Microtubule organizing center
Encyclopedia
The microtubule-organizing center (MTOC) is a structure found in eukaryotic
cells from which microtubule
s emerge. MTOCs have two main functions: the organization of eukaryotic flagella
and cilia
and the organization of the mitotic
and meiotic
spindle apparatus, which separate the chromosome
s during cell division
. The MTOC is a major site of microtubule nucleation
and can be visualized in cells by immunohistochemical
detection of γ-tubulin. In animals, the two most important types of MTOCs are the basal bodies
associated with cilia and the centrosome
associated with spindle formation.
, usually located near the nucleus
, and generally associated closely with the Golgi apparatus
. The MTOC is made up of a pair of centriole
s at its center, and is surrounded by pericentriolar material (PCM) that is important for microtubule nucleation. Microtubules are anchored at the MTOC by their minus ends, while their plus ends continue to grow into the cell periphery. The polarity of the microtubules is important for cellular transport, as the motor proteins kinesin
and dynein
typically move preferentially in either the "plus" or "minus" direction, respectively, along a microtubule, allowing vesicles to be directed to or from the endoplasmic reticulum
and Golgi apparatus.
for more detailed information.
s and some algae, the MTOC is embedded into the nuclear envelope
as a spindle pole body. In these organisms, the nuclear envelope does not break down during mitosis and the spindle pole body serves to connect cytoplasmic with nuclear microtubules.
and spindle organization during plant cell mitosis.
Eukaryote
A eukaryote is an organism whose cells contain complex structures enclosed within membranes. Eukaryotes may more formally be referred to as the taxon Eukarya or Eukaryota. The defining membrane-bound structure that sets eukaryotic cells apart from prokaryotic cells is the nucleus, or nuclear...
cells from which microtubule
Microtubule
Microtubules are a component of the cytoskeleton. These rope-like polymers of tubulin can grow as long as 25 micrometers and are highly dynamic. The outer diameter of microtubule is about 25 nm. Microtubules are important for maintaining cell structure, providing platforms for intracellular...
s emerge. MTOCs have two main functions: the organization of eukaryotic flagella
Flagellum
A flagellum is a tail-like projection that protrudes from the cell body of certain prokaryotic and eukaryotic cells, and plays the dual role of locomotion and sense organ, being sensitive to chemicals and temperatures outside the cell. There are some notable differences between prokaryotic and...
and cilia
Cilium
A cilium is an organelle found in eukaryotic cells. Cilia are slender protuberances that project from the much larger cell body....
and the organization of the mitotic
Mitosis
Mitosis is the process by which a eukaryotic cell separates the chromosomes in its cell nucleus into two identical sets, in two separate nuclei. It is generally followed immediately by cytokinesis, which divides the nuclei, cytoplasm, organelles and cell membrane into two cells containing roughly...
and meiotic
Meiosis
Meiosis is a special type of cell division necessary for sexual reproduction. The cells produced by meiosis are gametes or spores. The animals' gametes are called sperm and egg cells....
spindle apparatus, which separate the chromosome
Chromosome
A chromosome is an organized structure of DNA and protein found in cells. It is a single piece of coiled DNA containing many genes, regulatory elements and other nucleotide sequences. Chromosomes also contain DNA-bound proteins, which serve to package the DNA and control its functions.Chromosomes...
s during cell division
Cell division
Cell division is the process by which a parent cell divides into two or more daughter cells . Cell division is usually a small segment of a larger cell cycle. This type of cell division in eukaryotes is known as mitosis, and leaves the daughter cell capable of dividing again. The corresponding sort...
. The MTOC is a major site of microtubule nucleation
Microtubule nucleation
Microtubules are filaments of the cytoskeleton. They typically form through the polymerization of α- and β-tubulin dimers elongating existing microtubules...
and can be visualized in cells by immunohistochemical
Immunohistochemistry
Immunohistochemistry or IHC refers to the process of detecting antigens in cells of a tissue section by exploiting the principle of antibodies binding specifically to antigens in biological tissues. IHC takes its name from the roots "immuno," in reference to antibodies used in the procedure, and...
detection of γ-tubulin. In animals, the two most important types of MTOCs are the basal bodies
Basal body
A basal body is an organelle formed from a centriole, and a short cylindrical array of microtubules. It is found at the base of a eukaryotic undulipodium and serves as a nucleation site for the growth of the axoneme microtubules...
associated with cilia and the centrosome
Centrosome
In cell biology, the centrosome is an organelle that serves as the main microtubule organizing center of the animal cell as well as a regulator of cell-cycle progression. It was discovered by Edouard Van Beneden in 1883...
associated with spindle formation.
