The ultrastructure of nucleoli and chromosomes during the early stages of liver regeneration and the changes produced in these structures by X radiation

Abstract

A method of tissue preparation for electron microscopy is described which allows the observation of more nuclear structure than has previously been reported. Regenerating rat-liver cells were used for this study. During the early stages of regeneration the nucleolar size was found to be much greater than normal, although nucleolonema fibres and pars amorpha were still present. At 16 to 18 h after operation, however, when there is considerable production of new a-cytomembranes in the tissue, much of the material of the pars amorpha was lost from many cells, and the nucleoli consisted solely of nucleolonema fibres. Later the pars amorpha was re-formed, but just before the start of mitosis at 25 to 26 h after operation, many of the nucleoli became very irregularly shaped. The various mitotic stages seen in liver cells with the electron microscope were recognizably similar to those previously seen in living mammalian cells with the light microscope. In the earliest stages of prophase the chromosomes appeared as strands approximately 1000 A in diameter, which were made up of thin fibres approximately 100 A in diameter. By mid-prophase these strands could be seen running diagonally across the chromosomes in a manner suggestive of coiling, but by late prophase this coiling was no longer visible and the chromosomes appeared as homogeneous masses of fibres 100 A in diameter. The double nature of the chromosomes was, however, evident during metaphase. Telophase nuclei were seen to contain fibrous inclusions approximately 1000 A in diameter which it is suggested may represent stages in the re-formation of the nucleolus after mitosis. The effect of X radiation on the ultrastructure of the nuclear components of regenerating liver cells was also studied in these experiments. With small doses (450 r) no ultrastructural changes were found in the chromosomes during cell division, but some changes were visible in the nucleoli just prior to mitosis. Larger doses (2000 r) caused chromosome fragmentation and a loosening of the fibrous chromosome structure. In telophase it was found that when the separation of the two daughter nuclei is inhibited or delayed, the nuclear membrane re-forms around the two partially separated chromosome masses.