Presented by Dr. Hema KeswaniRajiv gandhi college of dental sciences,Bangalore

CONTENTS

• Definition • Structure and function• Classification• Cell polarity• Apical domain ,lateral domains, basal domains• Gland • Epithelial cell renewal• Oral epithelium

• Definition : epithelium covers body surfaces, lines body cavities and constitute glands.

• 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.

• In special situation epithelial cells lack a free surface • ( epitheliod tissue)Derived from progenitor mesnchymal cellsEx- interstitial cells of leydig, islets of Langerhans ,parenchyma of

adrenal gland, tumor derived epithelium , epithelioreticular cells of thymus, accumulation of CT macrophages in response to injury or infection,

FUNCTIONS•Secretion- columnar epithelium of stomach and gastric gland

•Absorption- columnar epithelium in intestine and PCT in kidney

•Transportation- material of cells along the surface of epithelium by motile cilia

•Protection- skin and urothelium in bladder

•Receptor- taste and olfaction

Simple squamous epithelium•A single layer of flattened cell ( “pavement”) disc shaped central nuclei

•Thin cytoplasm

Simple cuboidal cells

• single layer of cuboidal cell with large spherical nuclei

•Facilitates exchanges but are more involved in active mechanism that require extensive organelle and membrane system, which necessitate greater cell volume

•Ex – PCT and DCT

Simple columnar epithelium•single layer of tall cells with round or oval nuclei

•Function – exchange

•Ex – alimentary canal ( intestinal absorptive cells)

Stratified epithelium•Two or more layers•Basal layer is attached to basement membrane•Stratified squamous epithelium• CK- keratinocytes display abundant desmosomes•Function – protection• variation-•St sq keratinizing epithelium/ orthokeratinizing or cutaneous type•Increased CK•Skin and certain areas of masticatory mucosa

• Increased acidophilic surface layer, degree of keratinization is so high that nuclei and other organelle are absent

• Keratohyaline granules present

St sq Para keratinizing epithelium•Intermediate in CK

•Mostly found on masticatory mucosa

•Nuclei seen in surface layer but acidophilic is more prominent in these cells compared to basal layer

•Keratohyaline may or may not be present

•No is less than SSKE

St sq nonkeratininzing epithelium •Hypo keratinized because of less CK

•Mucous type

•Ex – oral cavity, esophagus, vagina

•Nuclei present in surface layer and among all 3 acidophilia is least prominent

•Lower CK

Pseudo stratified epithelium•All cells are attached to basal lamina but not all cell reach the free surface

•Structure

1- similarity to simple epithelium

•Varying height ,tallest cell reach the apical surface of the epithelium

2- similarity to stratified epithelium

•Stratified due to nuclei arrangement at different levels

• Cells rest on basement membrane the cell are not stratified only their nuclei appear stratified

• Ex- respiratory and urinary

TRANSITIONAL EPITHELIUM

• Resembles both stratified squamous and stratified cuboidal ,basal cell cuboidal or columnar, surface cell dome shaped or squamous like depending on the degree of organ stretch

CLINICAL CORRELATION

• Epithelial metaplasia is a reversible conversion of one

mature epithelial cell type to another mature epithelial cell

type.

Polarity•All epithelia are polarized and exhibit specific domains that are related to one of their surfaces

•Apical- located at free surface

•Lateral- located facing adjacent member of epithelium

•Basal- surface adjacent to CT

•Basal + lateral = basolateral

APICAL DOMAIN AND ITS MODIFICATIONS

• Apical surface occur at epithelium surface facing lumen also known as free surface

• Ex – microvilli

• Cilia

• glycocalyx

MICROVILLI

• They are finger like cytoplasmic projections on the apical surface of most epithelial cells

• Internal structure contain a core of actin filament that are cross linked by several actin bundling proteins

• Striated border : intestine

• Brush border : kidney

• Stereo cilia : ductus epididymis, cochlear and vestibular hair cell of internal ear

