Extended chromatin fibers and chromatin organization

Studies of chromatin extensibility have revealed the flow of chromatin and DNA from cell nuclei and chromosomes in response to gravity or mechanical stretch following lysis by hypertonic saline and detergent solutions. Since this phenomenon was first reported, the technical methods by which extended chromatin fibers (ECFs) may be analyzed have been improved. These methods include topochemical assays, fluorescence in situ hybridization, immunofluorescence, electron microscopy, and polarization microscopy. Chromatin and DNA "halos" also have been studied in materials subjected to lysis, especially in a horizontal position or after cytocentrifugation. The analysis of ECF formation is useful not only as a tool for detecting the positioning of certain DNA signals on chromatin filaments, but also for describing diverse DNA-protein associations that may be related to varying transcriptional activities and chromatin supraorganization. A brief review of the methods and applications of ECF formation is presented here. We focus on light microscopy studies of ECF formation in mouse hepatocytes under different chromatin supraorganization and physiological conditions and in sperm cells with different DNA-protein complexes.