STRUCTURAL-CHANGES IN OOCYTE NUCLEOLI OF XENOPUS-LAEVIS DURING OOGENESIS AND MEIOTIC MATURATION

Immunoelectron microscopy with anti-nucleolin defined substructures wi thin the multiple nucleoli of biosynthetically active stage II-III ooc ytes and within the nucleoli of relatively quiescent stage VI oocytes of Xenopus laevis. Dense fibrillar components (DFCs) of nucleoli from stage II-III oocytes consisted of nucleolonemas that radiated from a c ontinuous DFC sheath surrounding fibrillar centers (FCs). Discernible granular regions (GRs) were absent in these same nucleoli. Conversely, stage VI oocyte nucleoli displayed compacted DFCS and prominent GRs. Immunofluroescence microscopy then tracked fibrillarin, nucleolin, and condensed DNA through oogenesis and into progesterone-induced meiotic maturation and nuclear breakdown. In stage II-III oocyte nucleoli, fi brillarin was enriched near the FC-DFC boundaries, while nucleolin was distributed throughout these same DFCs. Both proteins were enriched w ithin the compacted DFCs of stage VI oocyte nucleoli. Staining with (D API) 4',6-diamidino-2-phenyl-indole showed condensed DNA within nucleo lar FCs of both stage II-III and stage VI oocyte. Upon nuclear breakdo wn, we found fibrillarin and nucleolin in small particles and in the s urrounding cytoplasm. Although we saw no trace of fibrillarin or nucle olin in nuclear remnants prepared just minutes later, DAPI-stained par ticles remained within these preparations, thus suggesting that FCs we re at least slow to disassemble.