Microtubule organizing center

Bronchiolar area cilia cross-sections 2

Microtubule Organizing Center (MTOC) is a structure found in eukaryotic cells from which microtubules emerge. MTOCs are critical for the spatial arrangement of microtubules and thus play a key role in shaping the cell's cytoskeleton, organizing the intracellular transport system, and facilitating the segregation of chromosomes during cell division. The most well-known type of MTOC is the centrosome, found in most animal cells. In plant cells, similar functions are carried out by structures known as microtubule organizing centers, but these do not contain centrioles.

Structure and Function

The structure of a MTOC includes a variety of proteins that nucleate the assembly of microtubules, the most notable of which is γ-tubulin. γ-tubulin combines with other proteins to form a complex known as the γ-tubulin ring complex (γ-TuRC), which nucleates the polymerization of tubulin subunits into microtubules. This process is essential for the dynamic reorganization of the microtubule cytoskeleton during cell processes such as mitosis and meiosis.

MTOCs play a crucial role in determining the orientation of microtubules within the cell. This orientation is vital for many cellular processes, including intracellular transport, where microtubules serve as tracks for the movement of vesicles and organelles, and cell division, where they form the mitotic spindle that segregates chromosomes into daughter cells.

Types of MTOCs

While the centrosome is the most prominent MTOC in animal cells, other types of MTOCs exist in different organisms:

• Centrosomes, which contain a pair of centrioles surrounded by pericentriolar material, are key for the formation of the spindle apparatus during cell division in animal cells.
• Spindle pole bodies (SPBs) are the yeast and fungal equivalent of centrosomes and serve as the MTOC during cell division.
• Basal bodies serve as the nucleation point for cilia and flagella formation and can be considered MTOCs due to their role in organizing microtubules in these structures.
• In plants and some algae, MTOCs are not as well-defined but are believed to consist of dispersed sites within the cell where microtubules are nucleated.

Regulation

The activity and location of MTOCs are tightly regulated within the cell. The cell cycle controls the duplication and segregation of MTOCs, ensuring that each daughter cell inherits a single MTOC. Additionally, the positioning of MTOCs is influenced by cellular signaling pathways that respond to both internal and external cues, directing cell movement and polarity.

Clinical Significance

Abnormalities in MTOC function can lead to diseases, including cancer. For example, errors in microtubule organization can result in improper chromosome segregation, leading to aneuploidy and tumorigenesis. Research into MTOCs continues to be a significant area of study in cell biology, with implications for understanding disease mechanisms and developing new therapeutic strategies.