Download Intermediate Filaments

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28/12/2015
Biology
Lec 1
‫ بثينة‬.‫د‬
Cell Biology
Cytoskeleton
The cytoskeleton is a network of fibers throughout the cell's cytoplasm
that helps the cell to maintain its shape and gives support to the cell and
enables cellular motion such as cilia and flagella and plays important
roles in both intracellular transport , )the movement of vesicles and
organelles, for example) and cellular division .The cytoskeleton is made
up of three kinds of protein filaments:
1- Actin filaments (also called microfilaments)
2- Intermediate filaments
3- Microtubules
Cytoskeleton
Microfilaments
Microfilaments are fine rigid hollow rods ,5- 7 nm in diameter,
represent the active or motile part of the cytoskeleton. Made of a
contractile protein known as actin. Actin appears in a globular form
(G-actin). Microfilaments are long polymerized chains of the
(1)
molecules are intertwined in a helix, creating a filamentous form of
protein (F-actin).
Microfilaments considered part of the cell cortex, which regulates the
shape and movement of the cell’s surface.
Functions of microfilaments:
1- Provides mechanical strength to the cell
2- Link transmembrane proteins (e.g., cell surface receptor) to
cytoplasmic proteins.
3-Anchors the centrosomes at opposite poles of the cell during
mitosis.
4-Pinches the dividing animal cells during cytokinesis.
5-Supports the plasma membrane.
7-Microfilaments association with the protein myosin is responsible
for muscle contraction.
8- They function in the maintenance of cell-shape.
9- These filaments are associated with membrane activities such as
endocytosis and exocytosis.
Microfilaments
Microtubules
Microtubules are hollow cylindrical tubes, 20- 25 nm in diameter
(lumen = approximately 15nm in diameter), most commonly
comprised of 13 protofilaments which, in turn, are polymers of alpha
and beta tubulin .They have a very dynamic behavior, binding GTP
for polymerization.
(2)
In animal cells, microtubules arise from a region of the cell called the
microtubule organizing center (MTOC) located near the nucleus.
In nine triplet sets (star-shaped), they form the centrioles ,and in
nine doublets oriented about two additional microtubules (wheelshaped) they form cilia and flagella.
They play key roles in:
 Determine cell shape and in a variety of cell movements, including
some form of cell locomotion, the intracellular transport of
organelles, and the separation of chromosomes during mitosis.
 The axoneme of cilia and flagella .
 The mitotic spindle .
 Providing a pathway for intracellular movement of organelles and
proteins.
Microtubules
(3)
Intermediate Filaments
Intermediate filaments have a diameter of about 10nm, which is
intermediate between the diameter of two other principle elements
of the cytoskeleton , actin filaments and microtubules. Like actin
filaments, they function in the maintenance of cell-shape, its form a
'basket' around the nucleus.
Filaments serving as structural components of the nuclear lamina and
sarcomeres. Different intermediate filaments are:
 Vimentins, being the common structural support of many cells.
Type I and type II: acidic keratin and basic keratin,
respectively.Produced by different types of epithelial cells (
bladder, skin,etc).
 Neurofilaments of neural cells .
 Lamin inside the inner nuclear envelope, lamins are vital to the
re-formation of the nuclear envelope after cell division.
 Desmin, found in muscle cells.
Intermediate filaments
(4)
Centrosome
In cell biology, the centrosome is an organelle that serves as the
main microtubules organizing center ( MTOC) of the animal cell ,it is
duplicated during S phase of the cell cycle .
Centerioles , found only in animal cells, these paired organelles are
located together near the nucleus. Each centerioles is made of nine
bundles of microtubules (three per bundle) arranged in a ring.
Just before mitosis, the two centrosomes move part until they are on
opposite side of the nucleus and organized into a spindle-shaped
formation that called spindle fibers.These spindle fibers act as
indicator for the alignment of the chromosomes as they separate
later during the process of cell division.
Centosomes
Cilia & Flagella
Cilia & flagella are motile cellular appendages found in most
microorganisms and animals, cilia function to move a cell or to help
(5)
transport fluid or materials past them. The respiratory tract in
humans is lined with cilia that keep inhaled dust, and harmful
microorganisms from entering the lungs. Cilia are usually shorter and
occur together in much greater numbers than flagella.
In eukaryotic cells, cilia and flagella contain the motor protein
(dynein) and (microtubles), the core of each of the structures is
termed the (axoneme) and contains two central microtubules that
are surrounded by an outer ring of nine doublet microtubules. Dynein
molecules are located around the axoneme. A plasma membrane
surrounds the entire axoneme complex, which is attached to the cell
at a structure termed the basal body.
Basal Body:
Basal body (also known as a kinetosome). Basal bodies maintain the
basic outer ring structure of the axoneme of the cilia and The basal
body is structurally identical to the centrioles.
Inclusions:
Inclusions: are considered to be nonliving components of the cell that
neither possess metabolic activity nor are bounded by membranes.
The most common inclusions are glycogen, lipid droplet, pigments,
and crystals.
1-Glycogen: Glycogen is the most common storage form of glucose in
animals and is especially abundant in cells of muscle and liver.
2-Lipids: is a storage forms of triglycerides, are stored in specialized
(6)
cells, adipocytes, also located as individual droplets in various cell
types. Especially those of the liver, lipids are source of energy.
3- Pigments: there are deposits of colored substances include:
A- Melanin: the most common pigment in the body, its dark brown
pigment present in the skin, hair, retina, and some parts of the
central nervous system (C.N.S)
B-Lipofuscin: its yellow to brown pigment found in long lived cells,
like neurons of the C.N.S and cardiac muscles. Lipofuscin pigments
are membrane-bound and represent the indigestible remnants of
lysosomal activity.
C- Hemosiderin: it’s a cold-yellow pigment. It’s the end product of Hb
degradation of old red blood cells. They are present in the liver,
spleen and bone marrow.
D- Crystals: crystals are structures of crystalline forms of certain
proteins . They are not commonly found in cells, with the exception
of steroid cells, and interstitial cells of testes, and occasionally in
macrophages.
Ultrastructure of basal body
(7)