In the last few years numerous papers have been published on the nucleolus and its relation to the nucleolus organizer region (NOR). Several, even very recent, publications still reveal differences in the interpretation of these results. It is, however, our opinion that at the present state most of the observations on NORs and nucleoli can be interpreted so as to achieve a general understanding of the nucleolus. For this purpose we shall summarize some morphological results and problems with particular reference to the example of resting and stimulated lymphocytes. This may supplement recent reviews covering molecular aspects (Perry 1981; Moss and Birnstiel 1982), the cytological visualization of nucleolar genetic activity (Miller 1981; Scheer et al. 1982), the amplification of ribosomal genes (Macgregor 1982), problems of nucleolus formation in plants (Flavell and Martini 1982; de la Torre and Gimenez-Martin 1982), as well as reviews dealing mostly with the functional morphology of nucleoli in vertebrate and also human cells (Goessens and Lepoint 1979; Jordan and McGovern 1981; Bouteille et al. 1982; Stahl 1982). A few notes on the history of the early work on nucleoli are given in the beginning, supplementing the introduction to the review by Miller (1981), since we feel that the very careful observations of the early cytologists should be remembered and reread, if not for their face value, then at least as an example of exact descriptive morphology. Needless to say, this short review article can give only a small selection of the many papers on our topic and will be far from complete.

Nucleolus organizer regions and nucleoli.

Variations of constitutive heterochromatin (heteromorphisms) appear to be a general feature of eucaryotes. A variety of molecular and cytogenetic evidence supports the hypothesis that heteromorphisms result from unequal double-strand exchanges during mitotic DNA replication. Constitutive heterochromatin consists of highly repeated DNA sequences that are not transcribed. Thus, heteromorphisms are tolerated without overt phenotypic effect. Several of the highly repeated DNAs that comprise constitutive heterochromatin have been shown to contain site-specific endonuclease recognition sequences interspersed at regular intervals dependent upon nucleosome structure. These interspersed short repeated sequences could mediate unequal crossovers, resulting in quantitative variability of constitutive heterochromatin and satellite DNA. De novo variations of constitutive heterochromatin may be useful as markers of exposure to mutagens and/or carcinogens.

Satellite DNA and heterochromatin variants: the case for unequal mitotic crossing over.

Human leukocyte cultures were pulse-treated with the trifunctional alkylating mutagen trenimon in a final concentration of 10-7M for 15–20 h after culture start, i.e., in the G1 phase of the cell cycle. At 24 h after culture start bromodeoxyuridine (BUdR) was added to the trenimon-treated cultures and to several untreated cultures running in parallel. The series treated with BUdR only and the series treated with BUdR+trenimon were each used to prepare two cultures at different culture times. Mitoses were collected during consecutive intervals of 12 h from 30 h up to 102 h after culture initiation by colcemid. For all preparation times (42 h, 54 h, 66 h, 78 h, 90 h, and 102 h) the frequencies of first, second, and third and further mitoses were determined in the BUdR- and in the BUdR+trenimon-treated series. In the trenimon-treated series a clear cell cycle delay was detected as compared with the normal distribution of different types of mitoses found in series treated with BUdR only. Spontaneous and trenimon-induced sister chromatid exchange (SCE) frequencies were determined in second mitoses occurring at 66 h, 78 h, 90 h, and 102 h after culture start. For all these preparation times about six SCE per metaphase were consistently found in BUdR-treated, and about 19 SCE per metaphase in BUdR+trenimon-treated series, indicating a homogeneous sensitivity of early- and late-proliferating cells with respect to the induction of SCE.

Sister chromatid exchanges in human leukocyte chromosomes: spontaneous and induced frequencies in early- and late-proliferating cells in vitro.

Publisher Summary The nucleolar organizer regions (NORs) have been demonstrated to be the morphological sites around which the nucleoli develop at the end of mitosis. This chapter describes the structural and functional aspects of NORs of human chromosomes and discusses the clinical significance of these regions. The chromosomal associations resulted from NORs play an important role in at least three types of chromosomal disorders, and the most frequent of them is the meiotic nondisjunction causing trisomic condition. The degree of NOR activity is represented by different types of nucleoli, such as small ring-shaped nucleoli with low activity and compact nucleoli with full activity. The human acrocentric chromosomes because of the presence of NORs on short arms have a tendency to remain associated with each other during the cell division, which have deleterious consequences through meiotic nondisjunction. However, the biological and clinical implications of NOR size heteromorphisms of human acrocentrics are poorly understood.

Structural and functional aspects of nucleolar organizer regions (NORs) of human chromosomes.

Stain intensity of human nucleolus organizer region reflects incorporation of uridine into mature ribosomal RNA.

The Ag-stainability of the nucleolus organizer region (NOR) was studied in the acrocentric chromosomes identified by Q-banding in cultured lymphocytes from seven children with trisomy 21 and their parents. The observed Ag-NOR patterns were in accordance with chromosomal inheritance except for a slight intraindividual variation which might be explained mainly by technical causes. In two cases the meiotic nondisjunction could be attributed to one of the parents, once to the father, and once to the mother. It is concluded that the Ag-stainability of the NORs is in general a heritable characteristic of the acrocentric chromosomes in maximally activated cells as, e.g., cultured lymphocytes. It may reflect individual differences in the amount of rDNA as well as differences in the capacity for NOR activation.

Inheritance of Ag-stainability of nucleolus organizer regions. Investigations in 7 families with trisomy 21.

Human female blood cultures were labeled with BrdU for detecting sister chromatid exchanges (SCEs) by the Hoechst 33258 fluorescence technique. Late labeling with 3H-thymidine and autoradiography allowed the identification of the late replicating X. The mean number of SCEs in the cells was 13. The isopycnotic X showed an exchange frequency according to its relative length in the karyotype; in the late replicating X a doubled number of SCE events was observed.

P. Wagenbichler

Papers

Inheritance of Ag-stainability of nucleolus organizer regions. Investigations in 7 families with trisomy 21.

Increased sister chromatid exchange events in the human late replicating X.