Cilia formation, or ciliogenesis, begins when the basal body, derived from the mother centriole, docks at the apical cell membrane. This initiates a cascade of events leading to the growth of the cilium.
The Ciliogenesis Process: A Step-by-Step Overview
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Basal Body Docking: The process starts with the mother centriole transforming into the basal body, anchoring itself to the cell's apical membrane. This crucial step triggers subsequent events. [Referencing: "Primary cilia are microtubule-based organelles that extend from the apical surface of most mammalian cells, forming when the basal body (derived from the mother centriole) docks at the apical cell membrane."]
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Axoneme Assembly: Next, the axoneme, the core of the cilium, forms. This structure consists of nine sets of microtubule doublets arranged around a central pair (9+2 arrangement). This is surrounded by a membrane continuous with the plasma membrane of the cell. [Referencing: "Cilia contain nine sets of microtubule doublets surrounded by a phospholipid membrane. This membrane is topologically continuous with the plasma membrane…"] and ["Cilia are built at the distal ends of centrioles through the formation of an axoneme that is surrounded by the ciliary membrane…"]
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Ciliary Membrane Formation: Simultaneously, the ciliary membrane, an extension of the cell's plasma membrane, envelops the growing axoneme.
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Intraflagellar Transport (IFT): The growth and maintenance of the cilium rely heavily on intraflagellar transport (IFT), a bidirectional movement of protein complexes along the microtubules of the axoneme. IFT delivers proteins and other molecules necessary for ciliary structure and function.
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Cell Cycle Dependency: Ciliogenesis is often linked to the cell cycle. Many cells form cilia during the G0 or G1 phase when cell division is paused. [Referencing: "Cilia form cells in G1 or G0 when the mother centriole docks at the apical cell membrane to form the basal body…"] and ["To induce formation of cilia, replace the medium to serum free (Opti-MEM medium, Invitrogen) and culture for 48 hrs. With hTERT-RPE1 cells, >85% of cells…"]
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Regulatory Proteins: Various proteins regulate the entire process, including proteins involved in docking the basal body, assembling the axoneme, and regulating IFT. [Referencing multiple sources which highlight the roles of specific proteins like CAMSAP3, NEK9, and Cep164 in different aspects of ciliogenesis]
Factors Influencing Ciliogenesis
Several factors influence cilium formation, including:
- Cell type: Different cell types have varying ciliogenesis mechanisms and regulatory proteins.
- Cell cycle phase: As mentioned above, many cells form cilia during specific cell cycle stages.
- Signaling pathways: Various signaling pathways regulate the initiation and progression of ciliogenesis.
- Environmental cues: Environmental factors, like serum concentration in cell culture, can impact ciliogenesis.
