Nucleolar cycle and localization of NORs in early embryos of Parascaris univalens

During early embryogenesis of the nematode Parascaris univalens (2n=2) the processes of chromatin diminution and segregation of the germ and somatic cell lineages take place simultaneously. In this study we analyzed the nucleolar cycle in early embryos, both in germinal and somatic blastomeres, by means of silver staining and antibodies against the nucleolar protein fibrillarin. We observed an identical nucleolar cycle in both types of blastomeres, hence, the chromatin diminution process has no effect on the nucleolar cycle of somatic blastomeres. We report the existence of outstanding differences between this cycle and those previously reported during early embryogenesis of other species. There is a true nucleolar cycle in early embryos that shows a peculiar nucleolar disorganization at prophase, and a preferential localization of prenucleolar bodies only on the euchromatic regions during nucleologenesis. Moreover, fibrillarin does not form a perichromosomal sheath in metaphase or anaphase holocentric chromosomes, probably owing to their special centromeric organization. The number and location of nucleolus organizer regions (NORs) in the chromosomal complement have been determined using silver impregnation, chromomycin A3/ distamycin A staining, and fluorescent in situ hybridization using an rDNA probe. There are only two NORs, one per chromosome, and these are located in the middle of the euchromatic central regions. This location implies that no rDNA sequences are lost in blastomeres after chromatin diminution. Moreover, the constant presence of two nucleoli in somatic blastomeres suggests that NORs are not affected during the fragmentation of euchromatic regions when this process occurs.

Development of the nucleolus in early goat embryos

In vivo nucleologenesis was studied in goat embryos from the pronuclear stage to the blastocyst stage by light and electron microscopy. Ultrastructural changes of the nucleoli were characterized by the following progression: homogeneous electron-dense fibrillar primary nucleoli in the pronucleus; small, dense fibrillar masses dispersed in clusters of chromatin at the two-cell stage; ring-shaped nucleoli made up of a fibrillar center surrounded by a layer of dense fibrillar components at the four-cell stage; reticulated nucleoli composed of a three-dimensional network of fibrillar components surrounded by small amounts of granular components at the eight-cell stage; fully developed compact-type nucleoli consisting of several fibrillar centers each surrounded by a layer of fibrillar components and abundant granular components in morulae and blastocysts. Moreover, it was concluded that activation of rRNA transcription, as evidenced by specific silver nitrate staining of the nucleolus organizer regions of metaphase chromosomes, occurs at the two- to four-cell stage and that the morphological changes accompanied a substantial increase in nucleolar transcriptional activity up to the blastocyst stage. This study provides evidence that a structure-function relationship exists during nucleologenesis in goat embryos.

Nucleologenesis in the human embryo developing in vitro: ultrastructural and autoradiographic analysis

Nucleolar development during cleavage of human embryos in vitro was analyzed using combined autoradiographic and ultrastructural approaches. Human 2- to 4-cell embryos lack functionally active nucleoli. They possess nucleolus-like bodies formed by a homogeneous mass composed of densely packed fine fibrils. The nucleolus-like bodies contain no recently replicated embryonic DNA and do not show any detectable RNA synthesis. Their transformation into nucleoli is a relatively rapid process characterized by progressive infiltration of these bodies by adjacent chromatin, followed by the beginning of RNA synthesis and processing reflected by the appearance of the first nucleolar granules. These changes take place in 6- to 8-cell embryos, probably starting after the third cleavage division. Finally, most frequently in 10- to 12-cell embryos, typical nucleolar structure is established as a result of intranucleolar differentiation giving rise to distinct fibrillar and granular components as well as to nucleolar interstices. It is suggested that this pattern of nucleologenesis, different from that pertinent to mouse embryos (M. Geuskens and H. Alexandre (1984). Cell Differ. 14, 125-134), might be related to the relatively late activation of embryonic rRNA genes transcription and to the expressed rapidity of this process.

Ultrastructural and autoradiographic studies of nucleolar development and rDNA transcription in preimplantation mouse embryos

The development of the nucleoli and the sites of rDNA transcription have been studied by high-resolution autoradiography during the cleavage stages of mouse embryos. The appearance of fibrillar centres at the periphery of the fibrillar primary nucleoli has been observed at the 4-cell stage. Several fibrillar centres, interconnected by electron-dense fibrillar strands, form a reticulated region around the fibrillar mass at the 6-to 8-cell stage. After a 10 min pulse with [3H]uridine, only this peripheral network is labelled. At the late morula and at the blastocyst stage, the fibrillar component (nucleolonema) of the reticulated nucleoli is labelled after 10 min [3H]uridine incorporation. When the embryos are reincubated for 2 h in cold medium, the label is localized mainly in the granular component. Fibrillar centres are not labelled. Autoradiograms of in vitro developed embryos pulsed for 2 h with [3H]uridine confirm that the central fibrillar core of the nucleoli of 6-to 8-cell embryos is never labelled. Thus, the fibrillar constituent of this core is not homologous to the fibrillar component of the nucleoli of later stage embryos, which is the site of active rDNA transcription. An interpretation of nucleologenesis during early mouse embryogenesis is proposed.