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Journal of Cell Science, Vol 8, 1-17, Copyright © 1971 by Company of Biologists
Submitted on April 24, 1970
1 Department of Biology, University of Oregon, Eugene, Oregon, U.S.A.
2 Department of Biology, University of Oregon, Eugene, Oregon, U.S.A.; Department of Zoology, University of Leicester, Leicester, England
Nucleoli in some large yolky oocytes of Plethodon cinereus and Ascaphus truei have attached to them a filament which may or may not have a beaded appearance. These filaments have been called ‘nucleolar tails’. One nucles may contain several hundred of them. They may be up to 200 µm long and are usually about 1 µm wide. The beads which are sometimes attached to the filament are round, refractile and up to 3 µm in diameter. The tails are a feature of nuclei in which the nucleoli have left the nuclear envelope and migrated inward to cluster around the chromosomes in the centre of the nucleus. The tails project radially outwards from the central mass of nucleoli. Tails may be attached to round solid nucleoli, as a usually the case in A. truei, or they may be attached to ring nucleoli, as is often the case in P. cinereus, or they may lie free in the nuclear sap. Those nucleoli which remain atttached to the nucleolar organizing site on the lampbrush chormosomes of P. cinereus never have tails. Nucleolar tails are Feulgen-negative and do not stain with gallocyanine when it is used under the proper conditions for staining nucleic acids. Tails are unaffected by deoxyribonuclease and ribonuclease but are destroyed by proteolyic enzymes.
Thin sections of oocytes whose nuclei contained nucleolar tails showed chains of membranous vesicles lying near to many of the nucleoli. These vesicles appeared empty, some of them were confluent with their neighbours, all were bounded by a unit membrane and all were coated on their outer surfaces by a fine fibrous material. The chains of vesicles commonly ended near a depression on the surface of the core of a nucleolus. Chains of vesicles sometimes extended into a long membranous tube. We have interpreted these chains of vesicles and tubes as sections through nucleolar tails.
It is suggested that nucleolar tails may be associated with the replication of nucleolar DNA, or with the division of nucleolar cores and nucleoli, or with the production of nuclear membrane at a time when the nucleus in enlarging rapidly. Each of these suggestions is discussed. A likeness between nucleolar tails and the mesosomes of bacteria is proposed and discussed.
Submitted on April 24, 1970
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