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Fig. 5. 3D reconstruction of Xi heterochromatin. (A) Non-processed, 60-nm serial sections. Arrows indicate the Xi; arrowheads in zoomed insets point to specific large-scale Xi chromatin motifs of different dimensions: 30 nm (section 7), 215 nm (section 8) and 550 nm (section 10). Nuclear pores (NP) are present in the nuclear-envelope region adjacent to Xi heterochromatin and connect with interchromatin tunnels of nucleoplasm within the Barr-body volume. (B-D) 21 intensity-inverted and normalized (to be proportional to electron scattering `mass') serial sections were used for 3D reconstruction of the Barr body. See supplementary material Movie 1 for the complete serial section stack. (B) Maximum intensity projection of aligned sections (1300-nm depth) gives the impression of a more-solid Barr body, similar to that visualized by LM. (C) Orthogonal views show large nucleoplasm channels penetrating throughout the Barr-body interior and interconnecting with NPs. Images for this reconstruction were median filtered and interpolated in z to provide uniform x-y-z voxel dimensions. Green line, y-axis; blue line, x-axis; red line, z-axis. (D) An example of the input image after additional median filtering (originally section 7) used for solid model rendering shown in D' and D'', and for stereo-pair projections shown in D''' (also see supplementary material Fig. S6). All provide further illustration of the porous internal structure of Xi heterochromatin. See rotating solid model of the Barr body in supplementary material Movie 2. Scale bar: 1 µm (three left-most panels in A; B-D).

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