Ultrastructure of clathrin lattices in the three cell types. (A) Platinum replicas of representative curved, spherical, and flat clathrin structures in hESCs, NPCs, and fibroblasts, respectively. Bars, 200 nm. (B) Area of clathrin structures in the three different cell types (mean ± SEM). ***, P < 0.001; **, P < 0.01; Mann-Whitney test (three to four replicas, total number of structures analyzed: hESCs, n = 245; NPCs, n = 100; fibroblasts, n = 337). (C) Relative frequency of spherical (black), curved (white) and flat (gray) clathrin structures in hESCs, NPCs, and fibroblasts (three to four replicas, total number of structures analyzed: hESCs, n = 245; NPCs, n = 100; fibroblasts, n = 337). (D) Representative EM micrograph showing clustering of curved clathrin structures in fibroblasts. Bar, 200 nm. (E and F) Tf uptake in hESCs (black line), NPCs (dashed line), and fibroblasts (dotted line). For each time point, at least 2,000 cells were analyzed. Fluorescently labeled Tf levels were measured by flow cytometer (mean ± SD). ***, P < 0.001; **, P < 0.01; unpaired two-tailed t tests (three to four independent experiments, n = 2,000 cells) (F) Normalized amount of Tf uptake relative to the amount of Tf bound to the receptor at the cell surface at time 0 (mean ± SD). ***, P < 0.001; unpaired two-tailed t test (three to four independent experiments, n = 2,000 cells per experiment).