Organization of the X and Y chromosomes in human, chimpanzee and mouse pachytene nuclei using molecular cytogenetics and three-dimensional confocal analyses

We used multicolour fluorescence in-situ hybridization on air-dried pachyte ne nuclei to analyse the structural and functional domains of the sex vesic le (SV) in human, chimpanzee and mouse. The same technology associated with 3-dimensional analysis was then performed on human and mouse pachytene nuc lei from cytospin preparations and tissue cryosections. The human and the c himpanzee SVs were very similar, with a consistently small size and a high degree of condensation. The mouse SV was most often seen to be large and po orly condensed, although it did undergo progressive condensation during pac hynema. These results suggest that the condensation of the sex chromosomes is not a prerequisite for the formation of the mouse SV, and that a differe nt specific mechanism could be responsible for its formation. We also found that the X and Y chromosomes are organized into two separate and non-entan gled chromatin domains in the SV of the three species. In each species, tel omeres of the X and Y chromosomes remain clustered in a small area of the S V, even those without a pseudoautosomal region. The possible mechanisms inv olved in the organization of the sex chromosomes and in SV formation are di scussed.