EVIDENCE AGAINST A LOOPED STRUCTURE OF THE INACTIVE HUMAN X-CHROMOSOME TERRITORY

Multicolor fluorescence in situ hybridization with a whole chromosome composite probe for the X-chromosome and microdissection probes for th e Xp and Xq arms, as well as for the Xp terminal, Xq terminal, and X c entromer specific subregional probes, was applied to three-dimensional (3D) preserved human female amniotic fluid cell nuclei. Confocal lase r scanning microscopy and three-dimensional image analysis demonstrate d distinctly separated Xp arm and Xq arm domains. 3D distance measurem ents revealed a high variability of intrachromosomal distances between Xpter, Xcen, and Xqter specific probes within both X territories. A 3 D distance measurement error of +/-70 nm was found in control experime nts using quartz glass microspheres labeled with different fluorochrom es. Our data argue against the hypothesis of Walker ct al. (1991, Proc . Natl. Acad. Sci. USA 88, 6191-6195) that a looped structure of the i nactive X territory is formed by tight telomere-telomere associations. (C) 1998 Academic Press.