J. Cell Sci. 67, 217-220 (1984)
217
Printed in Great Britain © The Company of Biologists Limited 1984
NUCLEOLAR NOMENCLATURE
E. G. JORDAN*
Biology Department, Queen Elizabeth College, University of London, Campden Hill
Road, London W8 7AH, U.K.
At the 8th European Nucleolar Workshop, which was organized by M. Bouteille
and colleagues at Banyuls-sur-mer in June 1983, a discussion was held on nucleolar
nomenclature. The terms that are italicized in the following report were agreed upon.
The short descriptions following each term were written to convey the consensus of
opinion that emerged. After the discussion a report was displayed for approval. The
following account is the approved, amended report of the meeting, incorporating a
few minor points made in later comments.
The names of those who wish to endorse the definitions of the terms listed, and, as
far as possible, intend to use them in their publications are listed in the Appendix. It
is hoped this attempt to standardize nomenclature will be useful to all workers in this
field, as well as to writers of text books.
In deciding on this terminology the active interphase nucleolus was foremost in our
considerations.
As far as possible an attempt was made to acknowledge current conventions and
avoid ambiguity. It was recognized that the precise molecular definition of the parts
of the nucleolus described by these terms might vary according to cell type and
physiology. There was particular concern to take into account the views obtained from
workers on both plant and animal cells.
TERMS
Fibrillar centres
Areas of low electron density in electron micrographs prepared by conventional
techniques. In functioning interphase nucleoli they are surrounded more or less
completely by the dense fibrillar component. They are distinct from nucleolar
vacuoles by their fibrillar nature. They may have condensed chromatin inclusions,
which can be in various configurations and states of condensation, especially in plant
nucleoli. Fibrillar centres that have areas of condensed chromatin besides the usual
material of low electron density may be termed heterogeneous fibrillar centres.
A list of the known features includes:
(i) Ribosomal DNA (rDNA). This has been demonstrated (some would prefer to
say suggested) only by light-microscopic in situ hybridization, and by inference from
• Chairman of discussion group.
218
E.G.Jordan
the location of polymerase I. However, no transcription has ever been demonstrated
in fibrillar centres by electron-microscopic autoradiography, even when intense
labelling in the surrounding dense fibrillar component is apparent; neither has the
presence of RNA been demonstrated by cytochemistry. For these reasons the fibrillar
centre is considered to be transcriptionally inert.
(ii) RNA polymerase I (nucleolar polymerase). This has been localized using the
labelled-antibody technique. The chromatin in the fibrillar centres is in a nonnucleosomal configuration as revealed by the Feulgen-like osmium-ammine
technique.
There is no evidence that the condensed chromatin seen in plant heterogeneous
fibrillar centres has a non-nucleosomal organization.
(iii) Ag—nucleolar organizing region (NOR) proteins. Fibrillar centres contain
protein with high silver affinity, staining by the various methods specific for the
protein present at secondary constrictions (metaphasic NORs). (Silver affinity can be
demonstrated in all nucleolar components or in the dense fibrillar component plus the
fibrillar centre, or in the fibrillar centre alone, depending upon the methods used. We
refer here to the procedure that is specific for the fibrillar centre.) The use of similar
staining procedures on gels has led to the identification of two nucleolar argyrophilic
proteins, C23 and B23, as yet incompletely characterized.
Dense fibrillar component
This component lacks granules and stains more intensely than other nucleolar
components (with the possible exception of condensed chromatin). It is commonly
held to be the site where active ribosomal transcription units reside. The absence of
granules such as those visualized in spread preparations on nascent ribonucleoprotein
fibrils of transcription units is not understood.
Granular component
This contains the 15 nm pre-ribosomal particles.
Nucleolar organizing region
The chromosomal region where genes for the major ribosomal RNAs (18 S, 5-8 S,
28 S of mammals) are located.
Nucleolonema
The threads and sheets, characteristically 0-1 /zm in diameter or thickness, which
are seen in some nucleoli. They may be made up of either dense fibrillar component
or granular component. (See note under Nucleolar vacuole on 'Interstices'.)
Nucleolar organizer track
This is the thick (about 1 [im) meandering structure that marks the course of the
nucleolar organizing region through the nucleolus. It is more characteristic of plant
nucleoli and has sometimes been called the nucleolonema. That term is best not used
Nucleolar nomenclature
219
for the organizer track. This will help to minimize confusion and acknowledges a
general convention among workers on animal cells.
Nucleolar vacuole
This is still the most satisfactory term to describe the larger, more or less spherical
'spaces' that have various inclusions and a generally nucleoplasmic appearance, despite the fact that they are not membrane bound. The use of the word nucleoplasmic
does not imply anything about the function of the vacuoles. The vacuoles usually
contain loosely dispersed pre-ribosomal-like particles and fibrils.
'Interstices' was considered to be a useful word to describe the smaller areas between the more or less closely applied nucleolonemal components.
Nucleolus-associated chromatin
Some condensed chromatin is always found adjacent to the nucleolus and
sometimes penetrating any of the other components.
'Peri-nucleolar chromatin' is used most frequently for the 'shell' of surrounding
chromatin often seen in animal nucleoli. The penetrating chromatin with a condensed
structure is usually called the 'intranucleolar chromatin', but no chromatin within the
nucleolus can be excluded from the term 'nucleolus-associated chromatin'.
Nucleolar matrix
The residual structure left after extraction procedures used to reveal the so-called
'nuclear matrix'. The term 'nucleolar matrix' was first used to describe the generally
amorphous background material in which the fibrils of the dense fibrillar component
and the granules of the granular component were considered to be embedded. The
presence of matrix proteins within the fibrillar centres is an interesting but open
question. We do not know whether the amorphous background material has any
relation to the residual nucleolar material.
APPENDIX
Chouinard, L. A.
Derenzini, M.
Fakan, S.
Fernandes Gomez, M. E.
Goessens, G.
Hadjiolov, A. A.
Hernandez-Verdun, D.
Jordan, E. G.
Kister, K. P.
Knibiehler, B.
Medina, F. J.
Mirre, C.
220
E.G. Jordan
Moreno Diaz De La Espina, S.
Muller, B.
Novello, F.
Olmedilla Arnal, A.
Pebusque, M. J.
Ploton, D.
Puvion, E.
Risueno, M. C.
Sanchez-Pina, M. A.
Scheer, U.
Seite, R.
Smetana, K. #
Stahl, A.
Stenram, U.
Stevens, B. J.
Valkov, I.
{Received 2 November 1983-Accepted 17 November 1983)
• Not present at the discussion but wishing to endorse the report.