Journal of Cell Science, Vol 16, 63-93, Copyright © 1974 by Company of Biologists

Submitted on February 11, 1974

A Correlated Light- and Electron-Microscope Investigation of the Structural Evolution of the Nucleolus during the Cell Cycle in Plant Meristematic Cells (Allium Porrum)

¹ Laboratoire de Biologie cellulaire et moléculaire, Département de Biologie, Faculté des Sciences, Université Laval, Québec G1K-7P4, Canada

The interphase nucleolus consists of particulate zones and of a number of skein-like regions each of which contains a complex filamentous structure or nucleolonema. DNA is known to be present within the lacunar portions of these nucleolar regions.

In the course of prophase the nucleolonemata become less tortuous and take the form of elongate structures extending across the nucleolar mass. Their continuity with the nucleolar organizing chromosomes can then be much more easily established than during interphase. When the peripheral granular portion of the late prophase nucleolus begins to disperse, the nucleolonemal lacunae appear smaller and less conspicuous. Examination of serial sections discloses that these convoluted filaments progressively contract during disorganization of the nucleolus but always remain less compact than the remaining portions of the nucleolar chromosomes. At very late prophase these chromosome loops have become quite small and have evolved into rather dense, homogeneous structures made up mostly of fibrillar material. By the time the nuclear envelope breaks down, these structures appear less compact under phase-contrast optics and, judging from their progressive contraction and their change in ultrastructural characteristics, it is evident that they have undergone extensive reorganization during the nucleolar dissolution process. This striking morphological evolution of the nucleolonemata is best interpreted by assuming that they lose a great deal of their constituent material during the later part of prophase and that the remaining portion, part of which is organizer DNA, retracts back to the chromosome axis and gives rise to nucleolar secondary constrictions. From metaphase to early telophase these constrictions are observed to consist of fine fibrillar material which stains much less intensely with heavy metals than other chromosome segments.

When first detected as structured bodies, the late telophase nucleoli are roundish in shape and, under phase-contrast optics, their density closely matches that of the late prophase contracted nucleolonemata. At higher magnification incipient nucleoli are found to consist of a tight glomerulus predominantly fibrillar in texture. During the early G₁ period, growth of the nucleolus first results from expansion of the nucleolonemal skein and concurrent formation of numerous lacunar areas. Electron microscopy reveals that these lighter nucleolar zones contain diffuse fibrillar material similar to that observed in the lacunae of mature interphase nucleoli. Favourable preparations also show that this lacunar, fibrillar material is continuous with immediately adjacent segments of the nucleolar chromosomes. Concomitant with this expansion of the central skein region and enlargement of its lacunae, the growing nucleolus also gradually acquires an irregular layer of particulate material at its periphery.

Since the formation of chromatin-containing lacunar regions and the appearance of peripheral RNP granules are found to coincide with a resumption in the RNA-synthesizing activity of the early interphase nucleous (authors' unpublished observations), growth of this organelle appears to be partly dependent on derepression of its nucleolar loops. Mid-G₁ nucleoli exhibit all the morphological characterized of mature interphase organelles.