Epithelium Surface Specializations

8/6/2019 Epithelium Surface Specializations

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1.POLARITY- structural and functional differentiationbetween the distal end (toward free surface) and

proximal end (toward CT).

This polarity is evident also in the arrangement of

organelles in the cell interior- the centrosome and

Golgi apparatus being in a supranuclear position. The cell axis, an imaginary line passing through the

centrosome and the center of the nucleus is usually

vertical or perpendicular to the basement lamina.

Oriented parallel to the cell axis and found in greater

numbers in the apical cytoplasm are the long

mitochondria of columnar epithelia.

In stratified squamous epithelia, there is less

evidence of cell polarity.

SPECIALIZATIONS OF EPITHELIAL TISSUE


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Juxta-luminal junctional

complex are sites of low

resistance to ion flow,

providing cell communicationand for coordination of

activities.

The 3 most common kinds of

cell junctions are adhesive

junctions, tight junctions and

gap junctions. Adhesive junctions

(desmosomes,

hemidesmosomes, adherens

junctions) link adjoining cells to

each other and to the ECM.

Although adhesive junctiontypes are similar in structure

and function, they contain

distinct intracellular

attachment proteins and

transmembrane linker proteins.

CELL JUNCTIONS


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The intracellular attachment proteinsform a thick layer of

fibrous material on the cytoplasmic side of the plasma

membrane called a plaque which binds actin microfilaments in

adherens junctions and intermediate filaments in desmosomesand hemidesmosomes.

The transmembrane linker protein is anchored to the plaque by

the cytoplasmic domain and binds the ECM or to the same

proteins on other cells.


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Distribution

of celljunctions in

3 domainsof epithelial

cells.


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ZONULA

OCCLUDENSextends

around the

entire

perimeter ofthe cell, but

typically

located near

the apex.

Also known as

terminal bars, tight or

occluding junctions


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Tight junctions consist of fused

ridges of tightly packed

transmembrane junctional

proteins.

Can be rapidly formed anddisassembled (e.g. during WBCmigration across endothelium).

Epithelia are classified either

as tight or leaky based on

the permeability of the zonula

occludens.


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Tight junctions block

lateral movement of

lipids and membrane

proteins to keep a cellpolarized. They leave

no space between

plasma membranes of

adjacent cells to

prevent the movementof molecules across

cell layers.

Sodium/glucose

symport proteins and

export by glucose

transport proteins onthe basolateral surface

and tight junctions

prevent the lateral

movement of these

transport proteins.


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ZONULA ADHERENS (intermediate junction,

belt desmosomes) is basal to the zonula

ocludens. The adjacent plasma membranes

are separated by a gap of 15-20 nm, filled withan electron dense plaque containing a

glycoprotein localized only in the membrane,

(adherens junction-specific cell adhesion

molecule or A-CAM or E-cadherin).


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Myosin,

tropomyosin,

-actinin, andvinculin, actin-

containing

microfilaments

insert into the

plaque tostabilize the

junction

between

epithelial cells,

fibroblasts,smooth muscle

cells and at

intercalated

discs.


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MACULA ADHERENS or

DESMOSOMES are bipartite

structures of apposing cell

membranes. An attachment

plaque on the cytoplasmic

side anchors tonofilaments

which are intermediate

filaments.

Desmosomes form strong

points of adhesion between

cells in a tissue such that

two adjoining cells are

separated by a thin spaceof 25-35 nm, the

desmosome core, in which

cadherin molecules

mediate cell-cell adhesion.


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The plaques on the inner surfaces of cells joined

by desmosomes have a mixture of intracellular

attachment proteins (desmoplakins andplakoglobin) which interact with the tonofilament

intermediate filaments.


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Adherens junctions called FOCAL ADHESION can join

a cell to the ECM, primarily through fibronectin

receptors.


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HEMIDESMOSOMES connect a cell, through a plaque, to

the basal lamina (ECM) by integrins. As in desmosomes,

hemidesmosomes interact with tonofilament intermediate

filaments. Adherens junctions resemble desmosomes

except two adjoining cells are

separated by a thin space of

20-25 nm and connect to actin

microfilaments

in the cytoplasm.

