Nucleolus Precursor Body (NPB): A Distinct Structure in Mammalian Oocytes and Zygotes

Abstract

Nucleoli in mammalian oocytes and zygotes, sometimes referred to as nucleolus precursor bodies (NPBs), are compact and morphologically different from nucleoli in somatic cells. We applied a unique NPB analyzing method "enucleolation" technique to zygotes to remove the NPBs. It has been reported that oocyte NPBs are essential for embryonic development; in their absence, the oocytes complete maturation and can be fertilized, but no nucleoli are formed in the zygotes and embryos, leading to developmental failure. However, we found that when NPBs were removed from zygotes, the zygotes developed successfully to live-born pups. These results indicated that oocyte NPBs are essential for embryonic development, but zygote NPBs are not. In addition, the enucleolated zygotes formed somatic-type nucleoli during early embryonic development, demonstrating that somatic-type nucleoli do not originate from zygote NPBs. We summarize our recent investigation on NPBs, and provide additional comments and findings.

Localisation of RNAs and proteins in nucleolar precursor bodies of early mouse embryos

“…However, recent analysis of developmental capacities of mouse embryos obtained from enucleolated GV oocytes (Ogushi et al 2008;Ogushi and Saitou 2010) and NPB-less zygotes (Kyogoku et al 2014a(Kyogoku et al , 2014b indicated that functional and biochemical similarities between NPBs and NLBs are most likely overestimated. Indeed, embryos obtained from the enucleolated oocytes stopped development at the two-or four-cell stages, whereas enucleolated zygotes were capable of producing live pups and reconstructing normal nucleoli (Ogushi et al 2008;Ogushi and Saitou 2010;Kyogoku et al 2014aKyogoku et al , 2014b. These findings support the idea that biochemical compositions of NPBs and NLBs differ: NLBs are comprised of molecular constituents that are necessary for early embryonic development, whereas NPBs do not contain them.…”

“…Recently, by modification of standard immunostaining assays it has been shown that zygotic NPBs, like oocyte NLBs, contain nucleolar proteins NPM1 (B23, nucleophosmin), C23 (nucleolin) and fibrillarin but apparently lack the RNA polymerase I cofactor UBF (Fulka and Langerova 2014;Shishova et al 2015). These observations support the above-mentioned idea but they do not explain why microsurgical removal of zygotic NPBs does not affect further embryonic development (Kyogoku et al 2014a;2014b), whereas enucleolated GV oocytes are incapable of giving rise to fully developed embryos (Ogushi et al 2008;Ogushi and Saitou 2010).…”

“…NPBs appear in both pronuclei shortly after fertilisation as numerous (,10 in each pronucleus), small (,1 mm in diameter) optically dense roundish bodies randomly distributed in the nucleoplasm. During the zygotic stage, the number of NPBs decreases while their size increases, so that before the pronuclei fuse, a single large (,10 mm in diameter) NPB is usually seen in each pronucleus (Fulka and Fulka 2010;Kyogoku et al 2014a;2014b). The time of NPB assembly and the pattern of their localisation are used as biomarkers of developmental potencies of human embryos (Fulka and Fulka 2010;Papale et al 2012).…”

The potential and pitfalls of de‐extinction

“…The nucleolus is another vital consideration, with the presence of a structure called the nucleolus precursor body (which has structural differences from the somatic cell nucleolus) being a necessary component of the oocyte in order for successful embryonic development (Kyogoku et al . ).…”

“…Centromeres are also vital components of chromosomes for replication, but even the sequencing of these regions is difficult (Hayden & Willard 2012) and the assembly of the proteins that comprise this region constitutes another barrier (Aldrup-Macdonald & Sullivan 2014). The nucleolus is another vital consideration, with the presence of a structure called the nucleolus precursor body (which has structural differences from the somatic cell nucleolus) being a necessary component of the oocyte in order for successful embryonic development (Kyogoku et al 2014).…”