SPECIALIZATIONS OF THE CELL SURFACE
MICROVILLUS, CILIA, FLAGELLA
Prof Dr Müjgan Cengiz
İÜ.Cerrahpaşa Tıp Fakültesi
Tıbbi Biyoloji AD



The surface of the most cells have
extensions
They are used in cell movement,
phagocytosis, absorbtion.
Most of these extensions are based on
actin filaments.
I- SPECIALIZATIONS OF FREE SURFACE
(APICAL SURFACE )
1-Microvilli
2-Cilia, flagella
3-Stereocilia
1-Microvilli
Finger-like extensions derived from the cell
surface
Function: increase the surface area for absorption,
cell movement and phagocytosis
Localisations( cells specialized for absorbtion )
1-İntestinal epithelium (Striated border )
2-Proximal tubule of the kidney (Brush border)
3- Gall bladder epithelium
Microvilli
Finger-like prolongations 1  in length 0,1 diameter
(can be seen with L.M.)
It can seen with E.M.
 Contains a bundle of straight parallel filaments
(20-30 Actin filaments )
 Actin filaments extend 0,5 down into the apical
cytoplasm
 Enclosed in an extension of the plasma membrane
Glycocalix is ticker around the microvilli
Microvilli
(Actin based cell surface protrusions)
A bundle of actin filaments are cross linked into
closely packed parallel arrays
 “Actin binding proteins” cross link actin
filaments ;
Fimbrin,
Villin

Striated borders (microvilli )
contain enzymes
i.e sucrase
maltase
lactase
lipase
aminopeptidase
They are involved in the terminal digestion of proteins,
carbohydrates and lipids
2-Cilia (kinocilia ) and Flagella
Cilias
have eyelash or hair like structures
-Motile
-Larger
than microvilli (5-10 long , 0.2 in
diameter)
-Have a complex internal structure (E.M )
250
or more cilia (in each cell)
arranged in parallel rows
Localisations:
1-Epithelial cells of the upper respiratory tract
2- Epithelial cells of the uterine tubes (oviducts )
3- Epithelial cells of the Ductus efferentes
Ciliary movement
is constant in direction
In living cells
Cilia beat in a rhythmical wave –like manner
(phase-contrast microscope)
Functions:
1-to move mucus and particles over epithelial surfaces
( in respiratory epithelium )
2-to transport the ovum toward the uterus
(in uterine tubes)
3- to drive the spermatozoa toward the epididymis
(in ductus efferentes )
Cilia


With L.M
Hair –like protrusions
No internal structure is detectable
 At the base of each cilium a dense granule
(Basal body) is seen

With E.M
Have a complex internal structure
A characteristic arrangement of microtubules called
“ Axoneme’’ =9+2 microtubule complex
Fawcett 1954
Cilia
Each cilium is covered by an extention of
plasmalemma
1-Tapering tip
2-Cylindrical shaft
3-Basal body (located in the apical
cytoplasm)
Dynein arms
are arranged along the length of the microtubule .
 They are formed by a protein called “ dynein’’ and
contain ATPase (ATP splitting enzyme)activity.
“Nexin links’’
attach each microtubule A to the microtubule B of
the adjacent doublet.
 Nexin links are composed of an elastic material
called “nexin’’
 Responsible for recovery stroke
Basal Bodies





Cylindrical structures about 0.2  in
diameter
and 0.4  in length
nine groups of three microtubules fused into
triplets form the wall of the basal body
Basal body resembles a centriol
basal body contains some accessory structures
such as rootlet and basal foot
Accessory structures anchor the cilium in apical
cytoplasm
MECHANISM OF CILIARY
MOVEMENT
Cilia beat in a rhythmical wave-like manner’’
old concept:
Ciliary movement is based on the contraction of
microtubules (Microtubules are capable of shortening )
Satır, examined cross sections near the tips of cilia in
different phases of beating
Doublets of the bend cilium terminate at different levels
Sliding Microtubule Mechanism (Satır)

If a cilium is straight
doublets terminate at the same level

If a cilium is bent toward doublets 5 and 6
5 and 6 project farthest
Doublet 1 terminates first
If movements were produced by microtubule shortening,
microtubules on the concave side (5 and 6)
Should be shorter .
Result: Bending occured without shortening
Sliding Microtubule
Mechanism
(Satır )
To day accepted mechanism:
A cilium bends a long the axis by a type of
” Sliding Microtubule Mechanism’’ between microtubules
It is similar to that seen between myofilaments in striated
muscle fibers
Kartagener’s syndrome;
1-Chronic sinusitis
2-Bronchiectasis (chronic dilation of bronchi )
3-Situs inversus totalis
4-Male infertility
Normal number of spermatozoa but no motility
Afzelius 1978
E.M
Afzelius’s Experiment 1978

Electron Microscopic observation of immotile sperm flagellum of patients
with Kartagener’s syndrome:


EM observation of bronchial biopsies



Showed the absence of dynein arms
showed no dynein arms in ciliary axonemes
Result: *Dynein is essential for motility of cilia and flagella
Kartagener is a genetic disease
Dynein is essential for motility of cilia and
flagella
Afzelius examination 1978
*Clinical and
*Electron Microscopic observation
on a congenital form of human infertility
showed that
dynein is essential for motility of cilia and flagella
DYNEİN
• Extremely large protein
• Dynein arms form temporary cross bridges between
microıtubule A of one doublet and microtubule B of the
adjacent doublet
•During sliding they undergo a cyclic break and
reattachment
•Formation of cross bridges is ATP dependent
Dynein –Walking Model
• Dynein appears to “ walk’’ along the adjacent doublet
Experiment:
• Proteolytic enzymes digest nexin links and radial links
• The addition of ATP produces a sliding movement
of the bridge along the B tubule of the adjacent doublet
• Doublets move relative to each other powered by the
motor activity of axonemal dynein
• Radial linkers convert the sliding of microtubules into
bending of cilia
Ciliary movement
Requires
1-ATP
2-Ca ions , Mg ions
STRUCTURE OF MICROTUBULE

Microtubule (13 protofilaments )

Proto filaments are composed of Tubulin
subunits (dimer)
Dimer→ Tubulin α ve Tubulin β
Flagella
(Same internal structure with cilia) (Axoneme 9+2 )
Long whiplike protrusion whose ondulations drive a cell
through a fluid medium.
Eukaryotic flagella are longer than cilia.
Bacterial flagella smaller and different mechanism of action.
FLAGELLA
1- Longer than cilia (100-200 μ )
2-Different type of movement
(undulating wave type of movement)
3- Less in number (one or two in a single cell)
4- Mammalian spermium contains 9 additional dense
fibers arround the axoneme (9+9+2)
(protective function)
Stereocilia
1-Long and irregular microvilli 8μ (EM)
2-Have no internal structure
3-Microfilaments are poorly developed
4-No motility
5-No basal body


Function: Increase the cell surface for absorbtion
Localisation: Ductus epididymis, ductus deferns