• The internal scaffolding of the cell

• responsible for the shape of cells -

• driving force behind cell movement

three major components:

1) intermediate filaments,

2) microtubules, and

3) microfilaments.

INTERMEDIATE FILAMENTS

General Chacteristics

•  termed intermediate filaments because their diameter (7 to 11 nm) is intermediate between that of the thick and thin filaments of muscle

• - they are smaller than microtubules (25 nm)
• larger than microfilaments (5 nm) - ropelike polymers of "coiled-coil"fibrous polypeptides

• relatively stable - cell type-specific

• - different types of intermediate filament proteins are associated with different cell types 
•  this can be used for typing of cells, e.g. keratins are markers for tumors of epithelial origin, while vimentin is a marker for tumors of connective tissue cells (sarcomas).

mesenchymal cells- and cells in culture

  desmin muscle cells 

  glial fibrillary glial cells (astrocytes

   acidic protein  some Schwann cells)

• the central domains of two individual proteins form a helical dimer

• -two dimmers line up side by side to form a tetramer 
• tetramers aggregate end-to-end to form a protofilament
• eight protofilaments form a cylindrical, 10-nm-thick intermediate filament

- the globular domains at either end project from the surface and accounts for variations between
individual types of intermediate filaments.

• extend from the nuclear envelope to the plasma membrane

• - they are stable, i.e., no evidence for a rapid polymerization or depolymerization

•  particularly prominent in cells that are subject to mechanical stress, such as epithelial and cardiac muscle cells

•  keratin filaments form a network that crisscross the interior of the cell and attach to spot desmosomes

• single tubules are 25 nm in diameter 
•  composed of two monomers called α- and β-tubulin, which polymerize to form a hollow single microtubule 25 nm in diameter -

• assembly is regulated by microtubule-associatedproteins (MAPs)

• - microtubules have a defined polarity

•  assembly and disassembly occurs preferentially on one end 

• occurs as single, double or triple tubles.

• transient in nature 
• principal structural elements of the cytoplasm during interphase and the mitotic spindle during mitosis 
• region surrounding the centrioles is termed the Microtubule Organizing Center (MTOC)
• microtubules radiate away from this region forming a star-like structure termed an
• aster – amorphous material in this region serves as a nucleating site (or primer) for the formation cytoplasmic microtubules

• provide tracks for movement of pigment granules and membrane vesicles - kinesin and cytoplasmic dynein
• promote vesicle translocation in opposite directions along microtubules
•  both proteins have globular heads that bind to microtubules, and hydrolyze ATP and tails that bind to transported vesicles
• responsible for axonal transport in neurons
• - during mitosis bundles of microtubules radiate from the poles of the cells and attach to the chromosomes to form the mitotic spindle - these microtubules are responsible for separating the chromosomes during mitosis. 

• a centriole located at the base of a ciliary axoneme is termed a basal body

• centrioles can convert into basal bodies and vice verse 
• both centrioles and basal bodies consist of 9 sets of triplet microtubules (1 complete, 2 incomplete) forming a short cylinder 0.2 µm x 0.3 µm 
• Centrioles usually arise by the duplication of preexisting

• cilia extend from the surface of many epithelial cells
•  line the trachea oviduct and parts of the male reproductive tract
•  function to move fluids, mucous, eggs and sperm

• axonemes  represent the core of a cilium or a flagellum
•  contains a bundle of parallel microtubules in a "9+2" arrangement,
•  i.e. 9 outer doublet (fused) microtubules surrounding 2 central single microtubules

• protein associated with individual microtubules in an axoneme. 

• The dynein arms on one set of microtubule doublets that interact with adjacent microtubule doublet and hydrolyzes ATP to generate a sliding force between the two sets of microtubules. 

• Sliding of microtubule doublets is converted into bending of the cilia or flagella

• microtubule doublets are held together by a networks of cross-links. 

• 5 nm in diameter
• - very dynamic 
• - rapid if polymerization and depolymerization
•  - responsible for cell movement 
• present in all vertebrate cells 
• can account For a significant fraction of total cellular protein.

• Composed of a helical chain of actin

• filamentous actin is termed F-actin, while

• monomeric actin is termed G-actin (for globular) 

• F-actin is polarized and grows faster on one end thanthe other. 

Cross-linkers -

• some protein can cross-link actin filaments into a 3 dimensional network

• this is responsible for the reversible gel-sol transitions of actin
• -may be an important factor in cell movement.

Fragmenting (or capping) proteins which:

1. prevent polymerization, regulating the loss or

addition of G-actin – regulated by Ca++ions.

• family of proteins that interact with actin to produce tension 

• bind to actin and possess an ATPase
•  ATP hydrolysis is essential for movement

Type I (also termed minimyosins)

• present only in non-musclecells 
•  contains a short tail that can bind to membrane vesicles, plasma membrane, and microfilaments 
• responsible for movement of organelles along actin filaments

• present in both muscle and non-muscle cells
•  contains a globular head that can bind actin and a fibrous tail that can form polymers
•  
•  in the case of striated muscle the myosin filaments are organized into thick filaments and bind to F-actin. 

1. ZONULA ADHERENS

2. CELL CORTEX

• regions of cells specialized for the attachment of microfilaments to the plasma membrane
• particularly prominent in muscle cells 
• these proteins localize at the actin-anchorage plaques on the plasma membrane of the cell.

• a layer of actin and associated proteins which is immediately beneath the plasma membrane of many cells
•  attached to the plasma membrane by linker proteins 
• gives mechanical strength to the cell surface and enables the cell to change shape  and to move 
• the thickness varies from cell to cell and between different regions of the same cell. 

