Ultrastructure of the cilium shaft
To fully understand the morphology of these structures, they must be examined in various planes. When studying a longitudinal section, the overall length of the cytoplasmic projection is clearly visualized. However, the cross-section of the cilium shaft is most informative for assessing internal organization.
The cross-section reveals a strict geometric architecture based on a microtubule framework. Externally, this entire complex is reliably bounded by the plasma membrane, which is a continuation of the cell's own membrane. The following elements are distinguished internally:
- Peripheral doublets: microtubules are grouped in pairs, forming 9 doublets arranged in a regular circle.
- Central microtubules: in the very center of the shaft is a solitary pair of microtubules, not connected to the peripheral ring.
Organization of the basal body
Every cilium or flagellum has a robust "anchor", which is the basal body. Its main localization is directly at the base of the cytoplasmic projection, where it transitions into the intracellular space.
The morphology of the basal body is radically different from the shaft part of the cilium itself. If we look at a cross-section of the basal apparatus, we will see that the peripheral microtubules here are organized not into pairs, but into triplets. Each structural unit consists of three microtubules closely adjacent to each other. It is precisely this construction that provides the necessary rigidity at the base.
Structural changes in the transition zone
The transition from the cilium shaft to the basal body does not occur abruptly; it is accompanied by regular structural rearrangements at the microtubule level. This region requires special attention when studying cytology.
- Change in the central axis: the pair of microtubules that ran in the center of the cilium shaft terminates. A different structure is formed in its place—it is replaced by a hollow axis.
- Transformation of the peripheral framework: the peripheral pairs (doublets) of microtubules descend lower and penetrate into the cell cytoplasm. During this immersion, a third microtubule attaches to each doublet. Thus, initially paired structures are transformed into fully developed triplets, characteristic of the basal body structure.
Briefly: answers to topical questions
Which protein are the microtubules of cilia and basal bodies made of?
The microtubules of cilia and basal bodies are made of the protein tubulin. The microtubule wall is single-layered and consists of globular subunits of this protein. On cross-section, a ring of 13 subunits is formed. The structural unit of microtubules is the tubulin heterodimer. The polymerization of this protein ensures the formation of both central and peripheral microtubules (doublets in the axoneme and triplets in the basal body), creating the structural basis of motility organelles.
Which motor protein ensures the sliding of microtubules and the movement of the cilium?
The sliding of microtubules and the movement of the cilium are provided by the protein dynein. Dynein arms extend from each peripheral microtubule doublet toward the adjacent one. The attachment and detachment of dynein bridges cause adjacent doublets to shift relative to each other, which leads to the bending of the axoneme. For the spermatozoon tail, it is also noted that dynein possesses ATPase activity: it cleaves mitochondrial ATP and converts chemical energy into mechanical energy; in the genetic absence of dynein, spermatozoa are immobile.
By what mechanism does ciliary beating occur?
Ciliary beating occurs through the attachment and detachment of dynein bridges. This process causes the displacement, or sliding, of adjacent peripheral microtubule doublets relative to each other. As a result, the entire axoneme structure bends.
What functions does the basal body perform in the cell?
The basal body is located at the base of a cilium or flagellum and anchors them in the cytoplasm. In the region of the basal body, the central pair of microtubules terminates and is replaced by a hollow axis, while the peripheral pairs of microtubules penetrate into the cytoplasm and attach a third microtubule, forming triplets. In the organ of Corti, the preserved basal body performs a supportive function. In the bacterial flagellum, the basal body is an anchoring structure: its rings secure the flagellum to the plasma membrane and cell wall.
To remember the microtubule formula, use a numeric association: "A cilium is 9 by 2 plus 2 (doublets and center), and a basal body is 9 by 3 (triplets in the foundation)."
Class and exam questions
How are microtubules organized in the cross-section of the cilium shaft?
They are represented by 9 peripheral pairs (doublets) arranged in a circle, and one pair of central microtubules in the middle.
What is the main difference between the cross-section of the basal body and the cilium itself?
In the basal body, microtubules are organized not into doublets, but into triplets (3 each), and the central pair is absent.
What happens to the peripheral microtubules upon transition into the cytoplasm?
Peripheral doublets penetrate into the cytoplasm and attach a third microtubule to themselves, thereby forming the triplets of the basal body.
What else to study on this topic
- Axoneme of the flagellum and cilium
- Organization of the epithelial apical membrane
- Detailed structure of the transition zone
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