Structural relations between nucleus and cytoplasm during mitosis in Nicotiana tabacum mesophyll

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Abstract
The changes in relation between the nucleus and the cytoplasm associated with mitosis were explored at the ultrastructural level in the mesophyll of Nicotiana tabacum L. The material was fixed in glutaraldehyde – formaldehyde – osmium tetroxide. During interphase and early prophase the nucleoplasm can be distinguished from the cytoplasm by its lack of granules comparable to those interpreted as ribosomes in the cytoplasm. After the nuclear envelope is disrupted the nuclear region shows a population of ribosomes identical with that in the cytoplasm, that is, the nucleoplasm and the cytoplasm become indistinguishable. The nuclear envelope differs from the endoplasmic reticulum located in the cytoplasm by the presence of pores. Pieces of the disrupted nuclear envelope assume the same appearance as the endoplasmic reticulum although they remain localized around the nuclear region until metaphase. Eventually the remnants of the envelope are carried to the spindle poles and serve as a source of envelopes for the daughter nuclei. The larger organelles are excluded from the nuclei during development of their envelopes but units of endoplasmic reticulum and apparently some cytoplasmic ribosomes become trapped among the chromosomes. Later, the extranuclear components disappear, probably by being disassembled.

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