An electron-microscope study of the intranucleolar chromatin during nucleologenesis in root meristematic cells ofAllium cepa

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Abstract
The various states of condensation of the chromatin material, contained inside the lacunar regions of the reforming nucleolus in Allium cepa, have been investigated by means of conventional electron-microscope techniques. The observations reveal that, in the emerging early to late telophase nucleoli, the intralacunar chromatin material in question appears both in an extended and a condensed condition; from late telophase to the mid G1 period of interphase, the intralacunar chromatin material of the rapidly growing and developing nucleoli is present in an extended state only.An attempt is made to interpret these morphological findings in the light of current knowledge concerning the structural relationship of the nucleolar organizing region 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 reforming 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 concluded that gradual uncoiling and subsequent decondensation of the chromatin of the nucleolar organizing region in the form of a convoluted loop structure are key morphological and functional events associated with the process of nucleologenesis in the species investigated.