Some of the

transmembrane

glycoproteins are

cadherins.


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Hemidesmosomes

occur at most basal

surface of stratifiedsquamous epithelia

where the

superficial layer lack

junctional

complexes, and the

basal cells are

exposed to the

underlying CT.

They serve mainly as sites of

attachment for the actin-based

contractile system of the

cytoplasm.


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GAP JUNCTIONS (NEXUS)

separate cells by 2-3 nm and

allow direct electrical andchemical communication.


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The nexus is a site where there is no actual fusion of

membranes, and the gap is bridged by a connexon. These are

tightly packed 7 nm wide hollow cylinders in two adjacent cellmembranes that form a 3 nm thin hydrophilic channel that

allows the passage of small molecules and ions.


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The connexons of each membrane link to form

continuous pores that bridge the intercellular gap,

allowing passage of ions, cyclic AMP, amino acids andother small molecules.

As sites of electronic coupling (reduced resistance to

ion flow), it is the only type of junction mediating flow

of current between cells important in intercellular

communication and coordination.

An influx of calcium ions results in the closure of their

channels, preventing spread of damage to other cells.

Also found between osteocytes, astrocytes, cardiac

muscle cells, smooth muscle cells, & endocrine cells. Cancer cells generally do not have gap junctions, so

that cells fail to communicate their mitotic activity to

each other, which may explain their uncontrolled

growth.


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CELL SURFACE MOLECULESSurface

glycoproteinsthat bind to

other cells, or

to components

of theextracellular

matrix; play a

role in mutual

recognition ofsimilar cell

types


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1. Cadherins- a family of single-pass transmembrane

glycoproteins which stick embryonic cells together in the

presence of calcium (E-cadherin are found in epithelial tissues,N-cadherin occurs in neural tissue).

Cadherin tails are anchored to actin bundles in the

cytoskeleton by a complex called catenins (F-catenin, a

component of the Wnt signaling pathway provides a potential

link between cell signaling & cell association).


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2.Cell adhesion molecules

(CAMs)- single-pass

transmembrane glycoproteinswhich do not require calcium

to bind to other cells.

N-CAMs are a large family of

proteins formed by alternative

splicing Expression of low sialic acid

N-CAM molecules on adjacent

cell surfaces promote junction

formation on the adjacent cell

membranes.

When the polysialic acid

residues are removed, the two

cells can adhere.


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Three glycoproteins that mediate Ca+2-dependent

cell adhesion (desmoglein I, desmocollin I & II) and 4

nonglycosolated proteins located in the attachmentplaque (desmoplakin I & II, pakoglobin & a basic

polypeptide) have been identified as desmosome

components.

Abundant in stratified squamous epithelium, which

are sites of attachment of the cytoskeleton to the

free surface

Although sites of cell to cell adhesion, they do not

hamper the flow of substances between cells.

Bullous pemphigold is an autoimmune disease inwhich antibodies against desmosomal proteins are

formed. This results in widespread skin & mucous

membrane blistering as desmosomal proteins fall

apart.


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3.Integrins (ICAMS)- cell

surface glycoproteins

that require (Ca+2 orMg+2), to interact with

ECM components

(fibronectin, laminin and

collagens). Important in epithelial

cell cohesion and

attachment to substrate

& cell migration during

tissue repair.

Bound integrins prevent

transcription of genes

that specify apoptosis.


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Binding of integrins to ECM also activatessignal transduction pathways.


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1.MICROVILLI

Closely packed, finger-

like projections of

cytoplasm that increase

surface area of the cell.

Number and shape on

cell surface correlate

with absorptivecapacity.

Can be seen under LM

(brush border or

striated border).

Usually present onsurface of microvilli is

an amorphous cell coat

of glycoprotein

glycocalyx.


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Contain a core of actin

filaments, which are

anchored to villin at tip.

Actin extends downwardinto apical cytoplasm

where it attaches to the

terminal web which is a

horizontal network of

actin filaments lying justbelow base of microvillus.

These actin filaments are

stabilized by spectrin.

Spectrin anchors terminal

web to apical membraneof cell.

