Nucleolar reorganization in cells of the kidney of the rat and its relation to aging

Accelerated aging and nucleolar fragmentation in yeast sgs1 mutants

The SGS1 gene of yeast encodes a DNA helicase with homology to the human WRN gene. Mutations in WRN result in Werner's syndrome, a disease with symptoms resembling premature aging. Mutation of SGS1 is shown to cause premature aging in yeast mother cells on the basis of a shortened life-span and the aging-induced phenotypes of sterility and redistribution of the Sir3 silencing protein from telomeres to the nucleolus. Further, in old sgs1 cells the nucleolus is enlarged and fragmented-changes that also occur in old wild-type cells. These findings suggest a conserved mechanism of cellular aging that may be related to nucleolar structure.

Nuclear and nucleolar damage in adriamycin-induced toxicity to rat sensory ganglion cells

A single dose (10 mg/kg) of Adriamycin was given to 23 adult Wistar rats and the spinal ganglia were studied from 6 h to 15 days after. As previously described, this drug results in the appearance of 'clear' areas in the nuclei of rat spinal ganglion cells as seen by light and by electron microscopy. The 'clear' areas become less conspicuous during the week before the onset of cytoplasmic degeneration. In addition, nucleolar changes become increasingly evident with time after injection. Fibrillar centres enlarge and nucleolar segregation is present from 24 h onwards, although the latter is invariably partial and more readily seen with the electron microscope. Nucleolar fragmentation is seen more frequently from 3 days onwards and nucleolar enlargement is common from 6 days. Early cytoplasmic abnormalities are associated with pronounced loss of Nissl substance. Adriamycin must bind extensively to nuclear DNA in spinal ganglion cells, causing the 'cleared' nuclear areas and the changes in dense chromatin. In addition, the binding of Adriamycin to nucleolar DNA with disturbance to nucleolar functions must be important in producing later cytoplasmic changes that precede cell death. There are thus similarities between the action of Adriamycin on these cells and those of Cisplatin, although in the latter intoxication the nuclear effects are significantly less prominent.

Morphometric analysis of the nucleolus during the life cycle of human odontoblasts

Developing first premolars were used as a model system to obtain information on the nucleolar structure of human odontoblasts at several stages of their life cycle. Four stages were defined by their location within the tooth: a) preodontoblasts were located at the growing tip of the root; b) secretory odontoblasts in the apical region; c) transitional odontoblasts in the middle region; and d) aged odontoblasts in the coronal region. Preodontoblasts have a small nucleolus (0.55 micron 2) with few strands of dense fibrillar material radiating from the fibrillar center. Secretory odontoblasts are characterized by a large (1.24 micron 2), irregular, and reticulated nucleolus. The fibrillogranular material, the largest component in all nucleoli, reaches maximal size at this stage (0.88 micron 2). Fibrillar centers occupy about the same area (0.1 micron 2) throughout the odontoblast's life cycle. As the formation of primary dentin is completed, the nucleolus of transitional odontoblasts is reduced in size (0.54 micron 2). Finally, the aged odontoblasts have a small, compact nucleolus (0.39 micron 2), with segregated components. Morphologic analysis and quantification of size and component areas of nucleoli obtained with an image analyzer indicated that secretory odontoblasts had the most active, and aged odontoblasts the least active, nucleolus.

Nucleolar reorganization in liver cells of the aging rat

The nucleoli of rat liver cells duplicate in great detail the life-long series of reorganizational changes encountered in kidney and intestinal epithelial cells. The ultrastructural components of the large, loosely organized polymorphous nucleoli, which are dominant in the rapidly multiplying stem cells of embryos, are readily accessible for chemical activities. Smaller, more compact amphinucleoli are dominant in more mature cells, which were characterized by Smetana ('70) as "idling" cells, showing slowly continuing ribosome formation and RNP synthesis. In older cells bipartite nucleoli become dominant and are reorganized in increasing numbers from the younger amphinucleoli. These, however, are not replaced in equal numbers from the shrinking pool of polymorphs of young cells which have greatly reduced mitotic potential. Paralleling the shifts in dominant nucleolar types, the high level of protein synthesis declines in older cells not only in the quantity of proteins synthesized but also in kinds of enzymes produced. These fail to meet the structural and functional requirements of aging cells leading ultimately to the onset of age-related degenerative changes. Again it is noted that separation of the karyosomal DNA from the plasmosomal RNA-protein complex of the nucleolus may lead to possible breakdown of the DNA-dependent RNA-protein transcription system ultimately bringing protein synthesis to a very low level in the senescent animal.