproductive tract.
27. GOBLET CELL
These are modified columnar epithelial cells which
synthesize and secrete mucin.
Scattered among the cells of many epithelial linings
particularly respiratory and GIT.
Mucus content stain for glycoprotein using PAS.
The part of the cell where secretory vesicle
accumulates bulges and compresses the
neighboring cells.
28.
29. PSEUDOSTRATIFIED COLUMNAR EPITHELIUM
The term pseudostratified is derived from the appearance
of this epithelium in section which conveys the erroneous
impression that there is more than one layer of cells.
True simple epithelium since all the cells rest on the
basement membrane.
Nuclei are disposed at different levels thus creating the
illusion of cellular stratification.
30. Exhibit polarity with nuclei confined to the basal two-third
of the epithelium, cilia are never present on stratified
epithelium
PRESENT IN:
Trachea & bronchial tree
Ductus deferens
Auditory tube and tympanic cavity
Nasal cavity & lacrimal sac
Male urethra
Large excretory ducts
33. STRATIFIED SQUAMOUS EPITHELIUM
It consists of a variable number of cells layer which
exhibit transition from a cuboidal basal layer to a
flattened surface.
Basal layer divide continuously.
Well adapted to withstand abrasion since loss of
surface cells does not compromise the underlying
tissue.
Nuclei become progressively condensed (pyknotic)
and flattened, before ultimately disintegrating.
39. STRATIFIED CUBOIDAL EPITHELIUM
Thin, stratified epithelium which usually consists of
only two or three layers of cuboidal or low columnar
cells.
Not involved in significant absorptive or secretory
activity
PRESENT IN:
Ducts of sweat glands
Large ducts of exocrine glands
Anorectal junction
42. STRATIFIED COLUMNAR EPITHELIUM
PRESENT IN:
Conjunctiva of eye
Some large excretory ducts
Portions of male urethra
FUNCTION:
Secretion
Absorption
Protection
43.
44. TRANSITIONAL EPITHELIUM
Form of stratified epithelium.
Highly specialized to accommodate a great degree of
stretch.
So named because it has some features which are
intermediate (transitional) between stratified cuboidal and
stratified squamous epithelia.
In relaxed state (contracted) state, transitional epithelium
appear to be about 4-5 layers thick. basal cells are
roughly cuboidal , the intermediate cells are polygonal, and
the surface cells are large and rounded and may contain 2
nuclei.
45. In the stretched state, it appears 2-3 cells thick (although
the actual number of layers remains constant) and the
intermediate and surface layers are extremely flattened.
PRESENT IN:
Urethra
Ureters
Bladders
Renal calyces
FUNCTION:
Protection
Distensible
46.
47. FUNCTIONS
1) Protection of underlying tissue of the body from
abrasion and injury
2) Transcellular transport of molecules across
epithelial layers
3) Secretion of mucus, hormones, enzymes and so
forth from various glands.
48. 4) Absorption of materials from a lumen
5) Control of movement of materials between body
compartments via selective permeability of
intracellular junctions between epithelial cells.
6) Detection of sensations via taste buds, retina of the
eye and specialized hair cells in the ear.
49. CELL POLARITY
The free or apical domain is always directed towards the
exterior surface or the lumen of an enclosed cavity or tube. It
is rich in ion channel, carrier protein and hydrolytic enzymes
as well as aquaporins, channel forming proteins that function
in regulation of water balance.
Lateral domain communicates with adjacent cells & is
characterized by specialized attachment areas.
The basal domain rests on the basal lamina anchoring the
cell to underlying connective tissue.
50.
51. MEMBRANE SPECIALIZATION OF EPITHELIA
The intercellular, luminal and basal surface of epithelial
cells exhibit a variety of specialization.
1) INTERCELLULAR SURFACE:
The apposed surface of epithelial cells are lined by
several different types of membrane and cytoskeletal
specialization. Cell junctions are:
a) Occluding or tight junctions:
Located immediately behind the luminal surface of
simple columnar epithelium
Intercellular spaces are oblitereted
Transmembrane adhesive protein- occludin, claudin,
junctional adhesive molecule
52. Each tight junction forms a continuous
circumferential band or zonules around the cell and
are thus known as zonula occludens.
FUNCTIONS:
Seal adjacent cells together
Involved in cell signaling
Defines apical and basolateral domain of
plasma membrane.
Tightness of the junction is related to the claudins
present
53.
54.
55. b) Adhering junctions:
Tightly binds the constituent cells of the epithelium
together and acts as an anchorage
Cytoskeleton of all the cells are effectively linked into a
single functional unit
Apoptosis, loss of cell polarity, unregulated cell
proliferation are absent.
