Microtubules

Microtubules are elements of the cytoskeleton and of specialized structures

involved in subcellular movements.

Microtubules are found in the:

-axoneme of cilia and flagella

-basal bodies of cilia

-mitotic spindle "fibers"

-centrioles from which the spindle fibers radiate

-elongating cell processes, such as axon

-cytoplasm, generally

Cilia and flagella

Cilia and flagella are motile processes with a highly organized microtubule core. This core consists of nine pairs of microtubules surrounding two central tubules. This sheaf of tubules, possessing the characteristic (9×2) +2 pattern is called an axoneme. At the base of each cilium of flagellum is a basal body.

Centrioles

T he formula of centrioles is (9×3) +0. It means 9 pares of microtubules in the periphery and 0 in the center.

Each rod-shaped centriole is about 0,2 nm long and consists of nine triplets of microtubules that are oriented parallel to long axis of the organelle. The three microtubules are fused to one another, with adjacent microtubules sharing a common wall. Centrioles and adjacent dense material (centriolar satellites) constitute a general microtubule organizing center in both interphase and mitosis.

Functions of microtubules:

1. Cell elongation and movement

2. Intracellular transport of secretory granules

3. Movement of chromosomes during mitosis and meiosis

4. Maintenance of cell shape

5. Beating of cilia and flagella

Microtubules are elongated polymeric structures made up of equal parts of α tubulin and β tubulin.

Filaments

There are two basic types of filaments:

-microfilaments

-intermediate filaments

Microfilaments are the thinnest cytoskeletal elements and are more flexible than microtubules. There are two types of microfilaments (myofilaments) present in muscle cells:

-6-7 nm microfilaments (called thin filaments) of actin

-15-nm microfilaments (called thick filaments) of myosin

Actin microfilaments are present in virtually all cell types. Actin filaments are often grouped as bundles close to the plasma membrane. These membrane-associated microfilaments have such functions as:

-anchorage and movement of membrane protein

-movement of plasma membrane (as in endocytosis, exocytosis and cytokinesis)

-formation of the structural core of microvilli on absorptive cells

-extension of cell processes

-locomotion of cells

Contraction in all cells involves interactions of actin and myosin.

Intermediate filaments are heterogeneous group of 8-10 nm. Cytoskeletal elements are found in various cell types

-cytokeratin

-vimentin

-desmin

-neurofilaments

-glial fibrillary acidic protein

Inclusions

Trophic inclusions are lipids, carbohydrates, glycogen, and proteins.

Pigment can be divided into endogenic (melanin, bilirubin) and exogenous.

Secretory inclusions are products of life activity of the cells necessary for an organism for example gastric juice.

Excretory inclusions are products of life activity of the cells, which should be removed from the cell.