Cytology · MU Pleven

Topic 18. Nonmembranous cell organelles – microtubules and cytofilaments.

Explains microtubules as hollow polymer tubes forming part of the cytoskeleton, covering their structure from tubulin heterodimers and protofilaments through to their polarity. It then describes the centrosome and microtubule organising centres, dynamic instability driven by GTP, and their roles in cell shape, intracellular transport, cell division, cilia and flagella. It closes with the motor proteins kinesin and dynein and the link to certain cancer chemotherapy drugs.

Inside: 2 sections · 3 labelled pictures · 15 flashcards · 3 min

Topic 18 of 59 in the MU Pleven syllabus for Cytology (Medicine, Year 1). The free AceMed note covers it in 2 sections, about 478 words (3 min of reading), with 15 flashcards.

In this note

  1. TOPIC 18. Nonmembranous cell organelles – microtubules and cytofilaments
  2. The cytoskeleton consists of the following components: -

The cytoskeleton consists of the following components: -

The cytoskeleton is a complex network of filaments and tubules that extends through the cytoplasm from the nucleus to the plasma membrane.

It is composed of protein polymers.

Microtubules: These are one of the three types of polymers, measuring 25 nm in diameter.

Microfilaments (Actin filaments): These are another type of polymer, with a diameter of 5-7 nm.

Intermediate filaments: These constitute the third type of polymer, having a diameter of 8-10 nm.

• Definition

Microtubules are fine, hollow tubular structures that form part of the cytoskeleton, a complex network of filaments and tubules extending throughout the cytoplasm from the nucleus to the plasma membrane. They are one of the three main types of polymers making up the cytoskeleton.

  • Structure and Composition of Microtubules have an outer diameter of 25 nm and a wall 5 nm thick.
  • They are polymers primarily composed of α and β tubulin heterodimers, with 13 protofilaments forming their wall.
  • They are polarised structures, meaning they have a distinct fast-growing (+) end and a slower-growing (-) end .
  • Centrioles, which are components of the centrosome, are themselves made up of nine sets of microtubular triplets arranged cylindrically.

Organisation

  • Microtubule assembly is directed by microtubule organising centres (MTOCs)
  • The primary MTOC in non-dividing cells is the centrosome, typically located near the nucleus.
  • The centrosome contains two cylindrical centrioles and surrounding pericentriolar material of tubulin subunits.
  • Centrioles are also involved in the formation of cilia and flagella on the surface of cells. Before cell division, the centrosome duplicates, and its halves move to opposite poles to organise the mitotic spindle.

Dynamic Nature

  • Microtubules exhibit dynamic instability, constantly undergoing cycles of polymerisation (assembly) and depolymerisation (disassembly).
  • Their stability can vary based on cellular location and function; for example, they are stable in cilia but have a shorter lifespan in the mitotic spindle. The energy for their assembly comes from GTP .

Functions

  1. 1. Provide cell shape and mechanical resistance to deformation due to their rigidity o Allow the cell to change shape.
  2. 2. Serve as tracks for intracellular transport of organelles, vesicles, and macromolecular complexes. This includes movements such as axoplasmic transport in neurons and vesicle movement towards/through the Golgi apparatus.
  3. 3. Facilitate chromosome movement and segregation during cell division via the mitotic spindle
  4. 4. Are the core structural component of cilia and flagella, enabling their movement

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