Important in cellular signaling
Intercellular space- 20nm
56.
57. CELL TO CELL ADHESIVE JUNCTION:
Transmembrane protein – cadherin
Cytoplasmic adaptar protein- catenin
Zona adherens- e-cadherin
A&B catenins, nectins
actin filaments
Macula adherens- A desmosome also known as macula
adherens is a cell structure specialized for cell-to-
cell adhesion.
Desmoglein & desmocollin
Desmoplakin, Plakoglobin,
Plakophollin
Intermediate filaments
58.
59.
60. CELL TO CELL MATRIX JUNCTION:
Focal adhesion which anchor actin filaments of the
cytoskeleton into the basement membrane
Integrin, A-actinin, vinculin, talin, actin filaments, remodelling
of actin filaments.
Hemidesmosomes which anchor the intermediate filaments
of the cytoskeleton into the basement membrane.
Hemidesmosomes are asymmetrical and are found in
epithelial cells connecting the basal face of the cell to basal
lamina. Similar in form to desmosomes when visualized by
electron microscopy
Integrin, A6B4, BP230, Plectin, intermediate filaments links
the cells to the basal lamina.
62. c) Gap junction:
Circular intercellular contacts areas containing hundreds
of tiny pores which permit passage of small molecules
between adjacent cells.
Intercellular space- 2-3 nm
Transmembrane protein- connexin (form aqueous
channels)
Function-:-
Creates a (nexus) adjacent cell conduct between two
adjacent cells for passage of small ions and
informational micromolecules.
63.
64. 2) LUMINAL SURFACE:
Luminal surface of epithelial cells may incorporate
3 main types of specialization:
a)cilia
b)microvilli
c)stereocilli
65. CILIA
Relatively long motile structure which are resolved by
light microscopy
They are hairlike extensions of the apical plasma
membrane containing an axoneme, the microtubule-
based internal structure.
Cilia give a “crew-cut” appearance to the epithelial
surface basal bodies.
66.
67. MOTILE CILIA
Active movement due to the presence of microtubule
associated proteins; rapid forward movement with slow
recovery stroke (half cone trajectory).
most commonly found on epithelia which function in
transporting secretions .
present on sperm cells as flagella; provides a forward
movement to the sperm cell.
Motile cilia are capable of moving fluid and particles
along epithelial surfaces eg. Tracheobronchial tree and
oviduct.
68. PRIMARY CILIA
found in almost all cells in the body .
transmit signals from extracellular space into the
cell.
No active movement; passively bend due to flow
of fluid.
Function: chemosensors
osmosensors
mechanosensors.
69. NODAL CILIA
Structure similar to primary cilia except they have
an ability for active transport, active rotational
movement
Found in the embryo during gastrulation on the
bilaminar disc near the area of primitive node.
Essential in developing left-right asymmetry of
internal organs.
70. MICROVILLI
Microvilli are fingerlike cytoplasmic projections on the
apical surface of most epithelial cells .
In intestinal absorptive cell this surface structure
was originally called the striated border; in the
kidney tubule cells, it is called the brush border.
Can not be individually resolved with the microscope
Internal structure contain a core of actin filament that
are cross linked by several actin binding protein
Increase absorptive capacity
71.
72. STREOCILIA
Stereocilia are unusually long, immotile microvilli.
Found only singly or in small number in odd sites
such as the male reproductive tracts.
Contains ezrin and A-actinin.
Treadmilling effect- structure renewal process
74. 3) BASAL SURFACE
The interface between all epithelia and underlying
supporting structures is marked by a noncellular
structure known as the basement membrane.
It provides structural support for epithelia and
constitute a selective barrier to the passage of
material between epithelium and supporting tissue.
75. Hemidesmosomes provides a mean of anchorage
of the cells via its cytoskeleton to the basement
membrane and underlying supporting tissue
Consists of 3 zones:
lamina lucida
lamina densa
lamina fibroreticularis or sublamina densa
76. LAMINA DENSA
The lamina densa is a component of the basement
membrane zone between the epidermis and dermis of
the skin, and is an electron-dense zone between
the lamina lucida and dermis.
Synthesized by the basal cells of the epidermis
Electron dense matrix 50nm thick between the epithelium
and the adjacent connective tissue
Exhibit a network of fine, 3-4nm filaments composed of
laminins, a type iv collagen molecule (chicken-wire) and
proteoglycans and glycoprotein.
77.
