Nuclear and cytoplasmic changes during early stages of cell differentiation in roots of the water-fern,Azolla pinnata

Summary

Selected nuclear and cytoplasmic changes associated with early differentiation of four cell-types—dermatogen, inner and outer cortex, and endodermis—have been analysed using montages of electron micrographs of median longitudinal sections of young roots ofAzolla pinnata. The area fraction of nucleoplasm occupied by chromocentres (CAF) is smaller in the apical cell than in the nuclei of its most recently formed daughter cells. The CAF also differs between the four cell-types: dermatogen nuclei have a lesser mean CAF and smaller chromocentres than nuclei of the endodermis; cortical cell nuclei have intermediate values. These differences may reflect changes in nuclear activity during cell differentiation. The area fraction occupied by the vacuome (VAF) differs between the apical cell and its daughters: the apical cell seems to retain most of the vacuome at division, while the daughter cells receive less vacuolate cytoplasm. Of the four cell-types analysed, the cortical cells develop a large VAF the quickest; the dermatogen is slower to become vacuolate. Cells in the dermatogen and outer cortex derive from common mother cells, as do cells in the endodermis and inner cortex, and even the most recently-formed cells in the files of inner and outer cortex are more vacuolated than their sister cells in the other two celltypes. The onset of vacuolation may be triggered by an inductive influence emanating from older vacuolated cells in the same file. The rate of vacuolation in each of the cell-types examined may also be negatively correlated to the intensity of synthesis of protein used to construct cytoplasmic materials.