We have recently taken some super-resolved images of live CHO-K1 cells expressing eGFP-tubulin. Some of the microtubule strands appear in the super-resolved image to be actually two tubules winding around each other, forming a sort double helix. Are microtubules expected to form such structures? Are there any good references (e.g. EM images) which we could compare with the structures we observe? Thanks!