Abstract
The improvement of the osmium-thiocarbohydrazide (OTO) technique which is necessary for utilizing chromosome preparations made by the air-drying method for scanning electron microscopy (SEM) was successfully carried out. It was found that air-drying at lower temperature, freshness of air-dried preparations and an appropriate amount of osmium deposit were desirable to make useful SEM preparations. In order to examine higher order structures of metaphase chromosomes, mammalian cells cultured under the different experimental conditions were used. The resultant SEM images together with stereo viewing revealed that metaphase packing is achieved by the compaction through helical coiling of a chromatid fiber of 200-400 nm in diameter which is in turn composed of radial loops of a 30 nm chromatin fiber and that the looped chromatin fiber is quite stable but highly flexible in nature.
