Ultrastructural and immunocytochemical analysis of the XY body in rat and Guinea pig.

The formation of the XY body involves the compaction of the extended chromatin to form a mesh of fibrogranular structures. During this process the ribonucleoprotein particles (RNP), which were associated with the chromatin filaments progressively disappear. High resolution immunolocalization indicates that the mature XY body does not contain RNA polymerase II, hnRNPs, or snURNPs. Occasionally chromatin fibrils extend outside of the XY body. These fibrils are frequently associated with nascent RNP fibrils and granules indicating that not all the DNA of the sex chromosomes is transcriptionally inactive. However, transcription is located outside the sex body. The recombination protein Dmc1 is present in nodules associated with the unpaired chromosomal axes of the sex chromosomes located in the XY body. Cytochemical staining methods and in situ hybridization at electron microscopic level show that RNA is present in the unpaired chromosomal axes suggesting that the presence of RNA in the chromosomal axes and in forming synaptonemal complexes is related with the process of final pairing. The sex body and the nucleoli associated with it do not interweave and do not exchange RNA or DNA-containing filaments. These observations indicate that the spatial relation between these structures is just a close proximity, which is, however, very frequent.