HETEROGENEITY OF COILIN-CONTAINING NUCLEAR DOMAINS IN EARLY MOUSE EMBRYOS.

The nucleus of mouse two-cell embryos houses the coilin-containing bodies of two types: 1) 1­3 large spherical structures of 1 mm and 2) small foci, which vary in number in different blastomeres. The largest coilin-containing structures, unlike the smallest ones, contain RNA polymerase I, nucleic acid chaperon YB-1, and also actin. Neither large nor small coilin-positive domains contain symplekin, one of the signature components of histone locus bodies. In the nuclei of late two-cell embryos, symplekin localizes to 1­2 well-formed roundish bodies that are observed both in close proximity to the coilin-positive structures and far away from them. Large coilin-containing bodies were not observed in embryos at the morula stage as well as in the nuclei of late two-cell embryos after artificial suppression of transcription activity. Thus, a population of coilin-containing bodies in the nuclei of late two-cell embryos of mice is heterogeneous in morphology and molecular composition. It could be assumed that the largest coilin-containing bodies are provisional nuclear domains that are formed at the background of significant changes of nuclear metabolism at the final stages of embryonic genome activation and the initial stages of reactivation of nucleolar transcription.

[NUCLEAR STRUCTURE IN THE SECRETORY CELLS OF MAMMARY GLANDS IN LACTATING AND NON-LACTATING RATS].

The features of structural and functional organization of the main nuclear compartments and distribution of their key molecular components (chromatin-remodeling protein ATRX, RNA polymerase I and II, and the splicing factor SC35) has been studied in the nuclei of mammary gland cells at different functional states. No significant differences between the nuclei of the cells in the lactating and non-lactating mammary glands have been revealed at the ultrastructural level. At the same time, photometric analysis has revealed higher intensity of nucleoplasmic immunofluorescent staining of mammary glands in the lactating animals when antibodies against the proteins ATRX and SC35 were used. Apparently, this observation reflects the changes of the structural and functional status of chromatin as well as the redistribution of splicing factors between the sites of their deposition and transcription.

Immunocytochemical study of YB-1 nuclear distribution in different cell types.

In the present work we studied the distribution of YB-1 in the nuclei of mouse hepatocytes, early embryos and human skin fibroblasts with the use of light and electron microscopy. To reveal YB-1, we applied rat polyclonal antibody against the C-terminal fragment of YB-1 molecule and rabbit polyclonal antibody against full-length YB-1 molecule. YB-1 distribution patterns varied significantly in different cell types. YB-1 was found to be colocalized with RNA polymerase I in mouse hepatocytes and embryos. Besides, YB-1 was revealed in a population of Cajal bodies in 2-cell mouse embryos but not in other cells studied.

Transcriptional activity of nuclei in 2-cell blocked mouse embryos.

Embryos of inbred mouse strains stop their cleavage at the late 2-cell stage and this phenomenon is known as the "2-cell block in vitro". In the present work we used microinjections of 5-bromouridine 5'-triphosphate (BrUTP) to examine the transcriptional status of blocked embryos. We also traced nuclear distribution of hyperphosphorylated form of RNA polymerase II in such embryos. We found that transcriptional activity is gradually decreased when the embryos stay in block. This is accompanied by prominent increasing of the size of SC35 nuclear speckles that begin to accumulate hyperphosphorylated RNA polymerase II.

Nuclear structure in early mouse embryos: A comparative ultrastructural and immunocytochemical study with special emphasis on the "2-cell block in vitro".

The embryos from many outbred and inbred strains of mice are arrested at the late 2-cell stage when cultured in vitro in simple culture media. This phenomenon is referred to as the "2-cell block in vitro". The ultrastructural morphology of the nuclei of the blocked embryos is not yet well described. In the present paper we documented the results of a comparative study on the nuclei of mouse embryos, both normally developing and arrested at the 2-cell stage. The blocked embryos maintain the morphological integrity of their nuclei. Main nuclear domains (nucleolus precursor bodies, interchromatin granule clusters, perichromatin granules, and perichromatin fibrils), typical for the control embryos, are observed in the blocked ones. A number and morphological characteristics of these nuclear substructures are not changed significantly in the blocked embryos. At the same time, RNA polymerase II and pre-mRNA splicing factors are redistributed in the nucleus of the blocked embryos. Although something goes to show that nuclear organization of the blocked embryos differ from that of the control, we could not reveal in the blocked embryos distinct signs of degeneration which might characterize aged or dying cells. Our data confirm a peculiar functional state of the 2-cell blocked embryos.