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PubMed1995

Genetic control of synapsis and recombination in Lolium amphidiploids.

Homologous bivalent formation in amphidiploids of Lolium is promoted during meiosis by diploidising genes carried by A-chromosomes, and by supernumerary B-chromosomes. The site and mode of action of these diploidising factors were investigated by comparing the relative frequencies of pairing configurations at meiotic prophase and metaphase I in several different hybrid genotypes. The results indicate that diploidising genes act predominantly by increasing the stringency of synapsis at early stages of meiotic prophase. By contrast, B-chromosomes appear to promote bivalent formation by ensuring that homoeologously paired chromosome segments within multivalents do not crossover. The results show that the additive effects of diploidising genes and B-chromosomes are to a certain extent separable in terms of their mode of action and timing during meiosis.

Chromosome Mapping↗
PubMed1984

Events associated with the initiation of mitosis in fused multinucleate HeLa cells.

Large multinucleate (LMN) HeLa cells with more than 10-50 nuclei were produced by random fusion with polyethylene glycol. The number of nuclei in a particular stage of the cell cycle at the time of fusion was proportionate to the duration of the phase relative to the total cell cycle. The fused cells did not gain generation time. Interaction of various nuclei in these cells has been observed. The nuclei initially belonging to the G1- or S-phase required a much longer time to complete DNA synthesis than in mononucleate cells. Some of the cells reached mitosis 15 h after fusion, whereas others required 24 h. The cells dividing early, contained a larger number of initially early G1-phase nuclei than those cells dividing late. The former very often showed prematurely condensed chromosome (PCC) groups. In cells with a large number of advanced nuclei the few less advanced nuclei could enter mitosis prematurely. On the other hand, the cells having a large number of nuclei belonging initially to late S- or G2-phase took longer to reach mitosis. These nuclei have been taken out of the normal sequence and therefore failed to synthesize the mitotic factors and depended on others to supply them. Therefore the cells as a whole required a longer period to enter mitosis. Although the nuclei became synchronized at metaphase, the cells revealed a gradation in prophase progression in the different nuclei. At the ultrastructural level the effect of advanced nuclei on the less advanced ones was evident with respect to chromosome condensation and nuclear envelope breakdown. Less advanced nuclei trapped among advanced nuclei showed PCC and nuclear envelope breakdown prematurely, whereas mitotic nuclei near interphase or early prophase nuclei retained their nuclear envelopes for a much longer time. PCC is closely related to premature breakdown of the nuclear envelope. Our observations clearly indicate that chromosome condensation and nuclear envelope breakdown are two distinct events. Kinetochores with attached microtubules could be observed on prematurely condensed chromosomes. Kinetochores of fully condensed chromosomes often failed to become connected to spindle elements. This indicates that the formation of a functional spindle is distinct from the other events and may depend on different factors.

Cell Division↗
PubMed1982

Cytophotometric and autoradiographic evidence for functional apomixis in a gynogenetic fish, Poecilia formosa and its related, triploid unisexuals.

Amounts of DNA in individual Feulgen-stained nuclei from squash preparations of ovaries and testes from wide-caught and laboratory-reared stocks of Poecilia spp. were determined with an integrating microdensitometer. The DNA content of primary spermatocytes (4C) at zygotene, pachytene, or at metaphase I (3.3-3.4 pg) was approximately twice that found in secondary spermatocytes (2C) and four times that found for young spermatids (1C). Rarely, mature sperm were found with 2C DNA amounts. Nuclei from follicular epithelium and oogonia from both bisexual and diploid unisexual fish contained about 1.6-1.7 pg DNA; whereas, the DNA content of primary oocyte nuclei was about 3.5-3.7 pg DNA, indicating that just one cycle of chromosomal replication had occurred in these cells during the period of DNA synthesis before the visible onset of meiotic prophase. Similar results were obtained for triploid unisexuals whose 6C primary oocyte nuclei contained 5.0-5.1 pg DNA, which was twice the DNA content of 3C oogonia and follicular epithelial cells (2.4-2.5 pg DNA). Autoradiographic studies, designed to monitor the incorporation of 3H-thymidine by oogonia and primary oocytes in vivo and in vitro, also showed that there is no additional synthesis of DNA during the course of meiotic prophase in these unisexual fish. Therefore, we conclude that apomixis, not endoreduplication, is the cytological basis of reproduction in Poecilia formosa and its related, triploid biotypes.

Animals↗
PubMed1993

Oocytes from pachytene to dictyotene can easily be analysed in neonatal rodents.

We have investigated the dynamics of meiotic prophase I in neonatal ovaries from different wild rodent species, from a laboratory strain of Mus musculus and from Mus musculus x Mus spretus F1 hybrids. We found that almost all stages of prophase I were regularly present in neonatal ovaries from these species and that their transcriptional activity can be assessed by [3H]-uridine incorporation, indicating that postnatal analysis of meiotic chromosomes and synapsis may be conducted as an alternative to the investigation of foetal ovaries.

