Cilia and flagella are complex cellular structures responsible for motility and fluid movement. Their structural organization is key to their function, and the arrangement of microtubules within them varies between different regions such as the basal body and the axoneme (the main shaft of cilia and flagella).

Structure of Cilia and Flagella

Basal Body

• The basal body acts as a nucleation site for the growth of the axoneme.
• It has a “9+0” arrangement of microtubule triplets. Each triplet consists of one complete microtubule (A-tubule) and two incomplete microtubules (B-tubule and C-tubule).

Axoneme

• The axoneme is the shaft of the cilium or flagellum and has a “9+2” arrangement of microtubules. This means it has nine doublets of microtubules arranged in a circle around two central singlet microtubules.
• Each doublet consists of one complete (A-tubule) and one incomplete microtubule (B-tubule).

Singlet Microtubules in the Hub

Presence in the Axoneme

The central pair of singlet microtubules (the “2” in the “9+2” arrangement) plays a crucial role in the movement of cilia and flagella. They are involved in regulating the dynein arms’ activity on the outer doublets, which generates the sliding motion that is converted into bending.

Absence in the Basal Body

The basal body does not contain the central pair of singlet microtubules because its primary role is to anchor the cilium or flagellum to the cell and to organize the microtubules’ formation in the axoneme. The presence of triplets in the basal body provides a sturdy and stable foundation necessary for the growth and structural integrity of the axoneme. The central singlets are not required in this anchoring and organizing function.

Functional Implications

Axoneme Functionality

The central pair of singlets is essential for the proper function of the axoneme, particularly in the coordinated movement and bending of the cilia and flagella. They help regulate the activity of dynein arms through interactions with radial spokes and other regulatory complexes.

Basal Body Structure

The basal body’s main function is to provide a robust base for the cilium or flagellum. The triplet structure is well-suited for this role, as it is more stable and can better withstand the mechanical stresses involved in the anchoring and nucleation process. The complexity of a “9+0” triplet arrangement does not necessitate central singlets since their function is purely structural and organizational, not motile.

Summary

In summary, the singlet microtubules are present in the hub of cilia and flagella (axoneme) to facilitate motility through their regulatory role, whereas they are absent in the basal body because their function is not needed for the structural role that the basal body plays.