Axoneme

Numerous eukaryotic cells carry whip-like appendages (cilia or eukaryotic flagella) whose inner core consists of a cytoskeletal structure called the axoneme. The axoneme serves as the "skeleton" of these organelles, both giving support to the structure and, in some cases, causing it to bend. Though distinctions of function and/or length may be made between cilia and flagella, the internal structure of the axoneme is common to both.

Structure

Inside cilia and flagella is a microtubule-based cytoskeleton called the axoneme. The axoneme of primary cilia typically has a ring of nine outer microtubule triplets (called a 9+0 axoneme), and the axoneme of a motile cilium has two central microtubules in addition to the nine outer doublets (called a 9+2 axoneme). The axonemal cytoskeleton acts as a scaffolding for various protein complexes and provides binding sites for molecular motor proteins such as kinesin II, that help carry proteins up and down the microtubules.

Motile cilia

The building-block of the axoneme is the microtubule; each axoneme is composed of several microtubules aligned in parallel. To be specific, the microtubules are arranged in a characteristic pattern known as the “9x2 + 2," as shown in the image at right. Nine sets of "doublet" microtubules (a specialized structure consisting of two linked microtubules) form a ring around a "central pair" of single microtubules.