Cytochemical Localization of DNA Loop Attachment Sites to the Nuclear Lamina and to the Inner Nuclear Matrix

The rat liver nuclear matrix, obtained by endogenous nuclease digestion and extraction with low and high ionic strength media, contains residual DNA fragments that are considered to represent the attachment sites of the chromatin domains to the nucleoskeleton. These sites, protected against nuclease digestion by their binding with the nucleoskeleton proteins, should be either mainly linked to the peripheral lamina or to the inner nuclear matrix. The DNA fragment distribution at the level of the different components of the nuclear matrix has been evaluated in samples embedded in Epon and in hydrophilic resins by means of the DNase-gold technique. The labeling obtained suggests that the chromatin loops are prevailingly associated with the interior of the matrix; in fact about twice of the label is present in the inner matrix with respect to the peripheral lamina area. These results confirm the hypothesis that in interphase the chromatin maintains an organization similar to that of chromosomes, with loops radiating from a central scaffold, instead of being mainly attached to the lamina as otherwise suggested.