Epithelial Cell Surface Modifications

Surface modifications of epithelial cells reflect functions of cells at that surface

Apical surface: microvilli, cilia, stereocilia

Basal surface: Basement membrane,

Hemi-desmosomes,

Plasma membrane interdigitations

Lateral surface: A-Junctional complex

i. Zonula occludens

ii. Zonula Adherence

iii. Macula adherence

iv. Fascia Adherence

B- Communicating Junction

C- Plasma membrane interdigitations

Cell-1 Cell-2 Cell-3z z z c z v

Apical surfacesL

ate

ral

surf

ace

Basal surfaces

Cilia

Microvilli

MICROVILLI

Extension of cytoplasm -

1mm long

Actin-containing

microfilaments

FUNCTION - increase

the surface area

LIGHT MICROSCOPE -

- Brush border &

Striated border

Microvilli

TW

*

**

*

*

*

Small intestine, plastic, toluidine blue

Apical surface with striated border made of microvilli

Basal

surface

Lumen

Simple columnar epithelium with striated border

Microvilli are anchored to an electron-dense structure, the terminal web.

microvilli

(duodenum) glycocalyx lumen

Attachment site

of the glycocalyx

on the plasma

membrane

Plasma

membrane

The glycocalyx is exceptionally well developed

on the surface of the intestinal epithelium.

The glycocalyx (cell surface coat)

Specializations of the cell surface • Microvilli

Scanning

EM.

Sea urchin

embryo

Microvilli are fine finger-like projection of the plasma membrane,

with limited capability of motion. Ø : 50 - 100 nm, length ~ 1 - 2 µm

Variation in morphology of microvilli

Uterine glands Placenta Intestine

MICROVILLI

SEM OR TEM ??????

The glycocalyx is a carbohydrate-rich zone on the

cell surface, made up of proteins and

oligosaccharides bound to the outer surface of the

plasma membrane.

The composition and arrangement of the glycocalyx is cell-

specific.

Microvillus and terminal web

Freeze-fracture preparation

Actin filaments(microfilaments)

TW*

TW*=Terminal web

Actin filaments(microfilaments)

Microvillus

• Stereocilia

Sensory

epithelium

✓are long microvilli

( 3 - 25 µm),

✓Stereocilia are capable of

moving passively.

✓e.g. epididymis and

the sensory cells of the

internal ear.

Stereocilia

Epididymis

• Pseudopodia

macrophage

B- Lymphocytes

Pseudopodia are foot-like, long cell projections.

Their inner structure resembles that of the microvilli,

but they are less strictly ordered.

• Pseudopodia are formed and withdrawn in several

minutes.

• Pseudopodia are capable of active, independent

movement. Their biological role is to ensure the

active movement of the cells.

Several cell-types use pseudopodia for locomotion:

For exampleplasma cells

granulocytes

macrophages (phagocytic cells!).

lymphocytes

Cilia • Long cytoplasmic extensions that have a core

of organized microtubules within them

• Length = 5-10µm ,Width= 0.2µm

• Have complex arrangement of microtubules (9+2) called “axoneme”

• Extend from basal body within cytoplasm

• Cilia are of two types, motile and nonmotile (primary) cilia

• Motile (beat synchronously) and thus facilitate flow of fluid over an epithelium (tubular organ like trachea, oviduct)

• A single non motile cilium (primary cilium ) is present on nearly every human cell that is in the G0 stage of the cell cycle.

• Similar in structure to flagella (sperm)

SEM LM

Cilia

Trachea, H & E

Cilia ( Kinocilia )

kinocilia

and

microvilli

trachea

Cilia are parallelly oriented, motile, finger-like protrusions of cell

membrane (Ø 300 nm, length 7 - 10 µm), connected to an

electron dense basal body.

basal body

Cilia and flagella are built up by highly ordered

microtubular system

Axoneme

Relationship of cilia and basal bodies

Plasmamembrane

Cytoplasm

Basal body

Cilium

TEM

Basal body

Basal body

Cilia

Tubulin subunit

ATPase

Ross, Fig. 5.5

Immotile Cilia Syndrome (Kartagener’s syndrome)

•Autosomal recessive diseases that are more prevalent in men•Several different syndromes that are due to varying modifications in ciliary structure

•Patients frequently have chronic respiratory problems resulting in abnormal mucociliary transport in the tracheobronchial tree

•Men are usually sterile

•Diagnostic EMs reveal abnormal cilia (absent/defective dynein

arms)

Dynein

Lateral surface specializations

Zonula Occludens

Zonula Adherenens

Macula Adherenens

Communicating Junction

Hemidesmosome

Lateral surface specializationsIntercellular junctions

Infoldings of plasma membranes

Terminal Bar (LM)

Junctionalcomplex

Junctional complex (TEM)

Epithelial cells are held together by intercellular junctions

Epithelia consist of closely apposed cells that are tightly linked both mechanically and functionally.

