So the microfilaments and intermediate filaments do not contain a hollow center but the micro tubules do contain a hollow center and that's exactly why they're called tubules because they create this hollow like tube. Now, just like our microfilaments, our micro tubules which are 23 nm in diameter so notice they are larger than the microfilaments or the intermediate filaments. The microtubules have a positive end as well as the negative end and the positive end is the end from where our microtubule grows. The negative end inside an animal cell is usually found within a region given by MTOC where MTOC stands for the microtubule organizing center. And within our eukaryotic animal cells this is known as our centrosome. So it's the centrosome region that contains our centrioles.