Epithelial Tissue 2003

THE TISSUES OF THE BODY THE EPITHELIUM

DR IRAM IQBAL

TISSUES Tissues are aggregates or groups of cells organized to perform one or more specific functions. 4 types of basic tissues Epithelium Connective tissue Muscle tissue Nerve tissue

EPITHELIUM 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).

EPITHELIAL TISSUE

CONNECTIVE TISSUE • Connective tissue consist of cells and extracellular matrix that includes structural (fibers) and specialized proteins that constitutes the ground substance.

CONNECTIVE TISSUE Cell nucleus

Cell Collagen fiber

Collagen fibers

Other fibers A. LOOSE CONNECTIVE TISSUE (under the skin)

D. FIBROUS CONNECTIVE TISSUE (forming a ligament)

Fat droplets

Cells

Cell nucleus

Matrix

B. ADIPOSE TISSUE

E. CARTILAGE (at the end of a bone)

White blood cells

Central canal Matrix

Red blood cells

Cells

Plasma C. BLOOD

F. BONE

MUSCULAR TISSUE • Muscular tissue is characterized by aggregates of specialized , elongated cells arranged in parallel array whose primary role is contraction.

MUSCULAR TISSUE

CARDIAC MUSCLE SKELETAL MUSCLE SMOOTH MUSCLE

Nervous Tissue • Densely packed tissue having Two main cell types • Neurons – transmit electrical signals • Support cells (neuroglial cells) – nonexcitable – Surround and wrap neurons

NERVOUS TISSUE

Nucleu s Cell body

Cell extensions

EPITHELIUM An epithelium can be defined as the layered collection of adherent cells, with very little intercellular material, usually covering internal & external surfaces of the body.

CHARACTERISTICS • Avascular tissue • Forms the secretary portion (Parenchyma) of Glands and Ducts • Receptors for certain Sensory Organs • Closely opposed adhere to each other by junctions

Epithelium is found everywhere Simple squamous Epithelium lines alveoli in lungs.

Simple cuboidal Epithelium Forms tubes in kidneys.

Simple columnar Epithelium lines the intestine.

Stratified squamous Epithelium lines the esophagus.

In addition, specialized epithelial cells function as receptors for the special senses (smell, taste, hearing, and vision).

Epithelium also forms the secretory portion (parenchyma) of glands and their ducts.

EPITHELOID TISSUES • • • •

Certain special situations Epithelial cells lack free surface Found aggregated in to each other Closed apposition and BM is present but no free surface hence epithelial like • Examples - Interstitial cells of leydig (testis) - Luteal cells (ovary) - Parenchyma of adrenal glands - Epithelioreticular cells (thymus) - Pathologic responses to injury and tumors

Functions of epithelium

FUNCTIONS Depend upon the activity of cell type • Selective barrier

- Forms a continuous sheet like investment separating CT from internal and external environment - Fascilitates or inhibits certain substances between the two compartments (metabolites and waste material pass through it not between them)

- Impervious barrier (urinary bladder)

• Secretory (Stomach) • Both Secretory and Absortive (intestine) • Transport system through motile Cilia on its surface (particulates/mucus in Trachea and Bronchi) • Serve to receive Sensory Stimuli (Taste Buds of Tongue or Retina of Eye)

How are epithelia classified? Depends on number of layers One layer

SIMPLE

Special

2 or more layers

PSEUDOSTRATIFIED TRANSITIONAL

STRATIFIED

The individual cells that compose an epithelium

Squamous

Cuboidal Columnar

Importance of layering

Simple squamous epithelium Locations Vascular system (endothelium) Body cavities (mesothelium) Bowman’s capsule (kidney) Respiratory spaces in lungs

Major functions Exchange, barrier in CNS Exchange & lubrication Barrier Exchange

Endothelium

Mesothelium

Respiratory spaces

Bowman’s capsule

Simple Squamous Epithelium 100X 1 cell Adjacent cell membranes held together with tight junctions Nucleus

Methylene blue

Single layer of flat cells

Simple Squamous Epithelium 400X 1 cell

Silver

Single layer of flat

Simple cuboidal epithelium Locations

Major functions

Small ducts of exocrine glands Surface of ovary (germinal epithelium) Kidney tubule

