Physiologically inactive nucleoli contain nucleoplasmic ribonucleoproteins: immunoelectron microscopy of mouse spermatids and early embryos.

Physiologically inactivating or reactivating nucleoli represent a good model to investigate modifications of the nucleolar activity, which influence to a great extent the morphology of this organelle. We have studied the nucleoli of mouse spermatids during gradual inactivation, as well as those of early mouse preimplantation embryos, which undergo reactivation. In the 2-cell systems, inactive nucleoli are represented by homogeneously, finely fibrillar spherical bodies. With the aim of clarifying the composition of these nucleoli, we have analyzed them by means of immunoelectron microscopy using specific antibodies directed against nucleoplasmic snRNPs, hnRNPs, ribosomal proteins, or fibrillarin as well as by cytochemical methods for visualizing DNA and RNA. Our results indicate that RNA is present in zygote and 2-cell nucleolus precursor bodies (NPBs) as well as in 4- to 8-cell or more advanced embryo nucleoli, but not in inactive spermatid nucleoli. DNA is absent from inactive spermatid nucleoli and NPBs but is present within the nucleolus-associated chromatin and the nucleolonema of active nucleoli. The dense masses constituting the NPBs of the zygote and 2-cell embryos contain ribosomal proteins and fibrillarin but also hnRNPs and nucleoplasmic snRNPs. Of these, only fibrillarin is present in the spermatid residual nucleoli.