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 pachytene nuclei to analyse the structural and functional domains of the sex vesicle (SV) in human, chimpanzee and mouse. The same technology associated with 3-dimensional analysis was then performed on human and mouse pachytene nuclei from cytospin preparations and tissue cryosections. The human and the chimpanzee 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 poorly condensed, although it did undergo progressive condensation during pachynema. These results suggest that the condensation of the sex chromosomes is not a prerequisite for the formation of the mouse SV, and that a different specific mechanism could be responsible for its formation. We also found that the X and Y chromosomes are organized into two separate and non-entangled chromatin domains in the SV of the three species. In each species, telomeres of the X and Y chromosomes remain clustered in a small area of the SV, even those without a pseudoautosomal region. The possible mechanisms involved in the organization of the sex chromosomes and in SV formation are discussed.