Absorption, conduit Barrier

Absorption & secretion

Ducts of exocrine glands

Simple Cuboidal Epithelium 400X

Cells

Single layer of cube shaped cells; large

Simple columnar epithelium Locations

Major functions

Small intestine & colon

Absorption & secretion Secretion

Stomach lining & gastric glands Gallbladder

Absorption

Simple Columnar Epithelium 400X Single layer columnar cells nuclei in a line

Kidney collecting duct

Simple Columnar Epithelium 1000X

Single layer of cube shaped cells; large nucleus

Simple Columnar Epithelium 400X

Stomach

SPECIAL CATEGORIES OF EPITHELIUM

• • • • •

PSEUDOSTRATIFIED EPITHELIUM Actually a simple epithelium All rest on BM Some cells do not reach the surface Appearance of being stratified Limited distribution

Pseudostratified epithelium Locations

Major functions

Trachea & bronchial tree Ductus deferens

Secretion, conduit

Efferent ductules of epididymis

Absorption, conduit

Trachea (monkey) 400X Goblet cell Pseudostratified ciliated columnar epithelium

Lamina propria

Smooth muscle

Seromucous glands

Stratified squamous epithelium Locations Epidermis Oral cavity & oesophagus Vagina

Major functions Barrier Protection

Stratified squamous epithelium, oesophagus (non-keratinized)

Stratified Squamous Epithelium 400X (keratinized) Squamous cells

Skin epidermis

Dermis

Outermost layers of cells are squamous shape

Stratified Squamous Epithelium (keratinized)

Stratified cuboidal epithelium Locations Sweat gland ducts Large ducts of exocrine glands Anorectal junction Growing follicles in ovary

Major functions Barrier Conduit

Stratified Cuboidal Epithelium 400X

2 layers of cuboidal cells

Stratified Cuboidal Ovary follicle

Stratified columnar epithelium Locations Largest ducts of exocrine glands Anorectal junction

Major functions Barrier Conduit

Stratified Columnar, sweat gland duct

SPECIAL CATEGORIES OF EPITHELIUM

• • • •

TRANSITIONAL EPITHELIUM (UROTHELIUM) Stratified epithelium Specific morphologic characteristics Functionally accommodates well to distension Lower urinary tract extending from the minor calyces of the kidney down to the proximal part of the urethra.

Transitional epithelium (urothelium) Locations Renal calyces Ureters Bladder Urethra

Major functions Barrier Distensible property

Transitional epithelium

Distended bladder

Empty bladder

SPECIAL TYPES OF EPITHELIUM 1.NEURO-EPITHELIAL CELLS

Examples: Sense organs like taste buds 2.MYO-EPITHELIAL CELLS Examples Secretory acini of Mammary, Salivary glands

Epithelial Feature! (name and location on cell)

3 1

2

Tight junction 1 Microvilli 2 3 Cilia Basement membrane

4

4

CELL POLARITY • Cells whose apex differs from it,s base regarding the modification of plasmalemma and function is called as polarized cell. • Epithelial cells exhibit distinct polarity • They have an; Apical domain Lateral domain Basal domain

APICAL DOMAIN & ITS MODIFICATIONS Structural surface modifications include microvilli, stereocilia & cilia. Apical domain may contain specific enzymes (e.g., hydrolases), ion channels & carrier proteins (e.g., glucose transporters).

MICROVILLI Finger-like cytoplasmic projections on the surface of most epithelial cells. Vary widely in appearance. Short, irregular, bleb-like projections Tall, closely packed, uniform projections Number & shape of the microvilli of a given cell type correlate with its absorptive capacity.