Several classes of morphologically distinct intercellular junctions contribute to these functions:

1. Occluding junctions

form seals and are impermeable

2. Adhering junctions

glue cells together and are associated with specific cytoskeletal elements

3. Communicating junctions

permit x-talk ( metabolic & electrical Communications

Junctional complex

Terminal bar is LM term which refers to a series of

intercellular junctions at the lateral surfaces of cells. In TEM this

region is called a Junctional complex, which is made of a

few different types of intercellular junctions

Zonula: zone or belt-like

Macula: spot

Zonula occludens

Zonula adherens

Macula adherens

Junctional complex

Lumen

ZA

ZO

Cell A

MA

Cell B

**

TIGHT JUNCTION -

ZONULA OCCLUDENS

Zonula occludens (Tight junction)

Inhibition of paracellular

diffusion

Cell A

Cell Blumen

Cell B

Z. occludens

Cell A

•Integral membrane proteins of two cells link across the extracellular space and occlude the space between the two cells. These proteins form “sealing strands”.

•Occludin is one of the integral

membrane proteins involved.

Tight Junctions

(“sealing strands”)

Interacting plasma membranesof 2 cells

Cell A

Cell B

Lateral cell membrane junction:

interdigitation

Cell 1

Cell 2

Cell 1

Cell 2

Basal striation and basal lamina

Infoldings of basal portion of kidney tubule cell showing infoldings of the plasma membrane and alignment of mitochondria

Drawing of kidney

tubule cell

Adhering (anchoring) junctions

• Actin filament attachment sites

-cell-cell adhering junctions (Zonula adherens)

• Intermediate filament attachment sites

-cell-cell (Macula adherens = Desmosomes)

-cell-matrix (Hemidesmosomes)

• Associated with a “glue” that helps hold cells together

ZO

ZA

MA

Hemidesmosome

Fine structure

Intercellular anchoring proteins:

Cadherins (Ca 2+ -dependent cell

adhesion proteins)

Actin

cytoskeleton

Ca2+

Cadherin

Cell 1

Cell 2

Zonula adherens

• Belt-like junction which

circumscribes the cell, usually near

the apical surface.

• Mechanically links cells to one

another (using *Cadherins as the

“glue”).

• Cadherins are integral membrane

glycoproteins that form homodimers

and require Ca++.

• Anchors the thin (actin) filaments of

the cell to the plasma membrane

via attachment proteins within the

cell.

ZA

Actin filaments

ZO

Cell A

Cell B

*

*

EM

Plaques

Cadherin

Intermediary

filaments

Ca2+

Fine structure

1. cell

2. cell

Desmosomes: “spot welds” are abundant in epithelia that are exposed to much abrasion

Examples: stratified squamous epithelium of the gingiva, tongue,

skin, oral mucosa

Cytoplasmic

plaque has

attachment

proteins

Cadherins

Keratin

(intermediate)

filamentsPlasma membranes

Intercellular

space

Intercellularspace

Desmosome (Macula adherens)

Basal surface specializations

• Basement membrane

• Junctional specialization:

hemidesmosome (integrins)

• Plasma membrane interdigitations

HEMIDESMOSOME

LOCATION - contact zone

between epithelium and basal

lamina

DISK-SHAPED

MORPHOLOGY - half a

desmosome

FUNCTION - binds the

epithelial cell to basal lamina

Found at base of stratified epithelia and connect the plasma membrane to the extracellular matrix via integrins

Glue is NOT cadherins but integrins

Plasma membrane Hemidesmosome

Basal lamina

Hemidesmosomes

Basallamina

Plasmamembrane

Hemidesmosomes and basal lamina

Basal lamina

HD HD

GAP JUNCTION -

NEXUS

Fine structure

Plasma membrane proteins (connexins) form a

channel (connexon).

Connexons of two adjacent cell membranes join

together in the intercellular space to constitute a

continuous hidrophilic channel, the nexus.

Small molecules ( < 1000 Da):

ions, second messengers,

metabolites etc. can freely pass.

The two cells are metabolically

and electrically coupled ( in

excitable tissues: electrical

synapse).

Enhanced intracellular Ca2+

concentration (cell damage!)

results in closing of the nexus.