Doppelttubuli

Doppelttubuli, or doublet microtubules, are structural units of the axoneme that forms the core of most motile cilia and flagella. Each outer doublet consists of two microtubules: the complete A-tubule and the incomplete B-tubule, which shares part of its wall with the A-tubule.

In a typical motile cilium or flagellum, nine outer doublets are arranged in a ring around a central pair of single microtubules, creating the classic 9+2 axoneme pattern. The A-tubule is a full microtubule with 13 protofilaments, while the B-tubule is an incomplete microtubule derived from the A-tubule and usually contains about 10 protofilaments.

The doublets are interconnected by protein structures, including dynein motor arms that extend along the surface of the doublets. Dynein-driven sliding of adjacent doublets against each other, coordinated by nexin links and radial spokes, is converted into the bending motion that powers ciliary and flagellar beating.

The arrangement of outer doublets is characteristic of motile cilia and flagella (often 9+2). Some cilia, such as many primary (non-motile) cilia, exhibit a 9+0 configuration and may lack the central pair, though the outer doublets usually remain present. Variations in the assembly or function of doublets, dynein arms, or linking proteins can disrupt motility and contribute to ciliopathies.

The term Doppelttubuli is used in German-language literature to describe these structures; in English-language texts they are commonly referred to as doublet microtubules.