Intestinal absorptive cells/striated border

Electron micrographs showing variations in microvilli of different cell types

Molecular structure of microvilli

STEREOCILIA Unusually long, immotile microvilli. Also termed stereovilli. Not widely distributed among epithelia. Limited to: Epididymis Proximal part of the ductus deferens of the male reproductive system Sensory (hair) cells of the ear

Electron micrographs showing variations in stereovilli of different cell types

Sterocilia of the sensory epithelium of the ear are uniform in diameter and possess an internal structure similar to that of genital duct sterocilia however they lack both erzin and aactinin, and serve as sensory receptors rather than absorptive structure

CILIA Motile cytoplasmic structures capable of moving fluid & particles along epithelial surfaces. Hair like structures .25um in diameter and 2um-10um in length. Location: Trachea Bronchi Oviducts

78

Cilia contain an organized core of microtubules arranged in a 9+2 pattern

Molecular structure of cilia Microtubule-associated motor protein

Cilia develop from procentrioles

Electron micrograph of the cilia

Microvillia vs cilia

Microvillia vs cilia, SEM

EPITHELIAL CELL ADHESIONS • Epithelial cell not only in close apposition but also adherent (with exceptions) • Before electron microscopy it was referred as intercellular cement and called Terminal Bar JUNCTIONAL COMPLEX • Specific structural components that make up the barrier and attachment device • Creat a long term behavior, compartmentalize and effects passage from and through the cells -

LATERAL DOMAIN & ITS SPECIALIZATIONS 3 types of junctional complexes in lateral domain i.e., occluding junctions anchoring junctions communicating junctions.

In addition, lateral cell surface membrane in some epithelia may form folds & processes, invaginations & evaginations that create interdigitating & interleaving tongue-andgroove margins between neighbouring cells.

Zonula occludens Also called “tight junctions”. Located at the most apical point. Forms a ring or circumferential band Created by localized sealing of adjacent plasma membranes. Separates Luminal space from intercellular space and CT Limit the movement of water & other molecules. • Maintain the integrity of apical & lateral surfaces.

At high resolution , • Zonula occludens appear not as a continuous seal but as a series of focal fusion b/w the cells. • These focal fusions are created by transmembrane proteins of adjoining cells that join in the intercellular space. • Best visualized by freeze fracture technique. • The ridges & grooves are arranged as a network of anastomosing particle strands thus creating a functional seal within the intercellular space.

Molecular organization of zonula occludens

Junctional adhesion molecule, associated with claudin ,involved in formation of occludens junction in endothelial cells

Claudin form backbone of each strand

Junctional adhesion molecule, associated with claudin ,involved in formation of occludens junction in endothelial cells

Freeze fracture preparation of zonula occludens

E-face reveals complementary grooves P-face reveals ridge like structures for junctional proteins

The zonula occludens separates the luminal space from the intercellular space & connective tissue compartment

• It is now evident that the zonula occludens play a essential role in selective passage of substances from one side of epithelium to the other. • Creating a diffusion barrier which is controlled by 2 pathways. – The transcellular pathway • Occur across the plasma membrane of the epithelial cell • Requires specialized energy-dependent membrane transport proteins and channels

– The paracellular pathway • Occur across the zonula occludens b/w 2 epithelial cells

Paracellular pathway

Transcellular pathway

• CLAUDINS not only form the backbone of the Zonula Occludens strand but also are responsible for the formation of extracellular aqueous channels that control the tightness of the seal b/w cells.

ANCHORING JUNCTIONS Provide lateral adhesions between epithelial cells, using proteins that link into the cytoskeleton of the adjacent cells. Band or belt like configuration around the cell 2 types of anchoring cell-to-cell junctions can be identified on the lateral surface. Zonula adherens, Which interacts with the network of actin filaments inside the cell.

Macula adherens/desmosomes Which interacts with intermediate filaments inside the cell.

ZONULA ADHERENS

ZONULA ADHERENS Homotypic binding, occur b/w CAMs of the same type

Heterotypic binding, If the binding occur b/w diff type of CAMs

The integrity of the epithelial surface depends in large part on the lateral adhesion of the cells with one another and their ability to resist separation.

Homotypic binding

The morphologic and functional integrity of zonula adherens is Ca dependent

CAMs

15-20nm

Fuzzy plaque

DESMOSOMES/MACULA ADHERENS

• Gr. Desmo-bond, Somabody (L. Macula- spot) • Strong attachment structure on the lateral sides • In epidermal cells only desmosomes present • other epithelia cuboidal/columnar all others present • Localized sites on cell surface(not continuous around cell) • Intercellular space is wider then others

MACULA ADHERENS/ DESMOSOMES

Molecular structure of macula adherens

Desmosomal attachment plaque(desmoplakins,plakoglobin )

30nm

Transmembrane glycoproteins Ca dependent

COMMUNICATING JUNCTIONS “Gap junctions” or “nexus”. Present in wide variety of tissues, including epithelia, smooth & cardiac muscle, & nerves. Consist of an accumulation of transmembrane channels or pores in a tightly packed array. Also called “low resistance junctions”.

