Ciliogenesis
From Wikipedia, the free encyclopedia
| Ciliogenesis | |
|---|---|
| Details | |
| Identifiers | |
| Latin | Ciliogenesis |
| TH | H1.00.01.1.01033 |
| Anatomical terminology | |
Ciliogenesis is defined as the building of the cell's antenna (primary cilia) or extracellular fluid mediation mechanism (motile cilium).[1] It includes the assembly and disassembly of the cilia during the cell cycle. Cilia are important appendages of cells and are involved in numerous activities such as cell signaling, processing developmental signals, and directing the flow of fluids such as mucus over and around cells. Due to the importance of these cell processes, defects in ciliogenesis can lead to numerous human diseases related to non-functioning cilia known as ciliopathies.[1]
Primary cilia are found to be formed when a cell exits the cell cycle.[2] Cilia consist of four main compartments: the basal body at the base, the transition zone, the axenome which is an arrangement of nine doublet microtubules and considered to be the core of the cilium, and the ciliary membrane.[2] Primary cilia contain nine doublet microtubules arranged as a cylinder in their axenome and are denoted as a 9+0 pattern.[2] Motile cilia are denoted as a 9+2 pattern because they contain two extra microtubules in the center of the cylinder that forms the axenome.[2] Due to differences between primary and motile cilia, differences are seen in the formation process.
Ciliogenesis occurs through an ordered set of steps.[3] Basal bodies migrate to the surface of the cell and attach to the cell cortex. Along the way, the basal bodies attach to membrane vesicles that fuse with the plasma membrane of the cell. The alignment of cilia is determined by the positioning and orientation of the basal bodies at this step. Once the alignment is determined, axonemal microtubules extend from the basal body and forming the cilia.[1]
Proteins must be synthesized in the cytoplasm of the cell and cannot be synthesized within cilia. For the cilium to elongate, proteins must be selectively imported from the cytoplasm into the cilium and transported to the tip of the cilium by intraflagellar transport (IFT). Once the cilium is completely formed, it continues to incorporate new tubulin at the tip of the cilia while older tubulin is simultaneously degraded. This requires an active mechanism that maintains ciliary length. Impairments in these mechanisms can affect the motility of the cell and cell signaling between cells.[1]
There are two noted types of ciliogenesis: compartmentalized and cytosolic.[4] Most cells undergo compartmentalized ciliogenesis in which cilia are enveloped by extensions of the plasma membrane for the entirety of development.[4] In cytosolic ciliogenesis, the axenome must interact with proteins in the cytoplasm therefore it is directly exposed to the cytoplasm.[4] In some cells, cytosolic ciliogenesis occurs after compartmentalized ciliogenesis.[4]
Disassembly
Cilia disassembly is much less understood than cilia assembly. From recent discoveries, three distinct types of cilia disassembly have been identified. One variety of cilia disassembly occurs when the length of the cilia is gradually reduced until it is no longer functional.[5] Another category of cilia disassembly is shedding where cilia are severed from the main cell body.[5] An example of this is Chlamydomonas in which a severing enzyme known as katanin separates basal bodies from axenomes.[5]
In some organisms, a third method of cilia disassembly has been seen in which the entire axenome is internalized and then later disintegrated.[2]
Cilia presence is seen to be inversely related to the progression of the cell cycle which can be seen by assembly occurring during cellular quiescence and disassembly occurring when the cell cycle is stimulated.[2]