STEREOCILIA

• Unusually long, immotile microvilli

• Develops from microvilli by lateral addition of actin filaments to actin bundles as well as elongation of the actin filament

CILIA

• Cytoplasmic and plasma membrane projection from apical surface

• Motility

• Contain core of microtubules arranged in specific configuration called axoneme

• Axoneme extends from basal bodies centriole derived microtubule organizing center(MTOC) located in apical region of ciliated cell

• 1 motile cilia – flagella posses typical 9+2 axonemal organization

• 2 primary cilia (monocilia)- solitary projection on almost all eukaryotic cells (9+0)

• Immotile (lack of microtubule associated protein)

• Chemosensor,osmosensor,mechanosensor

• 3 nodal- found in embryo on bilaminar ebryonic disc at time of gastrulation (9+0)

• Conc at area that surrounds primitive node

• Rotational movement

CLINICAL CORELATION

mutations in two genes, ADPKD1 and ADPKD2,

appear to affect development of these primary cilia, leading

to polycystic kidney disease (PKD).

• When nodal cilia are immotile or absent, nodal flow does not occur, leading to random placement of internal body organs. Therefore, primary ciliary dyskinesia (immotile cilia syndrome) often results in situs inversus, a condition in which the position of the heart and abdominal organs are reversed.

JUNCTIONAL COMPLEXES• Specific structural components that make up the barrier and the

attachment device are readily identified by EM are collectively referred to as junctional epitheliumOccluding 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

• Anchoring junctions- provide mechanical stability to epithelial cells by linking the CK of one cell to CK of adjacent cell

It interact with both actin and intermediate filament

• Their signal transduction capability anchoring junction play imp role in cell to cell recognition , morphogenesis, and differentiation

• Communicating junction – allow detail communication between adjacent cells by diffusion of small ( less than 1200 daltons) molecules ( aminoacid, sugar, nucleotide, secondmessenger metabolite )

ZONA OCCLUDENS• ZO is created by localized sealing of plasma membrane of

adjacent cells

• Ocludin – 60 kilodalton protein.It participates in maintaining the barrier between adjacent cells as well as the barrier between the apical and lateral domains

• Claudin – family (20-27 kilodalton)

• Claudin 2 and claudin 16 form aqueous channel for paracellular passage of ion and molecule

24 members

mutation – hereditary deafness(claud 14)

• JAM-40 kd

IgSF- associated with claudin

the zonula occludens appears not as a continuous seal but as a series of focal fusions between the cells

• All 3 protein contain unique amino acid sequence that attract regulatory and signaling protein called PDZ domain protein

• ZO1- tumor suppressor

• ZO2-epidermal growth factor

• ZO3- protein interact with ZO1 and cytoplasmic domain of ocludin

• CMV & cholera toxin- ZO1 & ZO2 –cause junction to become permeable

ANCHORING JUNCTIONS

• Two types of anchoring cell-to-cell junctions can be identified on the lateral cell surface:•

• zonula adherens ( pl., zonulae adherentes), which interacts with the network of actin filaments inside the cell; and

• macula adherens (pl., maculae adherentes) or desmosome, which interacts with intermediate filaments

ZONA ADHERANS(cell to cell)•Interact with networks of actin filament inside the cells

MACUL ADHERANS (CELL TO CELL)

• Desmosomes which interact with intermediate filament

CELL ADHESION MOLECULES

• Heterotypic binding

• Homotypic binding

• 50 CAMs

Major families

• cadherins

• Integrins

• Selectin

• Immunoglobulin superfamily

CADHERINS

• These are calcium dependent glycoprotein contain extracellular domain with 3-5 internal repeats, a single-spanning transmembrane domain, and an intracellular domain.

• Localized in ZA

• homotypic

• Catenin link to actin filament

Regulate growth and cell differentiation

INTEGRINS•15 alpha and 9 beta chains

•Heterotypic

•Interact with extracellular matrix molecules (collagen, laminin, fibronectin)and with actin & IF

•Regulate cell adhesion, control of cell movement and shape,growth and differentiation

• Integrins are composed of two noncovalently associated membrane-spanning subunits, designated α and β.