78. LAMINA LUCIDA
The lamina lucida is a component of the basement
membrane which is found between the epithelium and
underlying connective tissue.
Clear zone 40nm thick that attach the cells to the
basal lamina
Contain- collagen type xvii, integrins, laminin v
Anchoring fibrils consists of collagen type vii attach
basal lamina to connective tissue.
79. GLANDS
Typically glands are classified into:
1)Exocrine glands discharge their secretory product via a
duct onto an epithelial surface. Cells of which are
composed of highly specialized epithelial cells, the
internal structure of the cells reflecting the nature of the
secretory product and the mode of secretion.
Morphology: a)simple: single, unbranched duct.
b)compound: branched duct system.
2)Endocrine glands are ductless. Secrete their product
into the connective tissue where they enter the blood
stream to reach the target cells. The products of
endocrine glands are called hormone.
80.
81.
82. In some epithelia, individual cells secrete a
substance that does not reach the blood stream but
rather affects other cells within the same epithelia.
Such secretory activity is referred to as paracrine.
The secretory material reaches the target cells by
diffusion through the extracellular space or
immediately subjacent connective tissue.
83.
84. Cells of exocrine glands exhibit different
mechanism of secretion:
Merocrine secretion- involves the process of
exocytosis and is the most common form of
secretion, protein are usually the major secretory
product
Apocrine secretion- involves discharge of free,
unbroken, membrane bound vesicles containing
secretory product. This is an unusual mode of
secretion and appears to lipid secretory products in
the breasts and some sweat glands
85. Holocrine secretion- involves discharge of whole
secretory cells with subsequent disintegration of the
cells to release the secretory product. Occurs
principally on the sebaceous glands.
86.
87. EPITHELIAL CELL RENEWAL
The stratified squamous epithelium of skin is replaced in
approximately 28 days.
Cells in the stratum basale undergo mitosis to provide
for cell renewal.
As these cells differentiate they are pushed toward the
surface by new cells in the basal layer.
Ultimately, the cells become keratinized and slough off.
88. Thus a steady state is maintained within the
epithelium, with new cells normally replacing
exfoliated cells at the same rate.
Cells arising by division in the basal layer may
remain in the progenitor cell population or undergo a
process of maturation as they move to surface.
89. EPITHELIAL METAPLASIA
Epithelial metaplasia is a reversible conversion of
one mature epithelial cell type to another mature
epithelial cell type.
Metaplasia is generally an adaptive response to
stress, chronic inflammation, or other abnormal
stimuli.
91. TUMOURS ARISING FROM EPITHELIA
A tumour can arise from any tissue if there is uncontrolled
growth of cells.
A malignant tumour arising from an epithelia is a
carcinoma.
If it arises from squamous epithelium it is a squamous cell
carcinoma
If tumour arising from glandular epithelium it is called
adenoma.
Diagnosis can be made by Immuno histochemical
technique.
92. STRUCTURE OF THE ORAL EPITHELIUM
Stratified squamous variety.
May be keratinized (ortho or parakeratinized) or
nonkeratinized depending on location.
Keratinized: gingiva and hard palate (masticatory
mocosa). In many gingival epithelium is parakeratinized.
Non keratinized: cheeks, faucial and sublingual tissue.
Both keratinized and nonkeratinized contains 2 groups of
cells- keratinocytes and nonkeratinocytes.
93. TURNOVER TIME OF THE EPITHELIUM
Turnover time- time taken for a cell to divide and
pass through the entire epithelium.
E.g. – * skin - 52 to 75 days .
* gut - 4 to 14 days.
* gingiva - 41 to 57 days.
* cheek - 25 days.
Nonkeratinised buccal epithelium turns over faster
than keratinized gingival epithelium.
95. STRATUM BASALE
Single layer of cuboidal cells
Made up of cells that synthesize DNA and undergo
mitosis thus providing new cells
Basal cells and parabasal cells are referred to as
stratum germinativum but only basal cells can
divide.
Basal cells synthesize proteins
96. Hemidesmosomes are found in basal layer.
Lateral borders of the adjacent cells are closely
apposed and connected by desmosomes.
The basal cells contain tonofilaments and are
attached to the attachment plaque
Desmosomes consists of 2 principal proteins:
transmembranous protein and proteins within the
cells and related to attachment plaque.
98. STRATUM SPINOSUM
Irregular polyhedral cells larger than basal cells.
In light microscopy, it appears these are joined by
“intercellular bridges”
Tonofilaments seems to course from cell to cell
across the bridge.
