Distribution of the pericentromeres, centromeres, and rDNA FISH signals in late 1-cell stage embryos. 1-cell embryos at the PN4 stage (collected at 27 hphCG) were processed with pericentromeric probes (red) and either centromeric (green, upper panel A/B/C) or rDNA probes (green, lower panel D/E/F). DNA was counterstained with Yopro-1 (grey). Upper panel: (A/B) In both PNs, pericentromeres form more or less complete shells around the NPBs, in which the centromeres are embedded. Pericentromeres are also found at the nuclear periphery, associated with centromeric signals (see enlargement of A), and can form filaments with a “beads on a string” appearance (see enlargement of B). (C) 3D reconstruction of the same nuclei. Lower panel: (D, E) Most of the rDNA signals are around the NPBs. However, there are occasionally some signals associated with pericentromeric filaments (extending from the NPBs towards the nuclear periphery) as well as rDNA signals joining two NPBs. (F) 3D reconstruction of E. Bar = 5 μm.