SELECTIN• Expressed on WBCs (leukocytes) and endothelial cells and

mediate neutrophill endothelial cell recognition

• Heterotype

• Accumulation of lymphatic tissues (homing procedure)

• In selectins, the extracellular domain consists of a calcium-dependent lectin domain, an epidermal growth factor-like domain, followed by a domain homologous to epidermal growth factor.

• selectins contain a hydrophobic transmembrane domain and a short cytoplasmic tail

IGSF

• Glycoprotein

• Homotypic

Intercellular ICAM

Vascular VCAM

Down syndrome DSCAM

Platelet endothelial cell PECAM

JAM

• They have imp role in ca and tumor metastasis , angiogenesis, inflammation, immune response, microbial attachment

E cadherin +catenin +vinculin + α actinin

fuzzy plaque formation

• Desmocollin and desmoglein-linkage to plasma membrane of adjacent cells

2 domain in antiparallel orientatiom – cadherin zipper

They interact with plakoglobin, desmoplakins,

FOCAL ADHESION (CELL TO MATRIX)

HEMIDESMOSOME (CELL TO MATRIX)

GAP JUNCTION

• Communicating junctions, also called gap junctions

or nexuses, are the only known cellular structures that permit

the direct passage of signaling molecules from one cell

to another.

• Accumulation of transmembrane channel or pores

• Cx 46, Cx 50 – inherited cataracts

BASEMENT MEMBRANE

• discrete layer of electron dense matrix material 40-60 nm thick between epithelium and CT called basal lamina/ lamina densa

• Between basal lamina and cells is a relatively clear or electron lucent area called lamina lucida (40 nm wide) contain CAMs fibronectin and laminin receptors

• Basal lamina in non epithelial cells is referred to as the external lamina

• They consist of approx 50 protein that can be classified into 4 groups

• Collagens. At least three types of collagen species are

• present in the basal lamina

• type IV collagen.

• Two nonfibrillar types of collagens, type XV collagen and type XVIII collagen, are also found in the basal lamina. Type XV collagen plays an important role in stabilizing the structure of the external lamina in skeletal and cardiac muscle cells, whereas type XVIII collagen is mainly present in vascular and epithelial basal laminae and is believed to function in angiogenesis. In addition, type VII collagen forms anchoring fibrils that link the basal lamina to the underlying reticular lamina

• LAMININS- cross shaped glycoproteins molecules (140-400)

• 3 polypeptide chains

• It posses binding site for different integrin receptors in the basal domain of the overlying epithelial cell

• Involved in cell to ECM interaction

• Role in development , differentiation and remodeling of epithelium

• 15 variation of laminin molecules

ENTACTIN / NIDOGEN

• Small rod like sulfated glycoprotein (100 kd)

• Link between laminin and type IV collagen network

• Bind Calcium ,support cell adhesion, promote neutrophill chemotaxis and phagocytosis and interact with laminin ,perlecan ,fibronectin & type IV collagen

PROTEOGLYCAN

• A protein core to which heparn sulfate ,chondroitin sulfate or dermatan sulfate side chains are attached

• Highly anionic ,highly hydrated, negatively charged

• PERLECAN- most common heparn sulfate proteoglycan found in all basal laminae is the large multidomain proteoglycan

• Provide additional cross links to basal lamina by binding to laminin ,type IV collagen and entactin /nidogen

• ARGIN (500 kd)- glomerular basement membrane of kidney

• Basal lamina self assembly is initiated by polymerization of laminin in the basal cell domain & interaction with type Iv collagen suprastructure

• A layer of reticular fibers underlies the basal lamina

STRUCTURE FOR ATTACHMENT OF BASEMENT MEMBRANE TO CT•1 ANCHORING FIBRILS( TYPE IV collagen )- these fibrils entraps type III collagen (reticular)fibers in underlying CT which ensure sound epithelial anchorage

