ISSN:
1615-6102
Keywords:
Morphometry
;
Mitosis
;
Nuclear envelope
;
Nucleolus
;
Polytoma
Source:
Springer Online Journal Archives 1860-2000
Topics:
Biology
Notes:
Summary Quantitative electron microscopy of serial sections was used to study the cyclical changes in the nucleus ofPolytoma papillatum during its vegetative life cycle with special reference to mitosis. Particular attention was paid to the fluctuations in the nuclear volume, the nuclear envelope, and the nucleolus. Whereas a volumetric balance exists between the cell (100%) and the nucleus (ca. 8%) from early interphase to late anaphase, the nucleus-to-cell volume ratio is gradually reduced up to ca. 2% during telophase. This disproportion is gradually equalized during cytokinesis. The decrease in nuclear size is brought about by: (a) Constrictive abscission of hernia-like protrusions of the nucleus (“blebbing process”); (b) Ade novo production of membraneous septa across peripheral regions of the nucleus (“internal septation”). Just before or immediately after completion of this internal compartmentalization of the nucleus, the original envelope opens, releasing a portion of nucleoplasm into the cytoplasm (“membrane sluice process”); (c) Gaps in the nuclear envelope were occasionally found during telophase and may also permit nucleoplasm to leak out. Disorganization of the nucleolos, which is preceded by blending of its two major components (pars fibrosa, pars granulosa) starts at prophase via fragmentation. Subsequent dispersion continues until the nucleolus is more or less homogeneously distributed across the metaphase nucleus. During anaphase a good deal of the preexisting material recondenses around the separating chromosomes, but a considerable amount remains dispersed in the nucleoplasm and is extruded into the cytoplasm during nuclear volume reduction. Reorganization of the nucleolus occurs during telophase and early cytokinesis via further coalescence of the recondensed material and simultaneous reconstruction of the spatial separation of pars fibrosa and pars granulosa. Morphofunctional aspects of nucleo- and nucleologenesis are also discussed.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1007/BF01284743
Permalink
