Calibrated intensity profiles from LM by electron tomography data show the distribution of soluble and polymerized tubulin. (A) Concentrations of soluble (yellow), polymerized (red), and total (blue) tubulin concentration along the spindle axis after calibration. The soluble tubulin concentration is enriched at the centrosome and shows a peak concentration of ∼405 µM at r = 0.8 µm. Error bars are SD. (B) Fractions of polymerized and soluble tubulin analyzed in intervals along the radial distance up to the data range of the EM reconstructions and the total concentration in the remaining part. The embryo is loaded with a total tubulin concentration of ∼47 µM. Up to 8 µm, the overall tubulin is on average 68 µM, of which 22 µM is polymerized.