Electron microscopy revels- intercellular bridges are
desmosomes and tonofibrils are bundles of
tonofilament.
99. Desmosome attachment plaques contain the
polypeptides desmoplakin and plakoglobin.
Intercellular space contains glycoprotein,
glycosaminoglycan and fibronectin.
Prickle cell layer- shrinks away from each other
remaining in contact at the desmosomes.
Most active layer in protein synthesis.
101. STRATUM GRANULOSUM
Flatter and wider cells larger than spinous cells
Contains basophilic keratohyalin granules
Nucleus show degeration and pyknosis.
Tonofilaments are more dense in quantity and are often
seen associated with keratohyalin granules.
Cell surface are more regular and more closely
attached to adjacent cell surface.
102. Lamellar granules:
keratinosome or odland body-
membrane coating acts as permeability barrier.
Involucrin (keratolin)- protein present at
the upper half.
Membrane coating granules are glycoprotein.
104. STRATUM CORNEUM
keratinized squamae which are larger and flatter than
granular cells.
Nuclei and organelles have disappeared.
Acidophilic and histologically amorphous layer.
Keratohyalin granules have disappeared.
Cells are composed of densely packed filaments coated
by basic protein of keratohyaline granules, filaggrin.
106. Orthokeratinized epithelium:
do not contain nuclei.
Parakeratinized epithelium:
the stratum corneum retains pyknotic nuclei.
Incomplete removal of the organelles from the
cells of the granular layer occur so that the nuclei
remain as shrunken pyknotic structure, and
remnants of other organelles also may be
present in the keratinized layer
110. Basal cells are similar.
Cells of stratum intermedium are larger than
spinosum and are attached by desmosomes and
other junction.
More closely attached than spinous cells.
No Stratum Granulosum
111. No Stratum Corneum.
Stratum Superficiale – nucleated cells
Less number of tonofilaments
Lack keratohyaline granules.
112. Have higher rate of mitosis than keratinized
epithelium.
Parakeratosis –physiologic
normally keratinizing tissue becomes
parakeratinized.
Keratosis- Pathologic
keratinization occurs in anormally nonkeratinized
tissue.
113. KERATINOCYTE
Epidermal/epithelial cells that synthesize keratin.
Characteristic intermediate filament protein is
cytokeratin.
Show cell division, undergo maturation and finally
desquamate
Increase in volume in each successive from basal to
superficial.
114. NONKERATINOCYTES
Donot possess cytokeratin filament
Do not show mitotic activity undergo maturation and
finally desquamate
Usually dendritic and appears unstained or clear in
routine H&E stains
Identified by special stain or Imunohistochemical
technique
Migrate to oral epithelium from neural crest or bone
marrow.
115. MELANOCYTES
Present in basal layer.
Arise from neural crest ectoderm.
Staining reaction- dopa oxidase- tyrosinase, silver
stains.
Stained by : Mason-Fontana stain
Dendritic, no desmosomes and tonofilaments.
Premelanosomes and melanosomes are present.
Function- synthesis of melanin pigment granules
(melanosomes) and transfer to surrounding
keratinocytes.
116.
117. LANGERHANS CELL
Present in suprabasal layer.
Arise from bone marrow.
Dendritic or clear cells with no desmosomes or tonofilaments.
Characteristic langerhans granule- Birbeck granules
Staining reactions- cell surface antigen markers
Stains by: gold chloride, ATPase & immunofluorescent
markers.
Function-
antigen trapping and processing.
120. MERKEL CELLS
Present in basal layer.
Arise from division of epithelial cell.
Staining reaction- PAS positive.
Seen in masticatory mucosa but are absent in lining
mucosa
Non-dendritic with less desmosomes and
tonofilaments.
sensory and respond to touch.
Characteristic electron-dense vesicles and associated
nerve axon.
123. REFERENCES
Michael H. Ross and Wojciech Pawlina;
Histology A Text & Atlas; 6th edition; p.105-146
Kumar GS , Orban’s Oral Histology and
Embryology, 12th Ed,2009,Elsevier,New Delhi,
p.210-226.
Nanci A , Ten Cate’s Oral Histology Development
structure and function, 7th Ed,2008,Mosby,New
Delhi,p.320-336.
124. Singh.I,Histology of Human Histology Colour Atlas,5th Ed
, Jaypee brothers , 2009,New Delhi, p.45-53.
Wheaters, functional histology, a text and colour atlas,
4th edition, page 80-96
BKB Berkovitz, oral anatomy, histology and embryology,
3rd edition, page 220-224
Leslic P. Gartner, colour textbook of histology, 3rd
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