Cytoskeleton and Cell Inclusions - Dr Muhammad Ali Rabbani - Medicose Academics

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0310-7990649
Cytoskeleton and Cell Inclusions
Dr. Muhammad Ali Rabbani
Asst. Professor Anatomy
MBBS (NMC, Multan)
FCPS (Anatomy)
CHPE (Certificate in Health Professional Education)
DHPE (Diploma in Health Professional Education)
MBA (Masters of Business Administration)
MPH (Masters of Public Health)

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Learning Objectives
At the end of this lecture, students should be able to
• Identify different components of cytoskeleton
• Describe structure, formation and function of all three components
• Define inclusion bodies and describe its various types
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• The cytoplasmic cytoskeleton is a filamentous array of cytosolic
proteins
• microtubules,
• microfilaments (actin filaments)
• intermediate filaments
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Functions
• Structural Framework of cell
• Cytoplasmic Streaming
• Motility of the cells
• Cell Division
• Cores of specialized cell projections
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Microtubules
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Structure of a Microtubule
• Rigid, Hollow, Non- branching Tube
• Outer diameter of 25nm, Wall of 5nm
• Length Varies (dynamic structure)
• Building block
• heterodimer of α and β tubulin
• 13 units in one complete turn
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Structure of a Microtubule
• The dimers polymerize in an end-to-end
fashion forming longitudinal protofilament
• It is a polar structure, with
• a growing plus (+) end, corresponding to β-
tubulin
• a minus (-) end, corresponding to α-tubulin
embedded in MTOC
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Microtubule Organizing Centre (MTOC)
• Polymerization of tubulins is directed
by microtubule organizing centers
(MTOCs)
• Contain tubulin assemblies that act as
nucleating sites for polymerization.
• The dominant MTOC in most somatic
cells is the centrosome
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Centrosome
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• 0.2 μm diameter
0.3-0.5 μm length.
• Nine microtubule triplet
arrangement
• Two arranged at right angles
• Paired centrioles organize
nearby tubulin complexes as
a pericentriolar matrix
• Duplicates during cell division

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Dynamic Instability of Microtubules
• Microtubules are in continuous state of
polymerization and depolymerization, condition
known as dynamic instability
• It depends on concentrations of tubulin, Ca2+,
Mg2+, and the presence of various microtubule-
associated proteins (MAPs)
• Discuss “GDP Cap”, “Catastrophe”, “Rescue”
• Dynamic instability allows for exploration
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• Microtubules are also organized into larger arrays called axonemes in
the cytoplasmic extensions called cilia and flagella
• Two or more microtubules are often linked side-by-side by protein
arms or bridges, which are particularly important in cilia and flagella
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Microtubules Organizations

