Patterns of Nuclear and Nucleolar Growth in Synchronously Dividing Explants from Tubers of Helianthus tuberosus L.

Publisher Website
Google Scholar
Abstract
The nucleus and nucleolus have been examined by phase contrast microscopy of isolated fixed nuclei from synchronously dividing cells of Helianthus tuberosus L. tuber explants grown in nutrient medium on filter paper. The volumes of nuclei and nucleoli were computed from their areas and perimeters obtained by digitizing images projected from film. The nuclei did not show a pattern of growth related to the S phase but enlarged at times of both de-differentiation and differentiation. There was also rapid post-mitotic nuclear enlargement. The size attained by nuclei in the three successive divisions following cell activation decreased progressively, but started to rise again at the time of cell differentiation. The changes are discussed in relation to nuclear size regulation, the nuclear matrix and hypotheses relating nuclear growth to DNA, protein and water in the processes of de-differentiation, mitosis and differentiation. Nucleoli showed a clear fusion and growth cycle. The pattern of fusion can be used to identify the position of a sample of cells, though not any particular cell, within the cycle. Nucleolar growth was different in the succeeding cell cycles that were induced in the de-differentiating tissue. Nucleolar enlargement was slow in the first cycle and continued until mitosis. There was rapid nucleolar growth in the second cycle and none in later cycles until the time of cell differentiation. Nucleolar changes are discussed in relation to rRNA gene dosage, replication and polymerase availability.