J. Cell Sa. 72, 75-87 (1984) 75 Printed in Great Britain © The Company of Biologists Limited 1984 NUCLEAR PORES AND INTERPHASE CHROMATIN: HIGH-RESOLUTION IMAGE ANALYSIS AND FREEZE ETCHING C. NICOLINI Temple University, Philadelphia, U.SA. and Eminent Chair of Biophysics, Faculty of Medicine, University of Genova, Italy G. VERNAZZA, A. CHIABRERA Istituto di Elettrotecnica, Sezione di Ingegneria Biofisica ed Elettronica, Universita di Genova, Italy I. N. MARALDI AND S. CAPITANI Istituto di Anatotnia, Universita di Bologna, Italy SUMMARY Computer-enhanced analysis of electron micrographs of thin-seCtioned rat liver nuclei, combined with three-dimensional reconstruction of the same Feulgen-stained nuclei, points to a unique clustering of chromatin DNA fibres near the nuclear border. Computer-enhanced image analysis has been applied to electron micrographs of the envelopes of the same rat liver nuclei prepared by freeze etching and a few essential geometrical parameters characterizing the pores and their distribution have been determined. During interphase, clusters of nuclear pores, closely paralleling the clustering of membrane-attached chromatin fibres, have been identified on the envelope, the number of these being similar to the number of homologus pairs of metaphase chromosomes. Furthermore, rapid changes induced in chromatin distribution appear to be associated with rapid changes in pore number, but not in the number of pore clusters. INTRODUCTION The double membrane surrounding the nucleus contains as its most conspicuous features the nuclear pore complexes, which have been assumed to be the sites of molecular and ionic exchange between nucleus and cytoplasm (Maul, 1977). Curiously, these channels in the nuclear envelope are referred to as pores, even if they are usually filled with a dense plug (Krohne, Franke & Schree, 1978; Unwin & Milligan, 1982). Biochemical characterization suggests that the nuclear pore complex is formed only of a few majdr polypeptides and some RNA (Krohne et al. 1978), but no direct identification has yet been possible within the boundaries of this structure and all speculations about its function are based on scanty, circumstantial and at times contradictory data obtained by limited, ultrastructural and autoradiographic methods (Franke & Scheer, 1970; Maul, 1977). While the complex has long been known to consist of a cylindrical assembly that spans the inner and outer nuclear membranes and is arranged with octagonal symmetry about a central axis, new detailed informa- tion has recently been obtained from electron micrographs of an intact oocyte nuclear Key words: nuclear pores, chromatin, image analysis.