Nucleolar proteins and ultrastructure in preimplantation porcine embryos developed in vivo

Ribosomal RNA genes are transcribed in the nucleolus. The formation of this organelle after fertilization is essential for embryonic protein synthesis and viability. We have examined nucleolus formation in in vivo-derived por cine embryos by light microscopical autoradiography following 20 min of H-3 -uridine incubation, transmission electron microscopy (TEM), and immunocyto chemical localization by confocal laser scanning microscopy of key nucleola r proteins involved in rRNA transcription (nucleolin, upstream binding fact or, topoisomerase I, and RNA polymerase I) and processing (fibrillarin, nuc leophosmin). During the first two postfertilization cell cycles, TEM reveal ed fibrillar spheres as the most prominent intranuclear entity of the blast omeres. Fibrillogranular nucleoli were established during the third cell cy cle. Initially, fibrillar centers, a dense fibrillar component, and a granu lar component were formed on the surface of the fibrillar spheres. At the s ame time, autoradiographic labeling over the nucleoplasm and in particular the nucleoli was detected for the first time. The nucleolar proteins were, in general, not immunocytochemically localized to the presumptive nucleolar compartment until late during the third or early during the fourth cell cy cle.