High resolution analysis of interphase chromosome domains

Chromosome territories need to be well defined at high resolution before functional aspects of chromosome organization in interphase can be explored. To visualize chromosomes by electron microscopy (EM), the DNA of Chinese hamster fibroblasts was labeled in vivo with thymidine analogue BrdU. Labeled chromosomes were then segregated during several cell cycles to obtain nuclei containing only 2 to 3 labeled chromosomes. Subsequent immunocytochemical detection of BrdU allowed analysis by EM of chromosome territories and subchromosomal domains in well preserved nuclei. Our results provide the first high resolution visualization of chromosomes in interphase nuclei. We show that chromosome domains are either separated from one another by interchromatin space or are in close contact with no or little intermingling of their DNA. This demonstrates that, while chromosomes form discrete territories, chromatin of adjacent chromosomes may be in contact in limited regions, thus implying chromosome-chromosome interactions. Chromosomes are organized as condensed chromatin with dispersed chromatin extending into the interchromatin space that is largely devoid of DNA. The interchromatin space, which is known to be involved in various nuclear functions, forms interconnecting channels running through and around chromosome territories. Functional implications of this organization are discussed.