Primary cilium

Primary cilia are small filamentous protrusions from the cell body. They are about 300-600 nm thick, and occure in many different lengths, ranging from a few to tens of micrometers, and emerge solitary on most body cells. In the middle of the primary cilium there is a characteristic microtubule cytoskeleton that emerges from the mother centriole, which serves as a template during ciliogenesis. This central ciliary cytoskeleton structure is called the axoneme and consists of nine microtubule doublets. The axoneme is also a template for protein transport in the cilium, which is driven by dyneins and kinesins and the intra-flagellar transport (IFT) machinery. The mother centriole with an attached cilium is commonly called a basal body.

The primary cilium represents a small compartment that is separated from the cell body by the transition zone, which is a sieve like barrier limiting exchange of proteins, molecules, and lipids between the cilium and the cytosol. Albeit the ciliary membrane is a continuum of the plasma membrane, the ciliary membrane composition is very different from the plasma membrane. Protein localization to primary cilia is tightly regualted by an activate transport machinery into which many components of the intra-flagellar transport (IFT) machinery engage. Thereby, primary cilia represent a unique cellular compartment. Since the protein transport mechanisms are dynamic, the composition of primary cilia is very dynamic as well.

It is believed that the primary cilium represents a tiny reaction vessel, where the cell puts certain receptors and signaling components together to build very sensitive signaling cascades. Albeit the underlying mechanisms and the function of the primary cilium is still incompletely understood, it is commonly accepted that a major function of the primary cilium is to serve the cell as an antenna, receiving signals from the environment through receptors, locally processing signals with signaling cascades, and relaying the signal in its processed form through the cell body.

Immunofluorescent staining

Since the primary cilium emerges from the mother centriole and is degraded before and build up after cell division it can be seen mostly in non-mitotic cells, where it appears as a a slender, often curved, line emerging from the centrosome. Since primary cilia stick into the extracellular space they often have various orientations and thus, it is required to investigate them in 3D and across multiple focus planes.

Read more about other substructures of the primary cilium at primary cilium.