Animals↗
PubMed1990

Oogenesis in the free proglottis of Trilocularia acanthiaevulgaris (Cestoda, Tetraphyllidea).

The early (ovarian) stages of oogenesis in Trilocularia acanthiaevulgaris have been studied by light microscope histochemistry and transmission electron microscopy. The process proceeds as far as meiotic prophase in the primary oocyte. The oogonia and early immature primary oocytes occupy the anterior and outer edges of the ovary and are typically undifferentiated cells, showing a high nucleo-cytoplasmic ratio. The scant cytoplasm is packed with free ribosomes and contains a small number of mitochondria and a few short strands of granular endoplasmic reticulum (GER). The oogonia undergo a number of mitotic divisions, marked by the presence of centrioles. The immature primary oocytes enter meiotic prophase, as evidenced by the appearance of nuclear synaptonemal complexes. The maturing primary oocytes, which occupy the inner, central region of the ovary, undergo a growth phase that is accompanied by an increased nucleolar volume and export of RNA to the cytoplasm via nuclear pores, and by an increase in the number of mitochondria. A GER network develops and, together with the Golgi complexes, is involved in the production of a small number of cortical granules. The GER often takes the form of concentric cisternae. Numerous lipid droplets are also present in the cytoplasm. The mature primary oocytes predominate in the posterior region of the ovary, near the oviduct. They represent a resting phase in development, in which cellular activity is minimal. Lipid droplets are abundant and the cortical granules remain in more central regions of the cell and do not migrate to the periphery of the cell. The ovary contains a second, non-germinal type of cell, the follicle cell. The cell body is smaller in size than the oogonia, and cytoplasmic processes from it ramify around the periphery of the ovary. The different cell types within the ovary are embedded in a cytoplasmic matrix that contains a number of organelles.

Animals↗
PubMed1990

Heteropycnosis of an underreplicating chromosome.

Drosophila nasutoides has an extraordinary genome since 62% of its DNA resides in chromosome 4. This element mainly consists of constitutive heterochromatin which does not polytenize. Earlier studies of heterochromatin attributed little attention to the fact that "condensed" chromosomes often vary in condensation. This paper reports that chromosomes of the same complement display different degrees and kinetics of condensation. In D. nasutoides, even sex specific differences can be observed. The results of a comparative microphotometric study on neuroblast metaphases in both sexes revealed the following picture. The process of chromosome condensation is not restricted to mitotic prophase but continues into the metaphase. The mean condensation is not equal for all chromosomes. In the metaphase of the female, Feulgen density increases from the X chromosome, via 3 and 2, to chromosome 4. In the male, the order is X, 2, 3, Y, and 4. During the metaphase of the male, chromosomes condense with similar kinetics. In contrast, chromosomes of the female display asynchrony as monitored by area and length determinations. The X chromosomes of the female probably have enhanced shortening during prophase. This would explain the metaphase of the female where the X chromosomes shorten less than the autosomes, and why each of the X chromosomes is 15% shorter than the X chromosome in the metaphase of the male. Further differences were observed in the longitudinal and lateral compaction of the chromosomes in males and females. The sex chromosomes and chromosome 3 condense by shortening, while chromosome 2 and 4 preferentially reduce their diameter. The large amount of DNA engaged in heteropycnosis and the isochromosome nature allow the identification of chromosome 4 during interphase. At this stage, a new category of extreme DNA packaging was detected. The interphase density of chromosome 4 can exceed that of metaphase by a factor of up to 8. Two events account for this high degree of condensation: (1) the homologues are particularly associated due to somatic pairing and (2) the arms are further tightened as a result of pericentric folding. The features of the isochromosome suggest that the interaction of chromatids during interphase is essentially caused by specific DNA sequences. The data confirm that heteropycnosis not only interferes with gene expression but also strongly inhibits DNA synthesis in endocycles.

Animals↗
PubMed1996

High-resolution fluorescence in situ hybridization of RBM- and TSPY-related cosmids on released Y chromatin in humans and pygmy chimpanzees.

Applying two-colour fluorescence in situ hybridization (FISH) we simultaneously hybridized RBM- and TSPY-related cosmids to Y chromosomes in prophase and to released Y chromatin in interphase nuclei of man and pygmy chimpanzee. Whereas, even on prophasic Y chromosomes, no resolution of the overlapping RBM and TSPY signal clusters could be achieved, the RBM and TSPY signals are completely separated from each other in our maximum released Y chromatin stretches in interphase nuclei. These results unequivocally lend support to the view that the RBM and TSPY families have an interspersed organization on the Y chromosomes of man and higher apes. Thus, the distribution of RBM and TSPY signals might well go back to a common organization of these genes next to each other on an ancient Y chromosome.

Animals↗
PubMed1996

Changes in chromosomal ultrastructure during the cell cycle.