•Mutation- collagen VII- dystrophic epidermolysis bullosa

2 FIBRILLIN MICROFIBRILS – 10-12 nm in diameter

•Attached to lamina densa to elastic fibres

•Mutation FBN1 causes marfans syndrome

Discrete projection of lamina densa-

•On its CT side interact directly with reticular lamina to form an additional binding site with type III collagen

Functions•Structural attachment

•Compartmentalization

•Filtration

•Tissue scaffolding

•Regulation and signaling

CELL TO EXTRACELLULAR MATRIX JUNCTION FOCAL ADHESION which anchor actin filaments of cytoskeleton into basement membrne

•Transmembrane protein –integrin

•Actin binding protein(alpha actinin,vinculin,talin,paxillin)+ focal adhesion kinase or tyrokinase

•Extracellular side-integrin+extracel matrix glycoprotein (laminin and fibronectin)

HEMIDESMOSOMES which anchor the intermediate filaments of cytoskeleton into basement membrane

•It exhibits an intracellular attachment plaque on cytoplasmic side of basal plasma membrane

•Desmosomal plaque-desmoplakin like family protein

•Plectin -450 kd

•It interact with microtubule actin filaments and myosin II

ERBIN 180 kd mediate association of BP230 with integrin

•Integrin class of cell matrix receptor

Alpha 4 beta 6

– heterodimer molecule contain 2 polypeptide chains

•Interact with typeIV collagen, laminin 5,entactin/nidogen or perlecan

•Laminin 5 + alpha 4 beta 6= hemidesmosomes

•Mutation of laminin= junctional epidermolysis bullosa

• Type XVII collagen (BPAG2, BP 180), a transmembrane

molecule (180 kilodaltons), that regulates expression

and function of laminin-5. In experimental models, type

XVII collagen inhibits migration of endothelial cells

during angiogenesis and regulates keratinocyte migration

in the skin

• CD151 (32 kilodaltons), a glycoprotein that participates

in the clustering of integrin receptors to facilitate cellto–

extracellular matrix interactions.

GLANDS

ORAL EPTHELIUM• The tissue that forms surface of the oral mucosa

• It constitutes primary barrier between oral environment and deeper tissues

• It is of stratified sqaumous type• Maintain structural integrity by continuous cell removal

The cells consists of 2 functional population

Progenitor population – performing epithelial proliferation

Maturing population – performing epithelial maturation

Progenitor cells present in the basal layer • Dividing cells tend to occur on clusters • Progenitor compartment consists of 2 functional

sub population of cells •Small stem cells which produce basal cells and retain the proliferate potential of the tissue •Large amplifying cells-increase the number of the cells for subsequent maturation • (cannot be distinguished by appearance)

EPITHELIUM MATURATION

• Maturation follows 2 main patterns

• Keratinization

• Non – keratinization

Keratinization • Occurs in masticatory mucosa which is tough & resistant to

abrasion

• Histologically , shows a number of distinct layers or strata

Stratum basale • Basal layer adjacent to basement membrane

• Formed of cuboidal or columnar cells

• The cells in the basal layer capable of division and so called as stratum germinivatum

Stratum spinosum • Above basal layer

• Occur as rows of large elliptical or spherical cells

• Contacts only at points known as intercellular bridges or desmosomes

• Microscopically looks spine like(prickle layer)

Stratum granulosm •Next to spinous layer

•Consists of large flattened cells

•Cells contain small granules that stain immensely with hematoxylin

•The granules are called as keratohyalin granules

• Non keratinization

• Usually the lining mucosa

• Basal & prickle layers resemble that of keratinized except the prickle cells of non- keratinized epithelium are slightly larger and intercellular bridges are less conspicuous

• Above the prickle layer, divided into 2 zones

• Stratum intermedium

• Stratum superficiale

• # No granular layer

• # Superficial layer contain plump nucleus

• # Not stain intensely with eosin

CELLULAR EVENTS• In both keratinized and non keratinized types the changes in cell

size and shape takes place by

• Synthesis of more structural protein in the form of tonofilaments

• Appearance of new organelles

• Production of additional intercellular material

• (maturation depends on the amount of keratins present )

