Epithelium 01

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Tissues Introduction

and Epithelium 1

MD1Histo log y: Fall 2012



Tissues An organized aggregation of cells that

function in a collective manner is called atissue.

Body tissues can be classified into four basic types according to their function andstructure:

1. Epithelial tissue covers body surfaces andlines hollow organs, body cavities, andducts. It also forms glands. It is composedof closely aggregated cells with very littleextracellular substance.

2. Connective tissue protects and supports thebody and its organs. Various types of connective tissue bind organs together,store energy reserves as fat, and helpprovide immunity to disease-causingorganisms. It is characterized by abundanceof extracellular material produced by its

cells.



Tissues

3. Muscle tissue generates the physicalforce needed to make body structuresmove. It is composed of elongatedcells that have the specialized functionof contraction.

4. Nervous tissue detects changes in avariety of conditions inside andoutside the body and responds bygenerating nerve impulses that helpmaintain homeostasis. It is composedof cells with elongated processes thatreceive, generate, and transmit nerveimpulses.



Tissues

All tissues of the body develop from three

primary germ layer

ectoderm

mesoderm

endoderm

(the first tissues that form in a human embryo)

All three primary germ layers contribute to

epithelial tissues. Mesoderm gives rise to all connective tissues and

most muscle tissues.

Ectoderm develops into nervous tissue.



Epithelial tissue Main characteristics:

Epithelia line and cover all body surfaces except thearticular cartilage, the enamel of the tooth.

Most epithelial cells renew continuously by mitosis

Epithelia lack a direct blood (avascular ) and lymph

supply. Nutrients are delivered by diffusion. Epithelia have no or very little extracellular

substance.

The cohesive nature of the epithelia is maintained by

cell adhesion molecules and junctional complexes. Epithelia are anchored to a basal lamina. The basal

lamina and connective tissue components cooperate

to form basement membrane.

Epithelia have structural and functional polarity.



Form and Shape

The form and dimensions of epithelial cells rangefrom columnar to cuboidal to low squamous cells.

The epithelial cell nuclei have distinctive shape,

varying from spherical to elongated to ellipticalcorresponding to the shape of the cell.

Nuclear form is used as a criteria to determine the

arrangement of the cells in layers, a primarycriterion for classification of epithelial tissues



Epithelial tissue

Functions:

protection, covering and lining of surfaces (e.g.,skin, intestines)

absorption (e.g., intestines)

secretion (e.g., glands)

gas exchange (e.g., lungs) sensation (e.g., gustative and olfactory

neuroepithelium)

contractility (e.g., myoepithelial cells).

Because epithelial cells line all external andinternal surfaces of the body, everything thatenters or leaves the body must cross anepithelial sheet.



The various surfaces of epithelial cells often differ instructure and have specializedfunctions.

The apical (free) surface. Apicalsurfaces may contain cilia or microvilli.

The lateral surfaces of an epithelial

cell face the adjacent cells on either side.

The basal surface of an epithelial cellis opposite the apical surface andadheres to extracellular materials.

The basement membrane is a thinextracellular layer that commonlyconsists of two layers, the basallamina and reticular lamina.

The basal lamina is closer to theepithelial cells and is secreted by

them

Epithelial polarity



1. The Basal domain and cell to extracellular

matrix adhesion: Basal lamina & Basement

membrane

•Epithelial cells are separated from the connective

tissue by a sheet of extracellular material called the

basal lamina.

•In EM, it appears as a dense layer, 20 –100 nm thick,

consisting of a delicate network of very fine fibrils

(lamina densa).

•Basal laminae may have an electron-lucent layer on

one or both sides of the lamina densa, called lamina

rara or lamina lucida.

•Between cell layers without intervening connective

tissue, such as in lung alveoli and in the renal

glomerulus, the basal lamina is thicker as a result of

fusion of the basal laminae of each epithelial cell

layer.

•The main components of basal laminae are type IV

collagen, the glycoproteins laminin and entactin,

and proteoglycans.



Basal laminaBasal laminae are attached to the underlying connective tissues by

anchoring fibrils formed by type VII collagen

These components are secreted by epithelial, muscle, adipose, and

Schwann cells.

In some instances, reticular fibers are closely associated with the basal

lamina, forming the reticular lamina. Connective tissue cells produce the

reticular fibers.



Basal lamina

Basal laminae have many functions. It

provide support to the cells

provide a barrier that limits or regulates theexchange of macromolecules between connective

tissue and cells of other tissues. is able to influence cell polarity, regulate cell

proliferation and differentiation by binding withgrowth factors, influence cell metabolism, and

serve as pathways for cell migration. contain the information necessary for certain cell-

to-cell interactions, such as the reinnervation of denervated muscle cells.



Basal Infoldings

Many cells that transport fluid have

infoldings at the basal surface. They significantly increase the surface

area of the basal cell domain allowing

for more transport proteins and

channels to be present.

These basal surface modifications areprominent in cells that participate in

active transport of molecules e.g. in

proximal and distal tubules of the

kidney and in certain ducts of the

salivary glands.

Mitochondria are typically concentrated

at this basal site to provide energy. The

mitochondria are usually oriented

vertically within the folds



2. The lateral domain and its specializations: Intercellular Junctions

Several membrane-associatedstructures contribute tocohesion and communication between cells.

The lateral membrane of epithelial cells has specializedintercellular junction that serveas site of adhesions and as aseal to prevent the passes of materials through theintercellular membrane. These

junctions are distributed from theapex to the base of the cells.



Intercellular junctions



Zonula occludens Tight junctions or Zonula occludens junctions: form the primary

intercellular difusion barrier between adjacent cells.

