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Membrane distribution in dividing endosperm cells of Haemanthus
Abstract:
Membranes in cell-wall-free dividing endosperm cells of Haemanthus were examined after postfixation with osmium tetroxide-potassium ferrocyanide. We found that preservation and staining of membranes in metaphase cells was highly variable. Even adjacent cells often showed different degrees of preservation of membrane. However, this method does reveal a much more extensive membrane system in the mitotic spindle of Haemanthus than has been revealed previously using glutaraldehyde-osmium fixation. At prometaphase a system of membranes becomes associated with the kinetochore bundles. By metaphase, membranes constitute a prominent feature of kinetochore bundles, terminating near the kinetichores. Minipoles, identified by converging microtubules and associated membranes, are distributed in a zone extending laterally across the polar regions of the cell. The microtubules appear to terminate at the minipoles, whereas the membrane system becomes oriented generally perpendicular to the spindle axis and interfaces distally with a region of amorphous electron-dense material, helical polyribosomes, and cell organelles. The role of this extensive membrane system, if any, in chromosome movement is unknown. However, its distribution is coincident with the distribution of calcium-rich membranes and kinetochore fibers at metaphase in these cells (Wolniak, S. M., P. K. Hepler, and W. T. Jackson, 1981, Eur. J. Cell Biol., 25:171-174). Thus, these membranes may function in creating calcium domains that, in turn, may play a regulatory role in chromosome movement.
Insights
A new fixation method reveals an extensive membrane system within the mitotic spindle of Haemanthus endosperm cells. This system may regulate chromosome movement through calcium domains.
Area of Science:
- Cell Biology
- Cytoskeleton Dynamics
- Plant Cell Division
Background:
- Understanding the intricate membrane systems within dividing plant cells is crucial for elucidating chromosome segregation mechanisms.
- Previous fixation techniques limited the visualization of membrane structures associated with the mitotic spindle.
Purpose of the Study:
- To investigate the distribution and potential role of membrane systems in the mitotic spindle of cell-wall-free Haemanthus endosperm cells.
- To compare membrane preservation using osmium tetroxide-potassium ferrocyanide postfixation with previous glutaraldehyde-osmium fixation methods.
Main Methods:
- Postfixation of Haemanthus endosperm cells with osmium tetroxide-potassium ferrocyanide.
- Microscopic examination of membrane structures within the mitotic spindle during prometaphase and metaphase.
Main Results:
- The osmium tetroxide-potassium ferrocyanide method revealed a more extensive membrane system in the mitotic spindle than previously observed.
- Membranes were found associated with kinetochore bundles, forming structures termed 'minipoles' near kinetochores.
- This membrane system's distribution coincided with calcium-rich regions, suggesting a potential role in calcium-mediated regulation of chromosome movement.
Conclusions:
- The study identified a previously underappreciated extensive membrane network within the Haemanthus mitotic spindle.
- These membranes, particularly at minipoles, may be involved in establishing calcium gradients that regulate chromosome dynamics during cell division.