Chromosoma (Berl.) 82, 55 64 (1981) CHROMOSOMA 9 Springer-Verlag 1981 Structure of Human Chromosomes Visualized at the Electron Microscopic Level John C. Hozier 1, Leo T. Furcht 2 and Gwen Wendelshafer-Grabb 2 Department of Biological Sciences, Florida Institute of Technology, Melbourne, Florida 32901, U.S.A., 2 Department of Laboratory Medicine and Pathology, University of Minnesota Medical School, Box 198 Mayo Memorial Building, Minneapolis, Minnesota 55455, U.S.A. Abstract. A newly developed technique allows cytological (light microscope level) chromosome preparations to be examined at the electron microscopic level. Ultrathin (50 rim) sections of highly condensed Hela cell metaphase chromosomes show the characteristic mitotic chromosome morphology. In addition a fibrous network (presumably chromosome fibers) can be seen within them. Fibers appear to be gathered at foci along each chromatid. Treatment of chromosomes with trypsin in a trypsin/G-banding procedure reduces the amount of staining material at the electron microscopic level and results in more prominent foci. Thicker (100 nm) sections of less con- densed chromosomes prepared from human lymphocytes display a banding pattern similar to G-banding, even without pretreatment with proteases. Introduction The morphology of mammalian mitotic chromosomes has been studied both by light and electron microscopy revealing two important general features of chromosome organization. At the light microscopic level the distinctive and specific chromosome banding patterns, observed in acetic acid/methanol fixed mitotic chromosomes using a wide variety of staining techniques, have greatly augmented our understanding of chromosome organization (Caspersson et al., 1968; Evans, 1978). These techniques have been applied with great success to the identification of individual chromosomes and have important clinical uses in identifying chromosomal defects associated with congenital and malig- nant diseases. At the electron microscopic level, mitotic chromosomes prepared in ways designed to preserve their native conformation show that chromosomes are fibrous structures (Stubblefield and Wray, 1971; Bahr and Golomb, 1974). Chromosome fibers, about 20 nm in diameter, are arranged in loops that extend transversely from the chromatid axis. However the relationship between chromosome bands seen by light microsco- py and the arrangement of chromosome fibers observed by electron microscopy is not readily analyzable by either technique alone. Whole-mount electron micros- 0009-5915/81/0082/0055/$02.00