An electron-microscope study of the intranucleolar chromatin in root meristematic cells of Allium cepa

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Abstract
The various states of condensation and configuration of the chromatin material, contained inside the lacunar regions of the interphase nucleolus in Allium cepa, have been investigated by means of conventional electron-microscope techniques. The observations reveal that in a number of lacunar profiles, the chromatin material in question appears in an extended state only; in other lacunar profiles of the same or different nucleoli, the chromatin material is present both in an extended and a condensed condition. Moreover, in some lacunar profiles, a single mass of chromatin in a condensed state is observed; in others, several discrete and often seemingly interconnected masses of condensed chromatin are visualized. An attempt is made to interpret these morphological findings in the light of current knowledge concerning the structural relationship of the nucleolar organizing segment of the nucleolar chromosome with the interphase nucleolus in plant cells. The relevant observational evidence would be consistent with the view that the chromatin-containing lacunar regions of the interphase nucleolus in Allium cepa correspond, in fact, to cross or oblique sections of a meandering channel through which the nucleolar organizing segment of the nucleolar chromosome passes. Assuming the applicability to intranucleolar chromatin of the general concept of condensed-inactive versus extended-active chromatin, it is hypothesized that the various states of condensation of intralacunar chromatin merely reflect variations in the functional activity of the nucleolar organizing segment during the interphase cell cycle in the species investigated. With regard to variations in the configurational state of the intralacunar condensed chromatin, it is postulated that they are the cytological expression of the gradual process of coiling or folding upon itself of the nucleolar organizing segment during late interphase and in preparation for the next mitosis.