In Xenopus the genes for ribosomal RNA are selectively amplified during the early stages of oogenesis and give rise to over 1000 extrachromosomal nucleoli. These oocyte nucleoli are unique in that they contain very high copy numbers of rRNA genes, are not attached to chromosomes, and lack nonribosomal DNA. How the amplified rRNA genes induce the formation of multiple nucleoli is as yet poorly understood. To gain some more insight into this assembly process we have studied nucleolar development in early previtellogenic Xenopus oocytes. By using light and electron microscopic immunocytochemistry, in situ hybridization, and the TdT-method for detection of DNA, we have identified three separate structural entities which are related to the three components of the functionally active nucleoli present in midsized oocytes. Besides fibrillar and granular bodies we describe a novel rod-like structure which contains the pol I-specific transcription factor UBF. Notably, rDNA emerges from these rods forming a filamentous layer. These results reinforce UBF's role as an architectural element involved in the spatial organization of the rRNA genes. We consider the rod-like rDNA/UBF complexes as extrachromosomal nucleolar organizers which are first juxtaposed to preformed fibrillar bodies before both elements gradually fuse into a coherent nucleolar structure. (C) 2002 Elsevier Science (USA). All rights reserved.