The axoneme is a structure present in the flagella and cilia of eukaryotic cells, responsible for their movement. The axoneme is composed of helical microtubules, which are connected by motor proteins and structural proteins. This structure is essential for cellular locomotion, allowing cells to move in a coordinated and efficient manner. In this context, the composition and characteristics of the axoneme are extremely important for understanding the functioning of eukaryotic flagella and cilia.
What is the importance of the Axoneme in cellular and ciliary function?
The axoneme is a fundamental structure for cellular and ciliary function. It provides support and movement for the cells' cilia and flagella. The axoneme is composed of organized microtubules that slide over each other to generate movement.
Furthermore, the axoneme is essential for the locomotion of various organisms, such as sperm and protozoa. It allows cells to move in a coordinated and efficient manner, performing functions vital to the survival of these organisms.
Another important function of the axoneme is the perception of external stimuli. The cilia present on sensory cells are capable of detecting changes in the environment and transmitting this information to the cell's interior. Thus, the axoneme contributes to the cellular response to external stimuli.
In summary, the axoneme is an essential structure for cellular and ciliary function, playing fundamental roles in locomotion, stimulus perception, and coordination of cell movement. Its composition and organization are crucial to ensuring proper cellular function and the survival of organisms.
Importance of Axoneme and Myofibril proteins in cellular and contractile function.
Axoneme and myofibril proteins are essential for proper cell function and muscle contraction. The axoneme is a structure present in cell cilia and flagella, responsible for the movement of these structures. Myofibrils are found in skeletal and cardiac muscles, and play a role in muscle contraction.
In the axoneme, proteins such as tubulin and dynein are essential for the movement of cilia and flagella. Tubulin forms the microtubules that make up the axoneme, while dynein is responsible for microtubule sliding, generating the characteristic movement of these structures. Without these proteins, the cell would be unable to perform movements essential for functions such as locomotion and perception of external stimuli.
In myofibrils, proteins such as actin and myosin are crucial for muscle contraction. Actin forms the thin filaments, while myosin forms the thick filaments. During muscle contraction, these proteins slide over each other, shortening the length of the myofibrils and causing muscle contraction. Without the action of these proteins, the muscle would not be able to contract and perform movements.
Thus, axoneme and myofibril proteins play fundamental roles in cellular function and muscle contractility. They are essential for cellular movement and muscle contraction, and are indispensable for maintaining homeostasis and the body's health.
Chemical composition of microtubules: what are they and how are they formed?
The chemical composition of microtubules is essential to understanding the structure and function of these important cellular structures. Microtubules are formed by tubulin, a protein composed of two main types of subunits: alpha and beta. These subunits are organized into dimers that bind to form protofilaments, which in turn associate laterally to form microtubules.
Microtubules play a fundamental role in cellular processes such as cell division, intracellular transport, and cell movement. Their unique chemical composition allows them to be highly dynamic and rapidly rearrange themselves as needed by the cell.
Axoneme: characteristics and composition
The axoneme is a specialized structure found in the cilia and flagella of eukaryotic cells. It consists of a characteristic arrangement of microtubules arranged in a "9+2" pattern, where nine pairs of peripheral microtubules surround two central microtubules.
In addition to microtubules, the axoneme also contains accessory proteins such as dyneins and nexins, which play a crucial role in the movement of cilia and flagella. These motor proteins allow the axoneme to contract and produce movement, essential for functions such as cell locomotion and the removal of particles from the environment.
Essential functions of microtubules in the cytoplasm: learn about the main ones.
Microtubules are essential structures in the cytoplasm of cells and perform several important functions. They are composed of tubulin dimers, forming hollow filaments that provide structural support, facilitate intracellular transport, and aid in cell division.
One of the fundamental roles of microtubules is to serve as tracks for the transport of organelles and vesicles within the cell. Motor proteins, such as dynein and kinesin, move along microtubules, enabling the movement of cargo essential for various cellular functions.
Furthermore, microtubules are essential for the formation of the mitotic spindle during cell division, ensuring the correct distribution of chromosomes to daughter cells. They are also involved in maintaining cell shape and the movement of cellular structures such as cilia and flagella.
Axoneme: characteristics and composition
The axoneme is a structure found in cilia and flagella, composed of microtubules organized in a specific way. It consists of nine microtubule doublets surrounding a central pair, giving it a characteristic "9+2" arrangement.
The axoneme's microtubules are composed of tubulin, a protein essential for its formation and function. The arrangement of microtubules in the axoneme allows the movement of cilia and flagella, providing motility to the cells that possess them.
Axoneme: characteristics and composition
O axoneme is an internal cytoskeletal structure of cilia and flagella based on microtubules that gives them movement. Its structure consists of a plasma membrane that surrounds a pair of central microtubules and nine pairs of peripheral microtubules.
The axoneme is located outside the cell and is anchored to the cell interior by the basal body. It is 0,2 μm in diameter and can vary in length from 5 to 10 μm in cilia to several mm in the flagella of some species, although it typically measures 50 to 150 μm.
The axonal structure of cilia and flagella is highly conservative in all eukaryotic organisms, from Chlamydomonas microalgae to the flagellum of human sperm.
Features
The axonemes of the vast majority of cilia and flagella have a configuration known as “9 + 2”, that is, nine pairs of peripheral microtubules around a central pair.
