Many microorganisms and cell types contain cilia or flagella, which are whip-like structures that propel a motile cell, move external materials around a fixed cell, or act as non-motile sensory elements. Cilia and flagella have the same basic structure and differ only in that flagella are longer than cilia. Both are rooted to the cell at the basal body, which is a specialized form of a structure called a centriole.
A basal body is a centriole, which is a cylinder-shaped structure composed of microtubules that in turn contain up to 13 protofilaments surrounding a hollow center. The protofilaments are polymers of the protein tubulin. The microtubules in a basal body appear as a set of nine triplets. Each triplet contains three microtubules, labeled A, B and C, attached to each other along their lengths. The nine triplets form a hollow cylinder situated just below the cell membrane. A basal body serves as the root from which flagella and cilia sprout.
Microtubule Organizing Center
The basal body is an example of a microtubule-organizing center, or MTOC. These structures are unique because they consist of the gamma form of tubulin. The tubulin proteins in flagella and cilia are of the alpha and beta variety. As an MTOC, the basal body stabilizes the microtubules and supports their movement. The gamma tubulin of an MTOC joins with other proteins to form ring complexes that provide a binding site for the microtubules.
The basal body transitions into a structure called an axoneme, which forms the skeleton of the flagellum or cilium. Within the transition zone, the C microtubules of the basal body terminate. The remaining nine sets of A and B tubules extend through the transition zone and help form the axoneme. Motile cilia and flagella, such as those found in the human trachea, have axonemes that contain two additional microtubules running up the central axis. Non-motile cilia lack the central microtubules.
Basal Body Functions
Basal bodies perform several functions important to cilia and flagella activities. The nine basal body microtubules provide the template for building the axoneme. The basal body also orients and positions the cilium or flagellum, which is critical to the correct movement of fluids within the axoneme. Basal bodies regulate the entry of proteins into the axoneme and play a role in cell division. Any basal body malfunctions can lead to various diseases.