BMED 2801 - Lecture 8: Study of one tissue in some detail – Epithelial Tissue
MODIFICATION OF THE APICAL PLASMA MEMBRANE
MICROVILLI
-
Are cytoplasmic finger-like protrusions from the apical surface of many epithelial cells that vary in
number and density.
They increase surface area for important transport and enzymatic activity needed at this surface.
Microvilli are filled with actin filaments that bind to a web of thin filaments below the microvilli. This
web is called the "terminal web"
FUNCTION
• Transport of fluids (secretion) & ABSORPTION e.g. in small and large intestine
• 1000 fold increase in functional surface area
APPEARANCE
Microvilli covered with
the plasma membrane
of the cell
High electron density along the
core from tip to terminal web –
are due to actin filaments.
Terminal web
• The appearance of microvilli relates to function.
- They can be small, irregular in height (brush border) OR taller, closely packed, even projections
•Can range from 0.5 - 1μm long
• The number & shape of microvilli correlates with their absorptive capacity.
MICROVILLI ARE MOTILE
• Sway slowly to increase absorptive power – by moving fluid along surface, allow for mixing/stirring
allows for more absorption – increase functional ability of plasma membrane.
STRUCTURE
- Microvilli are covered with the plasma membrane – dark line along the edge
- The core of actin filaments of cytoskeleton goes straight up through the microvilli - from tip to
Terminal Web
- Actin filaments – create the microvilli structure by pushing the plasma membrane forward. These
embed into more actin filaments around the top of the cell – called the terminal web (web of actin
filaments).
- Amorphous capping proteins (villin) are at the tip of the microvillus to prevent depolymerisation.
 gives - rigidity + motility
STEREOCILIA
Very few, long, immotile (due to length- high energy demand), same structure as microvilli.
10-25μm long
Location
Epididymis + vas deferens  absorption of sperm
Sensory cells of ear
Function
Facilitate absorption
Sensory receptors
CILIA
- MOTILE (sweeping action) cytoplasmic structures
- Invagination of plasma membrane
Image below: Respiratory tract - Pseudostratified columnar ciliated epithelium (all cells rest on basement
membrane but not all reach the surface)
Basement
membrane
LIGHT MICROSCOPE APPEARANCE
• Short, fine, hair like
• ½ length of cell
• All the same length/cell
• thin, dark staining band at base = Basal Bodies
• each Cilia associated with a Basal Body
FUNCTION
Respiratory tract  organised movement sweeping action towards mouth- sweep mucus + particulate
matter from lungs
Oviducts  transport Ova towards uterus for implantation (ectopic pregnancy – cilia not functioning
properly = implantation of Ova in the wall of the fallopian tube), move Ova from fimbri towards sperm.
CROSS-SECTION OF CILIA
ARRANGEMENT OF MICROTUBULES IN CILIA
• Central core (Axoneme) consisting of 9 doublets of microtubules + 2 central microtubules
• axoneme inserts into Basal Body
Microtubule doublets
= tubulin  is attached to a motor protein - dynein arms (ATPase enzyme activity)
Basal Body – slight change in microtubule structure
= modified centriole
• 9 microtubule triplets
• form a short cylinder
MOVEMENT OF CILIA
Regular, synchronous & undulating
Creates a WAVE motion
SWEEPS across the apical epithelial surface
MODIFICATION OF THE LATERAL PLASMA MEBRANE:
Terminal Bar (Junctional Complex)
IMPORTANT: Don’t confuse terminal bar (junctional complex) with terminal web (network of actin and
intermediate filaments that microvilli insert into)
Zonula Occludens (apical end)
Terminal bar = Junctional Complex = Zonula Adherens
Macula Adherens
There are 3 different types of junctions:
(1) zonnula occludens or tight junctions  acts as a diffusion barrier
- A region of occlusion (blocking)
There are tight bonds holding two cells together
(2) zonnula adherens (zones of adherence) & macula adherens (spots of adherence - also known as
desmisomes)  acts as an anchor via the cytoskeleton
(3) Gap junctions or Nexus  direct communication between adjacent cells (e.g. ionic
communication)
Other Structures:
Plicae
- Down the length of the plasma membrane  Are morphological changes to lateral cell surface
- Folds & processes create interdigitating cytoplasmic processes – help to adhere cells together.
- Function: increase cell surface area
Apical surface
Epithelial cell 1
Epithelial cell 2
Lateral plasma
membrane
-
(1)
Junctional complexes are seen as regions of increased density around the plasma
membrane of the two cells.
ZONULA OCCLUDENS (TIGHT JUNCTIONS)
Plasma membrane
– appear as railway
tracks
Occluding proteins
embedded on the
plasma membrane
= ring of plasma membrane union between neighboring cells
- Multiple sites of membrane fusion  involves a fusion of the two adjacent membranes with
fibrous connections
- The intercellular space between the two plasma membranes are not seen because they are so
close together – but these are not stuck together.
Location
Lateral border, close to apical membrane
Description
• Multiple sites of membrane fusion
• Intercellular space obliterated