Interphase MTOC
Most animal cells have one MTOC during interphaseInterphase
Interphase is the phase of the cell cycle in which the cell spends the majority of its time and performs the majority of its purposes including preparation for cell division. In preparation for cell division, it increases its size and makes a copy of its DNA...
, usually located near the nucleus
Cell nucleus
In cell biology, the nucleus is a membrane-enclosed organelle found in eukaryotic cells. It contains most of the cell's genetic material, organized as multiple long linear DNA molecules in complex with a large variety of proteins, such as histones, to form chromosomes. The genes within these...
, and generally associated closely with the Golgi apparatus
Golgi apparatus
The Golgi apparatus is an organelle found in most eukaryotic cells. It was identified in 1898 by the Italian physician Camillo Golgi, after whom the Golgi apparatus is named....
. The MTOC is made up of a pair of centriole
Centriole
A Centriole is a barrel-shaped cell structure found in most animal eukaryotic cells, though it is absent in higher plants and most fungi. The walls of each centriole are usually composed of nine triplets of microtubules...
s at its center, and is surrounded by pericentriolar material (PCM) that is important for microtubule nucleation. Microtubules are anchored at the MTOC by their minus ends, while their plus ends continue to grow into the cell periphery. The polarity of the microtubules is important for cellular transport, as the motor proteins kinesin
Kinesin
A kinesin is a protein belonging to a class of motor proteins found in eukaryotic cells. Kinesins move along microtubule filaments, and are powered by the hydrolysis of ATP . The active movement of kinesins supports several cellular functions including mitosis, meiosis and transport of cellular...
and dynein
Dynein
Dynein is a motor protein in cells which converts the chemical energy contained in ATP into the mechanical energy of movement. Dynein transports various cellular cargo by "walking" along cytoskeletal microtubules towards the minus-end of the microtubule, which is usually oriented towards the cell...
typically move preferentially in either the "plus" or "minus" direction, respectively, along a microtubule, allowing vesicles to be directed to or from the endoplasmic reticulum
Endoplasmic reticulum
The endoplasmic reticulum is an organelle of cells in eukaryotic organisms that forms an interconnected network of tubules, vesicles, and cisternae...
and Golgi apparatus.
Centrosomes
Before cell division begins, the interphase MTOC replicates to form two distinct MTOCs (now typically referred to as centrosomes). During cell division, these centrosomes move to opposite ends of the cell and nucleate microtubules to help form the mitotic/meotic spindle. Refer to centrosomeCentrosome
In cell biology, the centrosome is an organelle that serves as the main microtubule organizing center of the animal cell as well as a regulator of cell-cycle progression. It was discovered by Edouard Van Beneden in 1883...
for more detailed information.
Basal body
In epithelial cells, MTOCs also anchor and organize the microtubules that make up cilia. As with the centrosome, these MTOCs stabilize and give direction to the microtubules, in this case to allow unidirectional movement of the cilium itself, rather than vesicles moving along it.Spindle pole body
In yeastYeast
Yeasts are eukaryotic micro-organisms classified in the kingdom Fungi, with 1,500 species currently described estimated to be only 1% of all fungal species. Most reproduce asexually by mitosis, and many do so by an asymmetric division process called budding...
s and some algae, the MTOC is embedded into the nuclear envelope
Nuclear envelope
A nuclear envelope is a double lipid bilayer that encloses the genetic material in eukaryotic cells. The nuclear envelope also serves as the physical barrier, separating the contents of the nucleus from the cytosol...
as a spindle pole body. In these organisms, the nuclear envelope does not break down during mitosis and the spindle pole body serves to connect cytoplasmic with nuclear microtubules.
Plant MTOCs
Plant cells lack centrioles or spindle pole bodies. Instead, the nuclear envelope itself appears to function as the main MTOC for microtubule nucleationMicrotubule nucleation
Microtubules are filaments of the cytoskeleton. They typically form through the polymerization of α- and β-tubulin dimers elongating existing microtubules...
and spindle organization during plant cell mitosis.