Also contains myosin II

and tropomyosin

filaments, which allows

microvillus to contract.


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2.STRIATED/BRUSH

BORDER

Delicate vertical striationsseen in a refractile border

of columnar epithelia.

Plasma membrane-

covered microvilli

extensions 80-90 nm indiameter and 1-2 Qm long.

Non-contractile, and have

a purely supportive

function.

Cytoplasm shows parallelbundles of actin micro-

filaments anchored in a

dense mat of filaments in

the terminal web.


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Prominent in cells whose

principal function is to

enhance the surface area

of membranes exposedto substances for

absorption and digestion

Biochemical analysis

show that they containdigestive enzymes in

their glycocalyx.


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3.STEREOCILIA- long

pyriform tuft of slender

processes projecting intothe lumen.

Seen in the epithelium

lining the epididymis and

the hair cells of the inner

ear. Individual processes are

not true cilia but are non-

motile, very long microvilli.

Amplify the cell surface in

absorptive epithelium. In the inner ear they

generate an electrical

signal that is conveyed to

the CNS.

In the testes, theyconcentrate the seminal

plasma during its

passage through the

epididymal duct


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4.CILIA- numerous motile

processes larger than microvilli

Arranged in parallel rows

projecting from the free surfaces

of some epithelial cells.

Cilia moving in waves and

sweeping over the epithelial

surface serve to propel fluid or

a coating of mucus towards the

exterior.

At the base of each cilium is a

dense elongate granule called

the basal body.

They arise de novo, developing around small bodies

called deuterosomes or procentriole organizers.

These organizer lengthen by polymerization of tubulin to

form the 9 ciliary triplet microtubules; serve as templates

during ciliogenesis.


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Ciliary motion depends on

ATP-dependent dynein-

generated sliding of

doublet microtubules. Kartageners syndrome, a

rare disorder in which

dynein arms are lacking,

result in situs inversus

(organ reversal due tofailure of cells to migrate

properly during

embryogenesis),

recurrent

sinus/pulmonaryinfections (inability to

move mucous), and

sterility in males (retarded

sperm movement)


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1.BASEMENT MEMBRANE

(membrane propia, basal lamina)-

present in all epithelia except:

follicular epithelium of the thyroid

gland, the olfactory neuro-

epithelium and the transitional

lining epithelium of the excretory

passages of the kidney.

2.Functions:

Attachment of the basal cells of the epithelium,

providing stronger anchorage of the tissue

Form a barrier between epithelium & connective tissue

Under normal conditions, lymphocytes may pass the

basal lamina during immune surveillance

In cancer, neoplastic cells may pass the basal lamina

as a scaffolding during malignant invasion

A passive molecular sieve or ultrafilter.

A scaffolding in regeneration or wound healing


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It is a thin, homogeneous or fibrous layer composed

of type IV collagen (exclusive only to the basal

lamina), laminin (adherent to specific membranereceptors, and to collagen of the lamina densa) and

proteoglycans (heparan sulfate, fibronectin).

Under EM, 2 zones are distinguishable: the lamina

lucida adjacent to basal cell membranes, and the

lamina densa, an outer zone exposed to theunderlying CT.


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The lamina increases in density at its outer zone,

where small units of collagen and reticular fibers are

found.

It is believed to be a product of the epithelium, while

the outer dense layer of reticular fibers is a product of

CT fibroblasts.

The polysaccharide content of the basal lamina can

be well shown by the PAS technique; due to thereticular fibers, it stains + in silver dyes


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2.TUNICA or LAMINA PROPRIA

All epithelia possess a

tunica propria.

Maybe areolar or fibro-

reticular CT located

immediately beneath the

basement membrane.

Serves as a support to the epithelium, and as avascularized CT bed containing the blood vessels,

lymph vessels and nerves.

The blood vessels do not penetrate the basement

membrane, while nerve fibers do.

CT papillae are outgrowths of the tunica propiatowards the deep surface of thick epithelium such as

stratified squamous epithelium of the skin.

This facilitates nutrition by increasing surface area

thru which diffusion can take place.


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Epithelium Surface Specializations