Demonstration of fibrous components in hepatic interphase nuclei by high resolution scanning electron microscopy

Three-dimensional ultrastructure of apoptotic nuclei in rat prostatic epithelial cells revealed by a quick-freezing and deep-etching method

BACKGROUND

To clarify intranuclear apoptotic changes, we have investigated chromatin organization in apoptotic nuclei of castrated rat prostatic cells by a quick-freezing and deep-etching (QF-DE) method.

METHODS

The ventral prostates taken from intact and castrated adult male rats were investigated by light microscopy, in situ end-labeling (ISEL) technique, conventional electron microscopy, and the QF-DE method.

RESULTS

In control nuclei, the chromatin fibers were uniformly distributed and formed a network structure. In apoptotic nuclei, destruction of such chromatin networks was detected, which was clearly seen by the QF-DE method. Although it first appeared spotty in the apoptotic nucleus, definite destruction of the intranuclear network occurred in the nuclear center at later stages, and broken fibrous structures were condensed along the nuclear margin. The ISEL technique was applied to the QF-DE method. Localization of damaged DNA fragments could three-dimensionally be detected on replica membranes.

CONCLUSIONS

Intranuclear chromatin organization in apoptotic cell death of rat prostates was observed by the QF-DE method. We could examine early-stage apoptotic nuclei at an electron microscopic level, which would not be clarified by other conventional methods.

Neutron diffraction of chromatin in interphase nuclei and metaphase chromosomes

We have used neutron diffraction to study chromatin structure in interphase nuclei and metaphase chromosomes as a function of decreasing ion concentration. Aliquots of a suspension of rat liver nuclei prepared in a polyamine-free buffer were washed in buffers of 1/3, 1/6 and 1/12 if the original concentration of monovalent and divalent cations (40 mM KCl; 20 mM NaCl; 1.2 mM MgCl2). After the first dilution step (1/1 to 1/3), only small changes occurred in the diffraction pattern. They can be interpreted by a loosening of the original structure, i.e. by the formation of isolated buffer-filled spaces with an overall size of the order of 35-45 nm. Drastic changes in the diffraction pattern were observed, however, when the nuclei were washed in the more diluted buffers (1/6 and 1/12). The profiles of the distances distribution functions indicate the formation of supranucleosomal particles with an overall diameter of 40-50 nm. The compact chromatin structure disassembled directly into these fundamental structural units. Structural transformations in the Chinese hamster ovary metaphase chromosomes were induced by diminishing the Ca2+ ion concentration of the buffer from originally 3.0 mM to 0.3 mM and/or by increasing the pH value of the buffer from originally 7.0 up to 8.0. The neutron diffraction patterns remained essentially unchanged during these treatments, i.e. the decondensation of the chromosomes as observed in the light microscope is not accompanied by disassembly at the ultrastructural level between 2 nm and 150 nm.

Scanning electron microscopy of Giemsa-stained chromosomes and surface-spread chromosomes

Giemsa-stained chromosomes as prepared for light microscopy, and including G-banded, C-banded, and FPG-stained chromosomes, were examined by scanning electron microscopy. Although suitable for light microscopy, these chromosomes were too flat for a close examination of their fine structure by scanning electron microscopy. The surface of Giemsa-positive regions was rough and bright, whereas that of unstained or poorly stained regions was smoother and less bright. Giemsa-staining, therefore, seems to produce the bulkiness of the chromosomes. On topographical examination by scanning electron microscopy, the transparent chromosomes as observed with the light microscope proved to be "footprints". Stereographical examinations of surface-spread chromosomes showed that minimally stretched chromosomes were composed of a mass of nodular and twisted looping fibers with an average diameter of about 300 A. The substructure of these chromosome fibers was not determined. The kinetochore region was discernible as a constriction in the mass of the chromosome fibers, and was distinguishable from gaps by the presence of several chromosome fibers parallel to the axis of the chromatid. The organization of the chromosome fibers, however, was disordered rather than regular.

Fluorescence polarization of stretched polytene chromosomes stained with acridine orange

The molecules of the fluorescent dye acridine orange (AO) bind to DNA in such a way that the absorption and emission dipoles lie on a plane perpendicular to the DNA axis. For this reason, definite fluorescence polarization should correspond to each mode of spatial DNA packing. A chromosome, considered as an axially symmetrical ensemble of DNA, was characterized by two experimental parameters, P parallel and P perpendicular, i.e., by polarizations of fluorescence excited by light polarized parallel and perpendicular to the symmetry axis. In view of the sequential order in the packing levels of DNA fiber in a chromosome, it was suggested that, under mechanical stretching, the highest level is disrupted first, then the others, in the order of their sequence. Isolated chromosomes of Chironomus thummi were stained with AO and stretched with needles of a micromanipulator. From the changes of P parallel and P perpendicular measured during stretching it was concluded the polytene chromosome bands have three, at least, DNA packing levels, tentatively described as 100 A fiber, 250 A coil and chromomere.

The metaphase chromosome ultrastructure. II. Helical organization of the basic chromosome fiber as revealed by acute angle metal deposition

The acute angle metal depositon technique, discloses a helical chromosome fiber substructure. The microdensitometric analysis of the fiber parameters shows that a fibril 152 A in diameter coils to form the basic chromosome fiber 295 A in diameter with a pitch of 247 A. These figures should be corrected because of the thickening due to metal deposition, the real dimensions would be closer to 112 A for the fibril diameter and 255 A for fiber diameter. The real pitch value is probably somewhat smaller than 247 A (approximatively 200 A) as the values were obtained by analysing slightly stretched fibers. These results support the helical model of the chromosomes fiber organization and show that, most probably, the metaphase chromosome fiber as seen in electron microscopy is the second order of helical packing.