• KERATINIZED EPITHELIUM • These granules are • Small • Membrane bound • Size – 250 nm • Contain glycolipid • Originate from golgi system

• KERATINIZED EPITHELIUM

• In granular layer , these cells have greater size

• The membrane bound granules attach to the superficial cell membrane & discharge the contents into the intercellular space

• This discharge forms lipid rich permeability barrier

• KERATINIZED EPITHELIUM • Superficial cells of granular layer develop thickening on the

inner aspect of their membrane • This is responsible for the considerable resistance of

keratinized layer to chemical solvents • Main constituent of this thickening is a protein known as

involucrin

• Keratohyalin granules associated with tonofibrils , thought to form matrx

• The protein which form the bulk of these granules is filaggrin

• Sulphur rich loricrin is also present in some areas

• As the cells reach the superficial layer ,all the organelles including nuclei& kerato-hyalin granules disappear

• Dehydration occurs and cells become packed with filaments surrounded by filaggrin

• Cells are extremely flattened (squames )

• These squames are lost and replaced by cells from under lying layers

• This clearance is important – prevent colonization of pathogenic micro organisms (candida)

• Keratinized layer of gingiva may contain upto twenty layers of squames

Para keratinization• There is incomplete removal of organelles from the cells of

the granular layer• The nuclei remains as shrunken, pyknotic structures • Sometimes outermost squames of keratinized epithelium do

not look like the rest of the epithelium(incomplete keratinization – due to rehydration)• No pathologic significance

Non-keratinized epithelium• In prickle cell layer, increase in size is more than that of

keratinized epithelium

• Tonofilaments remain dispersed

• Contain membrane bound granules

• They are circular in shape, with an amorphous coat

• Granular layer – cells are flattened

• Thickening of inner aspect of cell membrane is not obvious

• Accumulation of glycogen on the surface cells

• Keratohyalin granules rarely seen. If present, they are regular spherical structures, surrounded by ribosomes ,not associated with tonofilaments

• No fillagrin but loricrin is present

• This loricrin contributes to the internal thickening of the cell membrane

• The cells of the superficial layer,

• Are more flattened

• Contain dispersed tonofilaments and nuclei

• Number of cell organelles are diminished

• Not dehydrated

PERMEABILITY AND ABSORPTION• Depends on the

• Thickness of the epithelium

• Pattern of maturation

• Thinnest epithelium allow better penetration

• Permeability barrier is due to the lipids derived from the membrane coating granules

NON KERATINOCYTES IN ORAL EPITHELIUM

• They together makes up 10% of cell population in the oral epithelium

• In light microscope – appear as clear cells , coz they lack desmosomal attachments to adjacent cells

• No tonofilaments

• No maturation

• Different non- keratinocytes in oral epithelium are

• Melanocytes

• Langerhans cells

• Merkel cells

• Inflammatory cells

MELANOCYTES

• Endogenous pigmentation most commonly contributing to the colour of the oral mucosa are melanin & hemoglobin

• Melanin produced by melanocytes

• They are present in the basal layer of the oral epithelium

• Melanocytes arise embryonically from neural crest ectoderm

• They enter the epithelium at about 8 weeks of gestation

• Lack desmosomes & tonofilaments

• Posses long dentric process that extend between keratinocytes ,often passing through several layers

• Melanin synthesized with in the melanocytes as small structures called melanosomes

• Melanosomes are inoculated into the cytoplasm of adjacent keratinocytes by the dendritic process

• In heavily pigmented tissues melanosomes groups identified through light microscope

• Colour differences in oral mucosa due 2

• Relative activity of melanocytes in producing melanin

• Rate at which melanosomes broken down in the keratinocytes

• In heavily pigmented persons melanin may be seen in con tissues , which is taken up by macrophages and called as melanophages

