In 1 cell and 2 cell stage embryos, we observed a tight association of this type of heterochro matin with NPBs/nucleoli, as previously described. This tight association does not however hold for all chromosomes, since pericentromeric heterochroma tin foci despite were also found at the nuclear periphery in inter Inhibitors,Modulators,Libraries phasic 1 cell embryos and escaping peripheral chromosomes Inhibitors,Modulators,Libraries are observed at condensation. Whether these are specific chromosomes remains unknown. this could be analyzed by chromosome territories painting. It was, however, quite surprising to find that, whenever pericentromeres were located at the nuclear periphery, rDNA signals were almost always associated with them. This confirms that rDNA genomic sequences are not automatically associated with NPBs.

Inhibitors,Modulators,Libraries It also suggests that, at least in early stages, NPBs are not basic nucleolar precursors, but may have another role and/or function. This hypoth esis is in agreement with the fact that oocyte nucleolar components are necessary for the reassembly of newly formed NPBs in both pronuclei after fertilization and for further embryonic development. However, the exact composition of these prominent compact fibrillar structures, which are present in fully grown oocytes and early embryos, is far from being com pletely deciphered. Different approaches have shown that they do not contain DNA, but rather RNA, nucle olar proteins, and non nucleolar spliceosomal factors. It is only during the first half of the 2 cell stage that components of the rDNA synthesis machinery are progressively assembled at the NPB surface, where the first rRNAs are synthesized at the mid/late 2 cell stage.

Remarkably, while a small but significant cell cycle dependent Inhibitors,Modulators,Libraries de crease in NPB number is observed Inhibitors,Modulators,Libraries at 1 cell and 2 cell stages, the decrease is more drastic in 4 cell embryos and is accompanied by a large increase in the median NPB volume. This might reflect a rapid transition in the NPBs function. Indeed, if the onset of rRNA synthesis was previously precisely timed, nothing is known concerning the dynamics of the other steps of rRNA maturation and pre ribosomal particles assembly. From a more structural point of view, the fact that the decrease in NPB number is associated with an increase in the median NPB volume, without a significant reduction in the overall volume, suggests the existence of a fusion process in early embryos.

A similar fusion process till could explain the slight decrease in NPB number at late 1 cell, as suggested by the rDNA bridges sometimes observed between 2 NPBs. Remarkably, the increase of the NPB volume stops at the 4 cell stage and is not observed anymore at the 8 cell stage. This would fit with the fact that active polymerase I transcription and related processing machineries are functionally organized at the NPB surface starting from the end of the 4 cell stage.