• non-contractile bundles of actin filaments maintain cell surface structures:

a. Microvilli of the intestinal epithelial cell - aids absorption (Fig. 15). 
b. Stereocilia of epididymis (aids absorption) and in cochlea (hearing). 

• mediated by the interaction between highly organized filaments ofactin and myosin
•  sliding filament model

cells can use different mechanisms for locomotion -

• one mechanism is "corticalflow" in which many types of cell movements are controlled by the flow of actin filaments in the cell cortex (Fig. 13)
•  this flow is governed by gradients of tension in the cortex generated by myosin
• elements flow from regions of low tension to regions of high tension

• - myosin may be responsible for maintaining the tension
• ----during initial stages of celldivision, there is an increase in the tension over the entire cell surface
• as cell division progresses, the tension at the two polar regions is relaxed 
• however, tension remains high in the equator leading to an indentation 

•  tension is controlled by the mitotic spindle - the contractile ring is a bundle of actin filaments and associated myosin located immediately below the plasma membrane of the cleavage furrow - contains a higher concentration of actin than other areas of the cell surface - cell surface components flow into the cleavage furrow during cytokinesis (i.e. separation of the cytoplasm)
•  a similar mechanism can account for cell migration - a continuous cycle is set up to return components of the cortical region 
•  in another mechanism for cell movement, the actin at the leading edge of the migrating cell under goes rapid remodeling

1. COMMUNICATING GAP JUNCTIONS

2. ZONULA OCCULDENS--TIGHT IMPEREABLE

3.DESOMOSOMES AND HEMIDESMOSEMS

4. ZONULA ADHERENS OR INTERMEDIATE JUNCTIONS

• on epithelial cells 
• and a variety of other cells, especially those that are electrically coupled (smooth and cardiac muscle) 
• most common type of junction. 

• under EM, membranes of adjacent cells are separated by a 2 nm gap

•  composed of several hundred connexons
•  - each connexon is composed of sixsubunits of a dumbbell-shaped protein called connexin.

• allows small molecules to pass from cell to cell
•  the pore size is large enough to pass ions, sugars and amino acids, but small enough to block the passage of proteins, nucleic acid and complex carbohydrates
• may be important for transmitting nutrients (metabolic cooperation) 
• - can transmit a wave of depolarization from cell to cell
• electrical coupling is important for contraction of heart and smooth muscle cells 
• regulates the coordinated beating of cilia on epithelial sheets.

•  membranes between adjacentcells appear to fuse 
• freeze fracture shows a series of parallel strands of intramembrane particles
•  
•  specific proteins in adjacent cells make contact with each other to form the tight junction. 

• serves as permeability barrier so that material cannot pass between cells -

• functionally divides the plasma membrane of cells into apical and basolateral regions
•  - many membrane proteins cannot pass this junction - requires divalent cations. 

• found between adjacent epithelial cells
•  -  button-like points of contact between cells
•  transmembrane linker proteins connect the two cytoplasmic plaques -

• hemidesmosomes are found in regions of epithelial cells in contact with the basement membrane
•  anchor the cells to the extracellular matrix of the basement membrane

• contain a pair of dense attachment plaques separated by a 20 nm space
• often has a dense line in the center
• - keratin filaments run into the plaque and the course back -

• there are filaments between the plaques of adjacent cells

act like spot welds

• - serve as anchoring sites for keratin filaments.

• occurs as a continuous band around the apical portion of epithelial cells -

• similar structures occur in intercalated disks

of heart muscle.

• contain two plaques into which actin filaments insert
• - EM shows an ill defined in material in the Zonula Adherens connecting adjacent cells together.

• contains cadherins 

• Proteins family of membrane proteinsinvolved in Ca++ - dependent cell to cell adhesion
•  - present on the cell surface and are closely associated with the Zonula Adherens. 

• Site of attachment of actin filaments to the plasma membrane
•  transmits contractile force from one cell to another. 

1. INTEGRINS
2. IMMUNOGLOBULIN SUPERFAMILY
3. SELECTINS
4. CADHERENS

• family of heterodimers
•  – function both cell-substratum cell-to-cell adhesion 

• - include the fibronectin receptor.

• cell-to-cell adhesion during embryogenesis, wound healing and inflammatory response - includes
• N-CAM   mediates the fasciculation of axons and is important in the attachment of neurons to skeletal muscle cells. 

• bind to specific carbohydrates on target cell
•  - expressed by some white blood cells and by endothelial cells
• mediates an inflammatory response in which leukocytes bind to blood vessels.

Ca++ dependent

- homophilic cell-cell adhesion molecules

- developmentally regulated

mediates cell-to-cell adhesion and serve as anchoring sites for actin filaments

• present on most cell types - the binding is probably homophilic in nature i.e. it binds to itself -
•  binding dependent on the presence of Ca++ions 
• - can be divided into a number of different subtypes based upon their distribution and cell binding specificity. 

mediates Ca++ dependent adhesion

- antibodies to these proteins can be used to
block cell-to-cell adhesion

E-cadherin holds most epithelial sheets together

-abundant at sites of cell-to-cell
contact in epithelia

- addition of antibodies to this cadherin cause cultured epithelial  cells to detach from each other - removal of Ca++ has a similar effect. 

 when different embryonic tissues are
dissociated into individual cells and then allowed to reaggregate,

they sort out, i.e. cells
from different tissues segregate into different regions of the aggregate. 

• during development, the expression of the variouscadherins undergoes dynamic changes
•  - the loss of cadherin may be responsible for the epithelial to mesenchymal transformation
•  - different germ layers express different forms of cadherins which may account for their separation.