The microtubules in each pair are different in size and composition, except for the central pair, which has similar microtubules. These tubules are stable structures, capable of withstanding ruptures.
Microtubules have polarity and all have the same arrangement, with the “+” end located towards the apex and the “-” end located at the base.
Structure and composition
As we've already noted, the axoneme structure is a 9 + 2 arrangement. Microtubules are long, cylindrical structures formed by protofilaments. Protofilaments, in turn, consist of protein subunits called alpha tubulin and beta tubulin.
Each protofilament has an alpha tubulin unit at one end, while the other end has a beta tubulin unit. The end with the beta tubulin terminal is called the "+" end, the other end would be the "-" end. All protofilaments of the same microtubule are oriented with the same polarity.
Microtubules contain, in addition to tubulins, proteins called microtubule-related proteins (MAPs). Of each pair of peripheral microtubules, the smaller one (microtubule A) is composed of 13 protofilaments.
Microtubule B has only 10 protofilaments, but is larger than microtubule A. The central pair of microtubules is the same size and each is composed of 13 protofilaments.
This central pair of microtubules is delimited by the central sheath, a protein that connects to the peripheral A microtubules via radial spokes. The A and B microtubules of each pair are joined by a protein called nexin.
Microtubules also include a pair of arms formed by a protein called dynein. This protein is responsible for using the energy available in ATP to drive the movement of cilia and flagella.
Externally, the axoneme is covered by a ciliary or flagellar membrane that has the same structure and composition as the cell's plasma membrane.
Exceptions to the “9 + 2” axoneme model
Although the “9 + 2” composition of the axoneme is highly conserved in most ciliated and/or flagellated eukaryotic cells, there are some exceptions to this model.
In the sperm of some species, the central pair of microtubules is missing, resulting in a "9+0" configuration. Flagellar movement in these sperm does not appear to vary much from that observed in axonemes with a normal configuration, so it is believed that these microtubules do not play a major role in the movement.
This axoneme model has been observed in the sperm of species such as fish Lycondontis and annelids of the genus Myzostomum .
Another configuration observed in axonemes is the "9 + 1" configuration. In this case, a single central microtubule is present, rather than a pair. In these cases, the central microtubule is extensively modified, presenting several concentric walls.
This axoneme pattern has been observed in the male gametes of some flatworm species. In these species, however, this axoneme pattern is not replicated in other ciliated cells or flagellated organisms.
Axoneme movement mechanism
Studies of flagellar movement have shown that flagellar flexion occurs without contraction or shortening of the axoneme microtubules. For this reason, cytologist Peter Satir proposed a model of flagellar movement based on microtubule displacement.
According to this model, movement is achieved by the displacement of one microtubule from each pair onto its partner. This model is similar to the sliding of myosin chains on actin during muscle contraction. Movement occurs in the presence of ATP.
The dynein arms are anchored to the A microtubule of each pair, with their ends directed toward the B microtubule. At the beginning of movement, the dynein arms adhere to the junction site on the B microtubule. Then, a change in the dynein configuration occurs that pulls the B microtubule down.
Nexin holds the two microtubules close together. Subsequently, the dynein arms separate from the B microtubule. They then rejoin to repeat the process. This sliding occurs alternately between one side of the axoneme and the other.
This alternating displacement of the axoneme from one side to the other causes the cilium, or flagellum, to bend first to one side and then to the opposite side. The advantage of Satir's model of flagellar movement is that it would explain the movement of the appendage regardless of the axoneme configuration of the axoneme microtubules.
Axoneme-related diseases
Several genetic mutations can cause abnormal axoneme development. These abnormalities can include, among others, the lack of one of the dynein arms, either inner or outer, of the central microtubules, or of the radial spokes.
In these cases, a syndrome called Kartagener syndrome develops, in which people suffering from it are infertile because sperm are unable to move.
These patients also develop viscera in an inverted position compared to their normal position; for example, the heart is located on the right side of the body and the liver on the left. This condition is known as situs inversus.
Those who suffer from Kartagener syndrome are also prone to respiratory and sinus infections.
Another disease related to abnormal axoneme development is polycystic kidney disease. In this condition, multiple cysts develop in the kidneys, eventually destroying the kidney. This disease is caused by a mutation in the genes that code for proteins called polycystins.
References
- M. Porter and W. Sale (2000). The 9 + 2 axoneme anchors several inner arm dyneins and a network of kinases and phosphatases that control motility. The Journal of Cell Biology.
- Axoneme On Wikipedia Retrieved from en.wikipedia.org.
- G. Karp (2008). Cellular and molecular biology. Concepts and experiments. 5 th Edition. John Wiley & Sons, Inc. Companies
- S. L. Wolfe (1977). Cellular Biology, Omega Publishing, Inc.
- T. Ishikawa (2017). Axoneme Structure of Motile Cilia. Cold Spring Harbor Perspectives in Biology.
- RW Linck, H. Chemes and D. F. Albertini (2016). The axoneme: the driving force of sperm and cilia and associated ciliopathies leading to infertility. Journal of Assisted Reproduction and Genetics.
- S. Resino (2013). The cytoskeleton: microtubules, cilia, and flagella. Retrieved from epidemiologiamolecular.com