The sphere organelle contains small nuclear ribonucleoproteins

We show by immunofluorescence microscopy of amphibian oocyte nuclei that small nuclear ribonucleoproteins (snRNPs) occur in lampbrush chromosome loops, in a few dozen extrachromosomal organelles previously described as "spheres," and in thousands of smaller granules. Spheres are variable in size (up to approximately 20 microns in diameter in the newt Notophthalmus and approximately 10 microns in the frog Xenopus) and are easily distinguishable from nucleoli by morphology and composition. Spheres occur both free in the nucleoplasm and attached to specific chromosome loci, the sphere organizers. Oocyte nuclei of a cricket and a spider contain essentially similar organelles, suggesting that spheres may be common throughout the animal kingdom. We suggest that spheres play a role in the assembly of snRNP complexes for the nucleus comparable to the way that nucleoli assemble ribosomal RNP complexes for the cytoplasm.