The microtubule-organizing center (MTOC) is a structure found in

eukaryotic Eukaryotes () are organisms whose Cell (biology), cells have a cell nucleus, nucleus. All animals, plants, fungi, and many unicellular organisms, are Eukaryotes. They belong to the group of organisms Eukaryota or Eukarya, which is one of the ...

cells from which microtubules emerge. MTOCs have two main functions: the organization of eukaryotic flagella and cilia and the organization of the

mitotic In cell biology, mitosis () is a part of the cell cycle in which replicated chromosomes are separated into two new nuclei. Cell division by mitosis gives rise to genetically identical cells in which the total number of chromosomes is maintai ...

and meiotic

spindle apparatus In cell biology, the spindle apparatus refers to the cytoskeletal structure of eukaryotic cells that forms during cell division to separate sister chromatids between daughter cells. It is referred to as the mitotic spindle during mitosis, a p ...

, which separate the

chromosome A chromosome is a long DNA molecule with part or all of the genetic material of an organism. In most chromosomes the very long thin DNA fibers are coated with packaging proteins; in eukaryotic cells the most important of these proteins are ...

s during

cell division Cell division is the process by which a parent cell divides into two daughter cells. Cell division usually occurs as part of a larger cell cycle in which the cell grows and replicates its chromosome(s) before dividing. In eukaryotes, there ar ...

. The MTOC is a major site of

microtubule nucleation In cell biology, microtubule nucleation is the event that initiates '' de novo'' formation of microtubules (MTs). These filaments of the cytoskeleton typically form through polymerization of α- and β-tubulin dimers, the basic building blocks of t ...

and can be visualized in cells by

immunohistochemical Immunohistochemistry (IHC) is the most common application of immunostaining. It involves the process of selectively identifying antigens (proteins) in cells of a tissue section by exploiting the principle of antibodies binding specifically to ant ...

detection of

γ-tubulin Tubulin in molecular biology can refer either to the tubulin protein superfamily of globular proteins, or one of the member proteins of that superfamily. α- and β-tubulins polymerize into microtubules, a major component of the eukaryotic cytoske ...

. The morphological characteristics of MTOCs vary between the different phyla and kingdoms. In animals, the two most important types of MTOCs are 1) the

basal bodies A basal body (synonymous with basal granule, kinetosome, and in older cytological literature with blepharoplast) is a protein structure found at the base of a eukaryotic undulipodium (cilium or flagellum). The basal body was named by Theodor W ...

associated with cilia and flagella and 2) the

centrosome In cell biology, the centrosome (Latin centrum 'center' + Greek sōma 'body') (archaically cytocentre) is an organelle that serves as the main microtubule organizing center (MTOC) of the animal cell, as well as a regulator of cell-cycle prog ...

associated with spindle formation.

Organization

Microtubule-organizing centers function as the site where microtubule formation begins, as well as a location where free-ends of microtubules attract to. Within the cells, microtubule-organizing centers can take on many different forms. An array of microtubules can arrange themselves in a pinwheel structure to form the basal bodies, which can lead to the formation of microtubule arrays in the cytoplasm or the 9+2 axoneme. Other arrangements range from fungi spindle pole bodies to the eukaryotic chromosomal

kinetochores A kinetochore (, ) is a disc-shaped protein structure associated with duplicated chromatids in eukaryotic cells where the spindle fibers attach during cell division to pull sister chromatids apart. The kinetochore assembles on the centromere and ...

(flat, laminated plaques). MTOCs can be freely dispersed throughout the cytoplasm or centrally localized as foci. The most notable MTOCs are the centrosome at interphase and the mitotic spindle poles.

Centrioles In cell biology a centriole is a cylindrical organelle composed mainly of a protein called tubulin. Centrioles are found in most eukaryotic cells, but are not present in conifers (Pinophyta), flowering plants (angiosperms) and most fungi, and are ...

can act as markers for MTOCs in the cell. If they are freely distributed in the cytoplasm, centrioles can gather during differentiation to become MTOCs. They can also be focused around a centrosome as a single MTOC, though centrosomes can work as an MTOC absent of centrioles.

In interphase

Most animal cells have one MTOC during

interphase Interphase is the portion of the cell cycle that is not accompanied by visible changes under the microscope, and includes the G1, S and G2 phases. During interphase, the cell grows (G1), replicates its DNA (S) and prepares for mitosis (G2). A c ...

, usually located near the

nucleus Nucleus ( : nuclei) is a Latin word for the seed inside a fruit. It most often refers to: *Atomic nucleus, the very dense central region of an atom * Cell nucleus, a central organelle of a eukaryotic cell, containing most of the cell's DNA Nucl ...

, and generally associated closely with the

Golgi apparatus The Golgi apparatus (), also known as the Golgi complex, Golgi body, or simply the Golgi, is an organelle found in most eukaryotic cells. Part of the endomembrane system in the cytoplasm, it packages proteins into membrane-bound vesicles ...

