Dendritic-like microtubule organization from varicosities is a conserved feature of human bRG cells. (A) Schematic of protocol as described in Materials and methods. (B) A slice of human fetal cortex electroporated with EB3-GFP. CP, cortical plate; SVZ, subventricular zone; VZ, ventricular zone. (C and D) The basal processes of human bRG cells display varicosities, visualized by overexpression of GFP (C) and immunofluorescence against Vimentin (D). (E) Live imaging of a human bRG cell showing appearance of de novo EB3 comets (arrowheads) in a basal process varicosity. (F) Quantification of the rate of EB3 comet formation in basal process shafts and varicosities of human bRG cells at gestational week (GW) 18 (n = 24 shafts and 30 varicosities from 20 cells). (G) Quantification of the overall directionality of EB3 comets in the basal process of human bRG cells (n = 205 comets from 7 cells). Mouse data from Fig. 1 F are shown for comparison. (H) Quantification of varicosity size in mouse (E14.5) and human (GW 14 and 20) tissue (n = 55, 129, and 772 varicosities from 11, 36, and 82 cells, respectively). (I) Quantification of the distance between individual varicosities along the basal process in mouse (E14.5) and human (GW 14 and 20) tissue (n = 260 and 605 varicosities from 36 and 82 cells, respectively). Error bars indicate SD. ****, P < 0.0001 by Mann-Whitney tests. Experiments were performed in two independent human fetal brains and three independent mice.