Questions about cilia

Q.I just had a quick question with regards to cilia configuration. On the slide it states that cilia insert into basal bodies with 9 triplets of microtubules. Does that mean 3 axoneme are bundled into a unit and a cilia contains 9 of those units (so a total of 27 axoneme in one cilia)?

A. The arrangement of microtubules at the insertion point into the basal body is a confusing (and mysterious) feature of cilia. So you’re right on in trying to make some sense out of it!

No – the axonemes aren’t bundled in threes – there is just one axoneme (with its characteristic 9 + 2 arrangement of microtubules) per cilium. But you’re right about the insertion point: each cilium inserts into its basal body with an arrangement of microtubules consisting of 9 triplets (and no central pair).

Just exactly how the 9 + 2 arrangement of microtubules in the axoneme changes to the 9 triplet arrangement at the basal body is unclear – there isn’t a consensus on how the transformation happens. So you’re right to wonder about it! Unfortunately, there isn’t a good answer – but at least now you know you’re not missing something important.

 

Q. In lecture we talked about apical structures such as microvilli and cilia. It says that cilia are inserted into basal bodies, what are those? They aren’t part of the basal lamina even though it has the word basal correct? I am a bit confused because they are apical structures.

A. Great question! The term basal means “bottom” – and we throw it around a lot! In “basal bodies” basal refers to the bottom of the cilium! So basal bodies are little structures at the bottom (base) of each cilium. Basal bodies are the places where axonemes (the central structures of cilia) start growing; they also serve as anchoring structures for the completed axonemes. So yes: the basal bodies are apical structures, and they are not part of the basal lamina.

 

Q. Why are the 2 central microtubules not a pair but the 9 pairs on the outside are considered pairs (instead of 18 microtubules) in the cilia? Is that because they are of a different type?

A. Thank you for bringing this up! It is confusing that we call this a “9+2” arrangement when it looks like there are 9 pairs of microtubules around the outside, and one pair of microtubules in the middle! Seems like it should be either “9+1” or “18+2.” There’s a good explanation, though. Here’s a diagram of an axoneme – we’ll talk about the “2” part and the “9” part separately below.

 

The “2” part of the axoneme
The two microtubules in the center of the axoneme are totally separate from each other. So they can be considered to be 2 separate, independent units.

The “9” part of the axoneme
The 18 microtubules around the outside are bound together, two at a time – so they can be thought of as 9 separate, independent units.

It turns out that in each of these 9 units, one of the microtubules is incomplete. If you look at the microtubules circled in red, you can kind of see that the top one looks round, and the bottom one looks like it’s not quite round, and has sort of just latched on to the top one. The bottom one is actually not a fully-formed microtubule (if you pulled the two apart, the top one would be round, and the bottom one would look like a C-shaped structure). So we really shouldn’t even call these guys “pairs” since they don’t consist of two fully-formed microtubules. The official name is “doublet” – and that is a little better than “pair,” I guess.

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