. The MTOC is made up of a pair of centrioles at its center, and is surrounded by

pericentriolar material Pericentriolar material (PCM, sometimes also called pericent matrix) is a highly structured, dense mass of protein which makes up the part of the animal centrosome that surrounds the two centrioles. The PCM contains proteins responsible for microtub ...

(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 A kinesin is a protein belonging to a class of motor proteins found in eukaryotic cells. Kinesins move along microtubule (MT) filaments and are powered by the hydrolysis of adenosine triphosphate (ATP) (thus kinesins are ATPases, a type of enzy ...

and dynein typically move preferentially in the "plus" and "minus" directions respectively, along a microtubule, allowing vesicles to be directed to or from the endoplasmic reticulum and Golgi apparatus. Particularly for the Golgi apparatus, structures associated with the apparatus travel towards the minus end of a microtubule and aid in the overall structure and site of the Golgi in the cell.

Centrosomes

Movements of the microtubules are based on the actions of the centrosome. Each daughter cell after the cessation of mitosis contains one primary MTOC. 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/meiotic spindle. If the MTOC does not replicate, the spindle cannot form, and mitosis ceases prematurely.

is a protein located at the centrosome that nucleates the microtubules by interacting with the tubulin monomer subunit in the microtubule at the minus end. Organization of the microtubules at the MTOC, or centrosome in this case, is determined by the polarity of the microtubules defined by y-tubulin.

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

yeast Yeasts are eukaryotic, single-celled microorganisms classified as members of the fungus kingdom. The first yeast originated hundreds of millions of years ago, and at least 1,500 species are currently recognized. They are estimated to constit ...

s and some

alga Algae (; singular alga ) is an informal term for a large and diverse group of photosynthetic eukaryotic organisms. It is a polyphyletic grouping that includes species from multiple distinct clades. Included organisms range from unicellular mic ...

e, the MTOC is embedded into the nuclear envelope as a spindle pole body. Centrioles do not exist in the MTOCs of yeast and fungi. In these organisms, the nuclear envelope does not break down during mitosis and the spindle pole body serves to connect cytoplasmic with nuclear microtubules. The disc-shaped spindle pole body is organized into three layers: the central plaque, inner plaque, and outer plaque. The central plaque is embedded in the membrane, while the inner plaque is an amorphous intranuclear layer, and the outer plaque is the layer located in the cytoplasm.

In plants

Plant Plants are predominantly photosynthetic eukaryotes of the kingdom Plantae. Historically, the plant kingdom encompassed all living things that were not animals, and included algae and fungi; however, all current definitions of Plantae exclu ...

cells lack centrioles or spindle pole bodies except in their flagellate male gametes, and they are entirely absent in the conifers and

flowering plants Flowering plants are plants that bear flowers and fruits, and form the clade Angiospermae (), commonly called angiosperms. The term "angiosperm" is derived from the Greek words ('container, vessel') and ('seed'), and refers to those plants th ...

.Marshall, W.F. (2009) Centriole Evolution. Current Opinion in Cell Biology 21(1), 14–19. Instead, the nuclear envelope itself appears to function as the main MTOC for

and spindle organization during plant cell mitosis.

Signal transduction

The MTOC reorients itself during signal transduction, primarily during wound repair or immune responses. The MTOC is relocalized to a position between the edge of the cell and the nucleus in cells like macrophages, fibroblasts, and endothelial cells. Organelles like the Golgi apparatus aid in the reorientation of the MTOC which can occur rapidly. Transduction signals cause microtubules to grow or contract, as well as cause the centrosome to become motile. The MTOC is located in a perinuclear position and contains the negative ends of microtubules while the positive ends grow rapidly towards the edge of the cell. The Golgi apparatus reorients along with the MTOC, and together cause the cell to seemingly send a polarized signal. In immune responses, upon interaction with a target cell in response to antigen-specific loaded antigen-presenting cells, immune cells, such as the

T cells A T cell is a type of lymphocyte. T cells are one of the important white blood cells of the immune system and play a central role in the adaptive immune response. T cells can be distinguished from other lymphocytes by the presence of a T-cell re ...

natural killer cells Natural killer cells, also known as NK cells or large granular lymphocytes (LGL), are a type of cytotoxic lymphocyte critical to the innate immune system that belong to the rapidly expanding family of known innate lymphoid cells (ILC) and represen ...

, and cytotoxic T lymphocytes, localize their MTOCs near the contact zone between the immune cell and the target cell. For T cells, the T cell receptor signaling response causes the reorientation of the MTOC by microtubules shortening to bring the MTOC to the site of interaction of the T cell receptor.

References

External links

MBInfo - MTOC Figure
* {{Organelles Cytoskeleton Centrosome