Here is a great question about the 9+2 arrangement of microtubules inside cilia.
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.”
I think it’s easier to think about the 9+2 thing if we forget for a moment about using the word “pair,” and use the word “unit” instead. Here’s a nice diagram of the axoneme (fancy name for the arrangement of microtubules inside cilia).
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.
Not only that, but in each of these 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.
I hope that helps. I think we should get rid of the word “pair” totally and tell all the textbooks to use “unit.”