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Motor Proteins
• Microtubules also form part of the system for intracellular transport
of membranous vesicles, macromolecular complexes, and organelles.
• They act like rail-roads on which different types of cargo is
transported
• Well-studied examples include
• axoplasmic transport in neurons,
• melanin transport in pigment cells,
• chromosome movements by the mitotic spindle,
• vesicle movements among different cell compartments.
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• Kinesins carry material away from
the MTOC near the nucleus toward
the plus end of microtubules
(anterograde transport)
• Dyneins carry material along
microtubules in the opposite
direction (retrograde transport),
generally toward the nucleus.
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Microfilaments
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• Microfilaments are thin (5-7 nm diameter), polarized
polymers, shorter and more flexible than
microtubules
• Composed of protein called actin
• They allow cellular motility and contractile activity by
reversible assembly of the actin fibres and interaction
between these and associated protein, myosin
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• The basic subunit of microtubule is
globular G-actin monomers
• They assemble (polymerize) in the
presence of K+ and Mg2+ into a double-
stranded helix of filamentous F-actin
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• Actin filaments are also highly dynamic.
• Monomers are added rapidly at the (+) or
barbed end, with hydrolysis of ATP at each
addition
• At the same time monomers dissociate at the
(−) or pointed end
• This leads to migration of subunits through
the polymer, which occurs rapidly in purified
filaments in a process called treadmilling
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Location
• Actin is very abundant in all cells
• It is usually concentrated as networks of actin filaments and abundant
free globular G-actin subunits concentrated near the cell membrane
(a region sometimes called the cell cortex)
• Also present as core proteins in cellular extensions, like microvilli.
• In cells attached to firm substrates, actin filaments may be
concentrated into parallel bundles called stress fibers
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Actin Binding Proteins
• A large number of proteins regulate the assembly of microfilaments
and the interactions of these filaments with one another.
• By altering microfilament length and cross-linking, such proteins
greatly influence physical properties of the local cytoplasm.
• Severing and/or capping the end of F-actin (eg, gelsolin, capZ)
• Cross-linking (eg, filamin) or bundling (eg, fimbrin, α-actinin) actin filaments
• Linking F-actin to membrane proteins and other cytoskeletal filaments (eg,
spectrin)
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Movement Across Actin
• Various myosin motors use ATP to transport cargo along F-actin.
• Movement is usually toward the barbed (+) end of actin filaments
• Myosin VI is the only known myosin that moves in the other
direction
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• Transport of various organelles, vesicles, and granules through the cell
(cytoplasmic streaming)
• Contractile rings of microfilaments and with myosin II that constrict to
produce two cells at the end of mitosis (cytokinesis)
• Membrane-associated molecules of myosin I whose movements
along microfilaments are important in the cell surface changes that
underlie phagocytosis and pinocytosis
• Contraction of cytoplasm that shortens cells or rapidly retracts
cellular extensions
• Stabilized arrays of actin filaments integrated with arrays of thicker
(16-nm) myosin filaments permit very forceful contractions in
specialized cells such as those of muscle
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Intermediate Filaments
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• In addition to microtubules and actin filaments, the cytoskeleton
includes class of filaments intermediate in size between the other two
and with a diameter averaging 10nm
• The intermediate filaments are much more stable than microtubules
and actin filaments.
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Structure
• Nearly all these subunits are coiled, rod-like dimers
• form antiparallel tetramers
• These self-assemble into large cable-like bundles
or protofibrils
• stabilized by further lateral interactions.
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• Taking into consideration the biochemical nature of the contituren
proteins many types of intermediate filaments have been identified in
different cells of the body.
• The most common varieties discussed are
• Keratin filaments
• Viemntin Filaments
• Desmin Filaments
• Neurofilaments
• Glial Filaments
• Lamin Filaments
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Inclusions
• non-living components
• do not possess metabolic activity
• contain accumulated metabolites and other substances
• not bounded by a membrane.

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Fat Droplets
• Prominent in
• adipocytes (fat cells)
• adrenal cortex cells
• liver and other cells
• Enclosed by
• a single monolayer of phospholipids
• various peripheral proteins, including enzymes for lipid metabolism.
• In routine processing of tissue for paraffin sections, fat droplets are
generally removed, leaving empty spaces in the cells.
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Glycogen Granules
• Aggregates of the carbohydrate polymer in
which glucose is stored, are visible in several
cell types, mainly liver cells, in the form of
irregular clumps of periodic acid-Schiff (PAS)–
positive or electron-dense material
• Glycogen is a ready source of energy, and such
granules are often abundant in cells with high
metabolic activity.
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Lipofuscin
• Lipofuscin is a yellowish-brown pigment
visualized by H&E staining in many cells,
especially in stable nondividing cells (eg,
neurons, cardiac muscle).
• Sometimes called “wear-and-tear pigment”
granules of lipofuscin contain a complex mix
of material partly derived from residual
bodies after lysosomal digestion.
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Hemosiderin
• Hemosiderin is a dense brown aggregate of
denatured ferritin proteins with many atoms of
bound iron.
• It occurs in phagocytic cells, especially macrophages
of the liver and spleen, where it results from
phagocytosis of red blood cells.
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