Ultrastructural association of the chromatin-containing lacunar spaces with the granular component of the interphase nucleolus in Allium cepa

Abstract

At the ultrastructural level, the interphase nucleolus in root meristematic cells of Allium cepa is characterized by the presence of chromatin-containing lacunar spaces associated with the dense-fibrillar component of the nucleolar mass. The present observations reveal that a number of these chromatin-containing lacunar spaces also exhibit partial association with the dense-granular component of the nucleolus. Under the electron microscope, such lacunar spaces are indeed seen to be enclosed or walled off, on one side, by dense-fibrillar material, and on the other side, by dense-granular material continuous with and indistinguishable from the dense-granular component of the nucleolus. The relevant observational evidence would be consistent with the view that loops of transcriptionally active chromatin, emanating from the nucleolar organizing region, project radially into either only the dense-fibrillar or both the dense-fibrillar and the dense-granular material bordering the lacunar spaces in question.