• Occluden (trans membrane protein) – integral part of the plasma membranes of the two cells– intertwine
and connect to each other – pulling the two cells together  associated within the cell with the actin
filaments of the cytoskeleton – which help with drawing the two cells together.
Function
• Barrier junction
• Completely seal the intercellular space off from lumen
E.g. epithelium in bladder – very acidic urine sitting in the bladder – if these seep into cells that line
epithelium = cause toxic damage to tissue and integrity of the organ itself -- >need tight junctions to hold
them together.
(2)
ZONULA ADHERENS
High electron density (fuzzy
plaque) – due to presence of
proteins + actin
microfilaments of the
cytoskeleton
= ring around the cell
Location
It is located below the tight junctions/occludens
Description
• The structure reveals a 20-30nm intercellular space  clearly visible extracellular space between the
two cells.
• The space contains E-cadherin (membrane Protein) + Ca2+ (form a bridge the two cells)
• The cytoplasm contains 6nm actin microfilaments (Fuzzy Plaque)
• It is continuous with Terminal Web
Function
- associate with the actin cytoskeleton
- Involved in adhesion (sticking cells together or sticking cells to surfaces).
- Transduce signals into and out of the cell, influencing a variety of cellular behaviours including
proliferation, migration, and differentiation.
(3) MACULA ADHERENS (DESMOSOME) – spot junctions
Desmisomes – two cells attached together with the same structure that bring them together
V. electron dense 
attachment plaque (protein
structure on cytoplasmic side
of plasma membrane)
Intermediate Filaments of
cytoskeleton
V. electron denserepresents interlocking of
cadherin proteins.
- Paler region – cadherin
just in intercellular space.
= localized spot attachments around cell periphery
VERY STRONG attachment
Cadherin protein- located
within the plasma membrane
of the cell
- Cadherin projects through
the gap.
Location
Lateral plasma membrane
Description
Cytoplasmic side has an ATTACHMENT PLAQUE  plaques on plasma membrane contain
desmoplakins which are intermediate filament binding proteins. Intermediate filaments loop into the
plaques connecting the desmosome with the cytoskeletal system of both plaques.
• 10nm intermediate filaments (Tonofilaments)
• anchor proteins
• 400nm x 250nm x 10nm disc
30nm wide intercellular space
• contains Cadherin protein molecules that connect one another via homophilic binding + Ca2+
(Binding requires calcium)  arranged like a ZIPPER
(4)
HEMIDESMOSOMES
= ½ desmosome
Location
• Basal plasma membrane  Hemidesmisomes– a connection between one cell and connective tissue 
function: Link cell to basement membrane at basal surface
• stratified squamous epithelium
Function
Connect epithelium TO basement membrane
(5)
FASCIA ADHERENS
• located in Cardiac Muscle
• cells attach at a BROAD FACE & not in a ring
• Otherwise same as Z. Occludens + M. Adherens
(6)
GAP JUNCTIONS
Plasma membrane
Gap junction –
between the two
plasma membranes
= broad areas of closely opposed plasma membranes (but do not fuse)
Connexins - integral protein
embedded on the plasma
membrane embed through the
plasma membrane of the
opposing cell  creating a
hollow channel.
Function
•Some degree of intercellular adhesion
• Intercellular exchange & communication 
• The physical travelling of ions, metabolites & regulatory molecules (epithelium) between cells.
• The spread of excitation (cardiac muscle)  Allow for the coordinated rhythmic contractions of
ventricles so it acts as a singular pump  irregular contraction – occurs when SA and AV nodes
and action potentials are not created properly due to dysfunction of gap junctions e.g. toxins
blocking the gap junction = this causes the heart to fibrillate – uncoordinated contractions as cells
can not communicate with each other.
• Gap junctions are dynamic structures – they change
Visualized with FREEZE-FRACTURE and TEM
Description

2 parallel plasma membranes separated by 2nm GAP

6 Connexin proteins arranged in circle  form a hollow channel going form one cell to another

1 Channel per membrane

Channels are aligned
Action
Changes in Connexin confirmation due to humeral or neural response received by the cell  results in a
Rapid change of arrangement  either increasing/decreasing the lumen of the channel (open/closing
controlled barrier)
Channels can OPEN 2nm wide & CLOSE
Lateral interdigitations:
- all junctional complexes occur
Basal-Lateral Interdigitations.
-
-
Epithelia that are important in regulated transport also may have specializations along their basal
surface.
These look as if the cell has multiple "legs" like an octopus.
The "legs" of different cells interdigitate with one another and form a space into which ions and
water are transported.
Even though these are at the basal surface, they are called "lateral interdigitations".
Also, any other substances that are moved across the cell to the blood supply are secreted into
this region.  The interdigitations of the membranes increase surface area available for the ion
and water transport.
The membranes have important ion pumps, like sodium/potassium ATPases.