• Skin colour , directly proportional to melanin deposition

Melanocytes involved in pigmented lesions of oral mucosa are

• Oral melanotic macule

• Nevus

• Melanoma

LANGERHANS CELL

• Seen in the suprabasal layer

• Lacks desmosomal attachments

• Characterized by the presence of a small rod or flask shaped granules called Birbeck granules

• Capable of limited division with in the epithelium

• Can move in and out of the epithelium

• Source – Bone marrow

• Have immunologic function (recognizing & processing antigenic material that enters the epithelium from external environment & present it to the helper T lymphocytes)

• Some times they migrate from epithelium to regional lymph nodes as they express Ia antigens and Fc receptors

• Important role in

contact hypersensitivity

anti-tumor immunity

graft rejection

MERKEL CELL

• Situated in the basal layer

• Not a dentritic cell

• Posses tonofilaments & occasional desmosomes , hence not always resemble clear cells in histologic sections

• Nucleus is often deeply invaginated and may contain a characteristic rodlet

• Characteristic feature is presence of small membrane bound vesicles in the cytoplasm sometimes situated adjacent to a nerve fiber associated with the cell

• Granules release neurotransmitter between the nerve and thus trigger an impulse

• Sensory & respond to touch

• May arise from division of a keratinocyte

INFLAMMATORY CELLS

• Mostly lymphocytes

• Some times PMN & mast cells

• Lymphocytes often associated with Langerhans cells

• Few inflammatory cells – normal component in non keratinocyte population

CYTOKERATINS IN ORAL EPITHELIUM

• Cytokeratins are protein of keratin-containing intermediate filaments found in the intracytoplasmic cytoskeleton of epithelial tissue. The term "cytokeratin" began to be used in the late 1970s (for example, see "Intermediate-sized filaments of human endothelial cells" by Franke, Schmid, Osborn and Weber) when the protein subunits of keratin intermediate filaments inside cells were first being identified and characterized. In 2006 a new systematic nomenclature for keratins was created and now the proteins previously called "cytokeratins" are simply called keratins.[2]

•There are two types of cytokeratins: the acidic type I cytokeratins and the basic or neutral type II cytokeratins. Cytokeratins are usually found in pairs comprising a type I cytokeratin and a type II cytokeratin. Basic or neutral cytokeratins include CK1, CK2, CK3, CK4, CK5, CK6, CK7, and CK8. Acidic cytokeratins are CK9, CK10, CK12, CK13, CK14, CK16, CK17, CK18, CK19 and CK2

INTERMEDIATE FILAMENT•Essential component o f cytoskeleton and nucleoskeleton of all cells•65 members of multigene family present known to encode 10-12 nm filament.•Cytokeratin : largest group of Ifs with about 50 genes•Based on gene structure and sequence homologues at least 5 classes of Ifs •I-acidic•II basic•III vimentin,desmin ,etc•IV neurofilaments•V nuclear lamins•VI beaded filaments

• 30 different types are present

• Lowest mol.wt is (40Kda) found in glandular and simple epithelia

• Highest mol.wt is (67Kda) found in keratinized stratified epithelium• The types of keratin present may vary between different epithelia and

even between different layers within the epithelia

• Assembly group 1&2 – cytoplasmic IFs

• Assembly group 3 – nuclear Ifs

• Organization- in epithelium ck referred to as tonofilaments ( ~10nm filament visualized with TEM )

• And bundles of tonofilaments are called as tonofibrils

• CLINICAL CORELATION changes in neurofilaments within brain tissue are characteristic of Alzheimer’s diseaseIn Alexander disease presence of cytoplasmic inclusionsin astrocytes (Rosenthal fibers) that contain accumulationof intermediate filament protein

• Keratinized epithelium has keratins 1, 6, 10, 16

• Non-keratinized epithelium has keratins 4, 13

REFERENCES

• Ross Histology and Text Atlas 6th edition

• Orbans Oral Histology and Embryology 13th edition

• Ten Cate’s Oral Histology 8th edition

• Oral Anatomy,Histology and Embryology (B.K.B Berkovitz, G.R Holland, B.J. Moxham) 3rd edition

• internet