Pre-Medicine Program - Cell Filament Systems

From CellBiology

Introduction to the Cytoskeleton

MT and MF movements 

J Cell Biol, 158:31-37, 2002

Red - actin (microfilaments).

Green - tubulin (microtubules).

2017 Lecture Slides

This lecture will provide an introduction to the 3 filament systems microfilaments, intermediate filaments, microtubules that together form the cell cytoskeleton. We have introduced the concept of dynamic processes with the eukaryotic cell, but how are these processes of change driven? The cell skeleton, unlike our own rigid skeleton, is the tool by which nearly all dynamic processes take place within the cell and by the whole cell. Key dynamic events of cell shape, surface specializations, cell division, development, adhesion, migration and intracellular transport can all be related to the cell cytoskeleton. All these processes require energy!

"Its not an artefact!"

The first electron micrograph taken of an intact eukaryotic cell from one of Keith Porter's original prints, a montage of five electron micrographs taken on July 6, 1944.

Size comparison of some common biological molecules (Myosin, Microtubule, Microfilament, Collagen)
Function Structure
  • functions based upon the filaments physical properties - each filament system has different properties
  • integral strength
  • cell shape
  • motility (inside the cell, whole cell, motor proteins associated with 2 filament systems MF MT)
  • signal transduction
  • Network of filamentous proteins - monomer protein forms polymer filaments
  • located in nucleus and cytoplasmic compartments - not within organelles (except nucleus)
  • location based upon cellular function and named on basis of physical size


  • Image - EM - Gut Epithelia cell (villi and junctions)
  • cytoplasmic - cortical meshwork under plasma membrane and three dimensional meshwork through cytoplasm
  • nuclear - cortical meshwork under nuclear envelope
  • assembly - some spontaneous, specific assembly sites
  • dynamic - variable stability, high to low stability, stability can be altered by associated proteins, signals and drugs

Cytoskeleton Movies: Play Movie - Exocytic transport | Play Movie - Vesicles Bidirectional Microtubule Movement | Play Movie - Respiratory Cilia | Play Movie - Axon Microtubules | Spermatozoa Motility | Kinesin motor

Cytoskeleton Filaments

  • non-muscle - cell cytoskeletons
  • muscle cells - have additional contractile filament system

Smallest to largest size:

  1. Microfilaments MF (7 nm)
  2. Intermediate Filaments IF (10 nm)
  3. Microtubules MT (25 nm)

  • 1 nanometre (nm) = 1 x 10 -9 m     (light microscope resolution is about 200 nm)


Microfilaments (MF)
  • Twisted chain 7 nm diameter
  • most abundant protein in cells (5% of all cell protein)
  • actin 43 Kd
  • Motility
  • Adhesion, focal adhesions
  • Actin binding proteins
  • myosin motors - walk along actin filament towards the barbed (+) end (exception myosin VI moves towards the pointed (-) end)
  • Muscle actin and myosin required for muscle contraction
  • microvilli, stereocilia (do not confuse with cilia MT)
Adherens junctions2.jpg

Adherens junctions

Actin Filament Assembly 
Muscle Myosin 

Intermediate Filaments

  • different cell types, different intermediate filaments (all eukaryotes nuclear cytoskeleton the same)
  • resist stresses applied externally to the cell
  • cytoplasmic network or meshwork - flexible intracellular scaffolding 10-nanometer diameter
  • cross-linking proteins allow interactions with other cytoskeletal networks
  • intermediate filament associated proteins (IFAPs) - coordinate interactions between intermediate filaments and other cytoskeletal elements and organelles
  • No motor proteins
  • human disorders - mutations weaken structural framework, increase the risk of cell rupture


Intermediate filament organization cartoon.jpg

Intermediate filament organization

Hutchinson-Gilford Progeria Syndrome.jpg

Hutchinson-Gilford Progeria Syndrome

Intermediate Filament Family 
Intermediate Filament - Primer 

Note this video does not include a description of the IF found in the nucleus.


Mammalian cell - prophase
  • 25 nm diameter, 14 nm internal channel
  • tubulin - cytoplasmic
  • All cells contain - Same core structure, Same motors Dynein (-) inwards (retrograde transport) and Kinesin (+) outwards (anterograde transport), Different associated proteins
  • Dynamic - Continuous remodelling
  • Movement - Intracellular > cellular, Cell division forms the mitotic spindle
  • Specialized structures - centrosome, basal bodies, spindle apparatus (mitosis, meiosis), Cell processes - cilia, spermatozoa tail (9+2)

Microtubules 01.jpg Dynein stain.jpg Centrosome cartoon1.jpg

Cell division phases 

J Cell Biol 168:21-28.

Cell division phases.jpg
Microtubule Kinesin Motor 

Links: Vesicles Display Bidirectional Motility along Microtubules | dyenin-stepping | Kinesin Walking | Dynein | Current Biology

Abnormal Cytoskeleton

tau fibrillary tangles
  • many mutations associated with human diseases
  • toxins can affect organization
  • infective bacteria and viruses can be appropriate (use)

Cytoskeleton Drugs

Many cancer therapies (chemotherapies) affect microtubule organization (mitotic spindle) required for cell division.


Science Lectures: Cytoskeleton Introduction | Microfilaments | Intermediate Filaments | Microtubules


Search Online Textbooks "cytoskeleton" Molecular Biology of the Cell | Molecular Cell Biology | The Cell- A molecular Approach | Bookshelf