AbstractThe intranuclear distribution of the transcription factor Oct‐4, which is specifically expressed in totipotent mice stem and germ line cells, was studied in mouse oocytes using immunogold labeling/electron microscopy and immunofluorescence/confocal laser scanning microcopy. The localization of Oct‐4 was studied in transcriptionally active (uni/bilaminar follicles) and inactive (antral follicles) oocytes. Additionally, the Oct‐4 distribution was examined relative to that of the unphosphorylated form of RNA polymerase II (Pol II) and splicing factor (SC 35) in the intranuclear entities such as perichromatin fibrils (PFs), perichromatin granules (PGs), interchromatin granule clusters (IGCs), Cajal bodies (CBs), and nucleolus‐like bodies (NLBs). It was shown that: (i) Oct‐4 is localized in PFs, IGCs, and in the dense fibrillar component (DFC) of the nucleolus at the transcriptionally active stage of the oocyte nucleus; (ii) Oct‐4 present in PFs and IGCs colocalizes with Pol II and SC 35 at the transcriptionally active stage; (iii) Oct‐4 accumulates in NLBs, CBs, and PGs at the inert stage of the oocyte. The results confirm the previous suggestion that PFs represent the major nucleoplasmic structural domain involved in active pre‐mRNA transcription/processing. The colocalization of Oct‐4 with Pol II in both IGCs and PFs in active oocytes (uni/bilaminar follicles) suggests that Oct‐4 is intimately associated with the Pol II holoenzyme before and during transcription. The colocalization of Oct‐4, Pol II, and SC 35 with coilin‐containing structures such as NLBs and CBs at the inert stage (antral follicles) suggests that the latter may represent storage sites for the transcription/splicing machinery during the decline of transcription. J. Cell. Biochem. 89: 720–732, 2003. © 2003 Wiley‐Liss, Inc.