COMMUNICATING JUNCTIONS • Along the lateral membranes • Aposition of adjacent cell membranes (2nm) • Individual unit – Connexons • Each gap junction is formed by tens/hundreds of Connexons • Each Connexon formed by six gap junction protiens – Connexins • Harmones,AMP,GMP can pass through (<1500 Da)

Structure of gap junctions:

Open & closed states of gap junctions

Gap junctions allow cells to exchange ions, regulatory molecules, & small metabolites through the pores

Electron micrograph showing the plasma membranes of the 2 adjoining cells forming a gap junction.

MORPHOLOGICAL SPECIALIZATIONS OF LATERAL SURFACE Lateral surface folds (plicae) create interdigitating cytoplasmic processes of adjoining cells. Increase lateral surface area of the cell. Particularly prominent in epithelia that are engaged in fluid & electrolyte transport, such as intestinal & gallbladder epithelium.

BASAL DOMAIN & ITS SPECIALIZATIONS Basement membrane: Located next to basal surface Cell-to-extracellular matrix junction: Anchor cell to extracellular matrix Plasma membrane infoldings: Increase surface area & facilitate morphological interactions between adjacent cells.

Basal lamina EM examination of the site of the basement membranes reveals a discrete layer of electron dense matrix material 40-60nm thick between the epithelium & adjacent connective tissue called the “basal lamina” or, sometimes “lamina densa”. Between the basal lamina & the cell is a relatively clear or electron-lucent area, the “lamina lucida” (also about 40nm wide).

“LAMINA DENSA” & “LAMINA LUCIDA”

BASEMENT MEMBRANE • Epithelium attached to underlying CT • Layer of variable thickness at the basal surfaces • Periodic acid Schiff technique is implied BASAL LAMINA (LAMINA DENSA) • Discrete layer of electron dense material between epithelium and adjacent CT BY ELECTRON MICROSCOPY • Between Basal lamina and cell is Lamina Lucida • Between Basal lamina and CT is Microfibrils (Type VII collagen) attached to the Reticular fibres of CT RETICULAR LAMINA • Reticular fibres (Type III Collagen) beneath Basal Lamina • Part of CT • Some investigators contend that its part of Basal Lamina

COMPONENTS OF BASAL LAMINA • • • • •

Consists of at least five components Collagen (type IV collagen) provide structural integrity to Basal lamina Proteoglycans much of the bulk volume of Basal lamina Laminin (glycoprotein molecule) bridges the Lamina Lucida to the overlying epithelial cells Enactin and Fibronectin role not clear Anchoring filaments (Type VII collagen) link Basal Lamina to underlying CT (Reticular Lamina

Several structures are responsible for attachment of the basal lamina to the underlying connective tissue.

FUNCTION • Not fully ellucidated • Structural attachment site for the overlying cells and underlying CT • Compartmentalization Separations/isolation of CT from epithelia ,Nerves ,muscle tissue • Filtration movement of substances to and from the tissues (ionic charges and integral spaces), kidney • Tissue scaffolding serves as a guide during regeneration. Newly formed cells help to maintain original tissue architecture

FUNCTION S OF BASAL LAMINA

Cell-to-extracellular matrix junction 2 major anchoring junctions are;

Focal adhesions: Anchor actin filaments of the cytoskeleton into the basement membrane.

Hemidesmosomes: Anchor intermediate filaments of the cytoskeleton into the basement membrane.

FOCAL ADHESIONS

HEMIDESMOSOMES

Plasma membrane infoldings

References: Text And Atlas Of Histology By MICHAEL H. ROSS 5th Edition BAILEY’S Textbook Of Microscopic Anatomy 18th Edition Textbook Of Histology By LEESON, LEESON, PAPARO Google search results for images anatomy.iupui.edu/.../cell.f04/cellf04.html