Architecture of the nucleolonema in root-tip cells of Raphanus sativus

The nucleolonema of Raphanus sativus root-tip cells was studied with light and electron microscopy especially focusing on its three-dimensional structural organization. Its countour was very intricate and the thickness showed a large variation in air-dried specimens. It usually appeared to coil or meander. Serial ultrathin sectioning of the nucleolonema in its longitudinal orientation first revealed the stack of short nucleolonema fragments, then a pair of knobbly frames with space in-between, and again the stack of the short nucleolonema segments. On the other hand, transversal sections revealed 'c'-shaped segments with the change of their openings' orientation. Morphometry of the nucleolonema indicated two categories of the nucleolonema; each had an average thickness of 510 nm and 1.19 μm, respectively. Taken together, the thin nucleolonema may coil or fold to form the thick one. The fibrillar centers including DNA-containing structures were closely associated with the thin nucleolonema and intermittently arranged along the nucleolonema.