Figure 2(A, B, C) Cryo-electron tomograms of the proximal regions of a P. tetraurelia centriole (A), a N. gruberi centriole (B), and a H. sapiens procentriole (C). White arrows denote a broken cartwheel; procentriole, PC; mature centriole, MC; Scale bar, 100 nm. Note that most N. gruberi and H. sapiens centrioles were heavily compressed during the cryo-EM preparation, as previously described (Guichard et al., 2010). The displayed N. gruberi centriole illustrates the damage caused by compression. The periodicities of N. gruberi and H. sapiens cartwheels were measured only on regions that were not damaged (D) Cross sections from cartwheel-containing region of P. tetraurelia centrioles. Scale bar, 50 nm. (E) Zoomed side view of cartwheel from P. tetraurelia, displaying the central hub (CH) and several cartwheel inner densities (CIDs), white arrow. Scale bar, 25 nm. (F) Cross section from cartwheel-containing region of N. gruberi, (G) Zoomed side view of cartwheel from N. gruberi displaying the central hub (CH) and several cartwheel inner densities (CIDs), white arrow. (H) Cross section from cartwheel-containing region of H. sapiens centriole. (I) Zoomed side view of cartwheel from H. sapiens, displaying the central hub (CH) and several cartwheel inner densities (CIDs), white arrow. (J) Longitudinal periodicity of the central hub and CIDs in P. tetraurelia, N. gruberi, and H. sapiens. Mean values are displayed above data range. Black lines indicate the mean and the standard deviation. Blue data points are measured distance between individual units of the central hub, red data points are measured distances between individual units of the cartwheel inner density. P. tetraurelia, central hub, n = 10, mean = 4.3 +/- 0.38 (S.D); N. gruberi, central hub, n = 10, mean = 4.4 +/- 0.53 (S.D); H. sapiens, central hub, n = 10, mean = 4.2 +/- 0.68 (S.D); P. tetraurelia, cartwheel inner density, n = 8, mean = 8.4 +/- 1.3 (S.D); N. gruberi, cartwheel inner density, n = 8, mean = 8.3 +/- 1.8 (S.D); H. sapiens, cartwheel inner density, n = 8, mean = 8.1 +/- 2.5 (S.D). (K Proximal protrusion length of the cartwheel beyond the microtubule triplets in C. reinhardtii, P. tetraurelia, N. gruberi, and H. sapiens. Internal cartwheel inside the microtubule barrel, dark blue (INT); external cartwheel beyond the microtubule wall, light blue (EXT). Mean values are displayed above each bar plot with the black lines indicating the standard deviation (K). Mature C. reinhardtii, n = 4, external cartwheel length = 37.6 +/- 3.4, internal cartwheel length 74.2 +/- 16.6; C. reinhardtii procentriole, n = 2, external cartwheel mean length = 52.8 +/- 22.0, internal cartwheel length 26.5 +/- 29.7; P. tetraurelia, n = 23, external cartwheel mean length = 11.5 +/- 9.2, internal cartwheel length 66.6 +/- 15.7; N. gruberi, n = 19, external cartwheel mean length = 41.0 +/- 22.5, internal cartwheel length 259.0 +/- 87.2; human procentriole, n = 7, external cartwheel length 23.1 +/- 10.3, internal cartwheel length 154.7 +/- 47.7. Error reported is standard deviation. Scale bar, 50 nm. The start of each microtubule wall is delineated by a dashed white line (L). Mature C. reinhardtii, n = 4, external cartwheel length = 37.6 +/- 3.4, internal cartwheel length 74.2 +/- 16.6; C. reinhardtii procentriole, n = 2, external cartwheel mean length = 52.8 +/- 22.0, internal cartwheel length 26.5 +/- 29.7; P. tetraurelia, n = 23, external cartwheel mean length = 11.5 +/- 9.2, internal cartwheel length 66.6 +/- 15.7; N. gruberi, n = 19, external cartwheel mean length = 41.0 +/- 22.5, internal cartwheel length 259.0 +/- 87.2; human procentriole, n = 7, external cartwheel length 23.1 +/- 10.3, internal cartwheel length 154.7 +/- 47.7. Error reported is standard deviation. Scale bar, 50 nm.