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The nature of the ‘nucleolus precursor body’ in early preimplantation embryos: a review of fine-structure cytochemical, immunocytochemical and autoradiographic data related to nucleolar function
- J.-E. Fléchon, V. Kopecny
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In mammals, the restoration of rRNA transcription after fertilisation is accompanied by a gradual differentiation of the nucleolar structure by a process called embryonic nucleogenesis. During cleavage, the nucleolar components appear sterically related to a class of nuclear bodies already detectable in pronuclei. These structures, due to their apparent function as centres of nucleolus formation, have been designated nucleolus precursor bodies (NPBs). It was found recently not only that the size and morphology of the NPBs differ among mammalian species, but that the pattern of embryonic nucleologenesis and even the molecular composition of different NPB compartments vary from one species to another. Accordingly we assumed that at least two definitely different types of NPBs exist, namely the mouse-type NPB and cow-type NPB. In the mouse-type NPB, the original compact material of the NPB remains detectable in the early functional nucleolus. This NPB core does not contain DNA or typical Ag-NOR nucleolar proteins. At the onset of rRNA transcription, the nucleolonema is formed at the periphery of the NPB. The cow-type NPB shows a homogeneous distribution of typical nucleolar proteins throughout its body from the pronucleolar to the early 8-cell stage. At the beginning of rRNA transcription, the cow-type NPB is penetrated by perinucleolar DNA and rRNA synthesis is detectable deep inside the nucleolus. In this case, the entire NPB is readily transformed into a typical nucleolus. These processes are recognisable using fine-structure analysis of preimplantation mammalian embryos. For this reason this approach is often used as a method of evaluating the state of experimental embryos; in such studies, the species differences must be taken into account.
Fine-structural cytochemical and immunocytochemical observations on nuclear bodies in the bovine 2-cell embryo
- V. Kopecný, M. Biggiogera, J. Pivko, A. Pavlok, T.E. Martin, S.H. Kaufmann, J.H. Shaper, S. Fakan
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Nuclear bodies occuring during the 2-cell stage of bovine embryos (obtained either by in vitro fertilisation of in vitro matured ovarian oocytes, or collection after fertilisation and cleavage in vivo) were studied using ultrastructural cytochemistry and immunocytochemistry to determine whether their occurrence may be linked with the onset of embryonic transcription. In addition, the species-specific ultrastructural features of the interchromatin structures of the 2-cell bovine embryo were displayed. Three different types of nuclear bodies were distinguished: (i) nucleolus precursor bodies (NPBs), (ii) loose bodies (LBs) and (iii) dense bodies (DBs). In order to determine their possible functional significance, we considered parallels between these three nuclear entities and interchromatin compartments reported in other cells. As detected by their preferential ribonucleoprotein staining, all types of nuclear bodies contained ribonucleoproteins. In contrast to the other types of nuclear bodies studied, NPBs contained argyrophilic proteins but in no case they did show morphological features of functional nucleoli. Both compact and vacuolated forms of NPBs were seen in both in vivo and in vitro embryos, sometimes simultaneously in the same nucleus. LBs and DBs reacted with antibodies to Sm antigen, indicating the presence of a group of nucleoplasmic, non-nucleolar small nuclear ribonucleoproteins (snRNPs). The immunoreactivity for Sm antigen was more intense and homogeneous in DBs than in LBs. DBs were seen in both categories of embryo. A possible kinship of DBs with the sphere organelle known from oocytes of different animal species or the prominent spherical inclusions of the early mouse embryo nuclei is suggested. The last type of intranuclear body, the LBs, showed a composite structure. Their granular component, occurring in clusters and displaying immunoreactivity for Sm antigen, was similar to interchromatin granules and was therefore named IG-like granules. Another component forming the LBs showed a much finer structure and a lower immunoreactivity with anti-Sm antibodies. We suggest that this amorphous component may be related to the IG-associated zone. All three types of intranuclear bodies were often seen close together, suggesting their possible mutual functional relationship. From these and other observations we conclude that the intranuclear bodies in 2-cell bovine embryos correspond, with the exception of the NPB, to similar structures/compartments supposed to accumulate inactive spliceosomal components in certain phases of somatic cell nucleus functions. Accordingly, the occurrence of such nuclear bodies does not represent cytological evidence for RNA synthesis. In contrast to this, an important morphological feature revealing the status of the bovine 2-cell embryo is the vacuol-isation of the NPB.