Nucleolus formation in pre-implantation cattle and swine embryos

Transcription of ribosomal RNA (rRNA) genes occurs in the nucleolus resulti ng in ribosome synthesis. In bovine and porcine embryos functional ribosome -synthesizing nucleoli become structurally recognizable towards the end of the 4(th) and 3(rd) post-fertilization cell cycle, respectively. In cattle, a range of important nucleolar proteins become localized in the nucleolar anlage over several cell cycles and this localization is apparently complet ed towards the end of the 4(th) cell cycle. In swine, the localization of t hese proteins in the anlage is more synchronous and occurs towards the end of the 3(rd) cell cycle and is apparently completed at the onset of the 4(t h). The rRNA gene activation and the associated nucleolus formation may be used as a marker for the activation of the embryonic genome in mammalian em bryos and, thus, serve to evaluate the developmental potential of embryos o riginating from different embryo technological procedures.