Activation of ribosomal RNA genes in pre-implantation in vitro-produced and nuclear transfer bovine embryos

Transcription of the ribosomal RNA (rRNA) genes occurs in the nucleolus res ulting in ribosome synthesis. The rRNA gene activation and the associated n ucleolus formation may be used as a marker for the activation of the embryo nic genome in mammalian embryos and, thus, serve to evaluate the developmen tal potential of embryos originating from different embryo technological pr ocedures. In bovine in-vitro-produced embryos, functional ribosome-synthesi zing nucleoli become structurally recognizable towards the end of the fourt h postfertilization cell cycle. A range of important nucleolar proteins (e. g. fibrillarin, nucleophosmin, nucleolin, topoisomerase I, upstream binding factor and RNA polymerase I) become localized to the nucleolar anlage over several cell cycles and this localization is completed towards the end of the fourth cell cycle. In nuclear transfer embryos originating from day 5-6 invitro-produced bovine embryos, fully developed nucleoli are apparently d etected one cell cycle later, and a substantial proportion of the nuclear t ransfer embryos display aberrations in protein localization in one or more blastomeres. This information may help to explain the abnormalities observe d in a proportion of fetuses and offspring derived from nuclear transfer em bryos.