By limiting the movement of water and other molecules throughthe intercellular space, they maintain physiochemical separationof the tissue compartments.

In EM, plasma membranes of adjoining cells come in closecontact to seal off the intercellular space. The local fusion of transmembrane proteins of adjoining cell form the junction.

Three major groups of proteins found in zonula occludens areoccludin, claudins and junctional adhesion molecules (JAM). The

cytoplasmic portion of these transmembrane proteins containzonula occludin proteins ZO-1, ZO-2 and ZO-3.

A number of bacteria, viruses and parasite products attack zonulaoccludens junction or associated molecules to gain entry to thebody.



Zonula occludens



Tight Junctions or Zonula occludens



Zonula adherens It encircles the cell and provides for the adhesion of one cell

to its neighbor.

Numerous actin filaments are inserted into electron-denseplaques of material on the cytoplasmic surfaces of the

junctional membranes.

There is gap of 15 -20 nm between the adjoiningmembranes.

It is composed of the transmembrane cell adhesionmolecule E-cadherin. On the cytoplasmic side the tail of E-cadherin is bound to catenin complex.



Zonula adherens



Macula adherens or desmosomes The desmosome is a complex disk-shaped structure at the surface

of one cell that is matched with an identical structure at the surface

of the adjacent cell. The cell membranes in this region are very straight and are

frequently somewhat farther apart (>30 nm) than the usual 20 nm.

On the cytosolic side of the membrane of each cell and separatedfrom it by a short distance is a circular plaque of material called an

attachment plaque, made up of at least 12 different proteins. In epithelial cells, groups of intermediate cytokeratin filaments are

inserted into the attachment plaque or make hairpin turns andreturn to the cytoplasm.

Because intermediate filaments of the cytoskeleton are very

strong, desmosomes provide a firm adhesion among the cells. In nonepithelial cells, the intermediate filaments attached to

desmosomes are made not of cytokeratin but of other proteins,such as desmin or vimentin.

Proteins of the cadherin family participate in the adhesion provided

by desmosomes.



Desmosome



Hemidesmosomes

In the contact zone between certain epithelial cells and the basal

lamina, hemidesmosomes are observed. These structures take the form of half a desmosome and bind the

epithelial cell to the subjacent basal lamina.

However, in desmosomes the attachment plaques contain mainlycadherins, whereas in hemidesmosomes the plaques are made of integrins, a family of transmembrane proteins that is a receptor

site for the extracellular macromolecules laminin and type IVcollagen.



Gap junction or nexus

Gap or communicating junctions (also called nexus) can occur

almost anywhere along the lateral membranes of epithelial cells.

Gap junctions are found in nearly all mammalian tissues, except inskeletal muscle.

They are seen, in conventional transmission electron micrographs,as a close (2-nm) apposition of adjacent cell membranes.

The individual unit of the gap junction is called a connexon. Eachconnexon is formed by six gap junction proteins called connexins.which join together leaving a hydrophilic pore about 1.5 nm indiameter in the center.

Connexons of adjacent cells are aligned to form a hydrophilic

channel between the two cells.

Signaling molecules such as some hormones, cyclic AMP andGMP, and ions can move through gap junctions, causing the cellsin many tissues to act in a coordinated manner.



Gap Junction

3 A i l d i S i li ti f th



3. Apical domain: Specializations of the

apical cell surface

In many epithelial cells

the apical surface exhibits special structuralsurface modifications to carry out specificfunctions.

In addition, the apical surface may containspecific enzymes, ion channels and carrier proteins.

The structural surface modification include:

Microvilli: cytoplasmic processes that extendfrom the surface.

Stereocilia: microvilli of large size

Cilia: motile cytoplasmic processes



Microvilli

•Microvilli are fingerlike extensions

measuring about 1 µm high and 0.08µm wide.

•They are found mainly on the free

cell surface.

•Hundreds of microvilli are found inabsorptive cells, such as the lining

epithelium of the small intestine and

the cells of the proximal renal tubule

•The complex of microvilli and

glycocalyx may be seen with the light

microscope and is called the brush

or striated border .

i i i



Microvilli

•Microvilli contain a core of actin

filaments, which are anchored to villin

at the tip of microvillous and extend

down to apical cytoplasm where they

interact with a horizontal network of

actin filaments, the terminal web.

•

The actin filament are cross linked at10 nm interval by actin bundling

proteins, fascin and fimbrin. This

provides support and rigidity to the

microvilli.

• Myosin I is also associated with theactin filaments of the core.

•Terminal web formed by horizontal

actin filaments at the base provide

support to the actin filaments of the

core.



Stereocilia

Stereocilia are long,nonmotile extensions of cellsof the epididymis and ductusdeferens that are actuallylong and branched microvilli

and should not be confusedwith true cilia. They are alsofound in sensory (hair) cellsin inner ear.

Stereocilia increase the cellsurface area, facilitating themovement of molecules intoand out of the cell.

Stereocilia



Stereocilia Stereocilia also have core of actin filaments that are

cross linked by fimbrin. In stereocilia, ezrin, a plasmamembrane associated molecule anchors actin to plasmamembrane. α-actinin cross bridges between actinfilaments are also present.

There is no villin present in the stereocilia.



Cilia

Cilia are motile structure capable of moving fluid andparticles along epithelial surfaces.

Cilia are cylindrical, 5 –10 µm long and 0.2 µm in diameter .

They are surrounded by the cell membrane and contain acentral pair of isolated microtubules surrounded by ninepairs of microtubules.

The two microtubules of the peripheral pairs are joined toeach other.

Cilia are inserted into basal bodies, which are smallcylindrical structures at the apical pole just below the cellmembrane.

Basal bodies have a structure similar to that of thecentrioles



Cilia

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