The surface structure of mitotic barley and rye chromosomes was studied by high-resolution scanning electron microscopy. Chromosomes with various degrees of chromatin condensation were prepared from untreated meristematic tissue of root tips. At lower magnifications the highly condensed chromosomes in metaphase and anaphase showed a compact structure with a smooth surface. The condensation starts from the centromeric region and the chromatids are often discernible in the still uncondensed telomeric region. Decondensation begins at the telomeric region during telophase. Parallel arrangement of fibres is a characteristic feature predominately seen in prophase and telophase chromosomes. Chromatin structures that resemble tiles on a roof or braided strands were often observed. Prophase and telophase chromosomes are particularly suitable for further studies of chromatin arrangement and organization in plant chromosomes.

Cell Cycle↗
PubMed1996

Organization of DNA topoisomerase II isotypes during the cell cycle of human lymphocytes and HeLa cells.

We have monitored the organization of DNA topoisomerase II (Topo II) in relation to chromatin disaggregation during mitogen stimulation of lymphocytes and to the mitotic chromosome condensation cycle by immunofluorescence microscopy with isozyme-specific antibodies. Labelling for both Topo II alpha and Topo II beta was diffusely nucleoplasmic and non-nucleolar in resting lymphocytes and the pattern changed little during stimulation. Topo II alpha labelling intensity increased in parallel with the extent of cell stimulation, but a fraction of fully stimulated cells was labelled very brightly. Topo II beta labelling intensity was also greater in stimulated cells, but all partially and fully stimulated cells were labelled at the same, higher, intensity. In addition, anti-Topo II beta detected a few small spots within nucleoli of stimulated cells that coincided with regions containing fibrillarin. In lymphocytes and HeLa, chromosome association of Topo II alpha began in prophase and lasted throughout mitosis. In contrast, Topo II beta stayed nucleoplasmic in prophase, was diffusely cytoplasmic during mitosis, and was first detected post-mitotically in nuclei with decondensing chromosomes and a reformed nuclear envelope. The results are consistent with a role for Topo II alpha, but not for Topo II beta, in mitotic chromosome condensation, and indicate that the isotypes may play independent roles in the reorganization of chromatin structure during lymphocyte mitogenic activation.

Cell Cycle↗
PubMed1996

Tomographic distribution of acetylated histone H4 in plant chromosomes, nuclei and nucleoli.

Root tip cells of broad bean (Vicia faba L. cv. 'Wase soramame') and barley (Hordeum vulgare L. cv. 'Minorimugi') were immunostained with antibodies specific for acetylated histone H4. With an antiserum that recognizes histone H4 acetylated at lysine-5, the nucleolar organizing region (NOR) in mitotic chromosomes was strongly labeled in both species. The broad bean had two signals in the metaphase and telophase chromosome complements and four signals in the prophase and anaphase chromosome complements, while the barley had four signals in the metaphase and telophase chromosome complements and eight signals in the prophase and anaphase complements. Five different patterns of signals were observed at interphase: in type I only nucleoli were wholly stained; in type II perinucleolar knob-like signals and/or fiber-like signals emanated from the nucleus; in type III aggregate signals appeared in the nucleolus; in type IV many small dot-like signals were distributed throughout the nucleus, except nucleoli; and in type V string-like or some granule-like signals appeared in the nucleoli. Type II was very similar to previous results by in situ hybridization with sense rDNA probes. Type III was similar to the patterns of DNA synthesis recognized as chromatin domains by anti-BrdU antibodies. Type V was very similar to the results of in situ hybridization with pTa71, rDNA probes and the appearance of the dense fibrillar components of the nucleolus.

Acetylation↗
PubMed2002

The temporal and spatial distribution of the proliferation associated Ki-67 protein during female and male meiosis.

We used immunolocalization in tissue sections and cytogenetic preparations of female and male gonads to study the distribution of the proliferation marker pKi-67 during meiotic cell cycles of the house mouse, Mus musculus. During male meiosis, pKi-67 was continuously present in nuclei of all stages from the spermatogonium through spermatocytes I and II up to the earliest spermatid stage (early round spermatids) when it appeared to fade out. It was not detected in later spermatid stages or sperm. During female meiosis, pKi-67 was present in prophase I oocytes of fetal ovaries. It was absent in oocytes from newborn mice and most oocytes of primordial follicles from adults. The Ki-67 protein reappeared in oocytes of growing follicles and was continuously present up to metaphase II. Thus, pKi-67 was present in all stages of cell growth and cell division while it was absent from resting oocytes and during the main stages of spermiocytogenesis. Progression through the meiotic cell cycle was associated with extensive intranuclear relocation of pKi-67. In the zygotene and pachytene stages, most of the pKi-67 colocalized with centromeric (centric and pericentric) heterochromatin and adjacent nucleoli; the heterochromatic XY body in male pachytene, however, was free of pKi-67. At early diplotene, pKi-67 was mainly associated with nucleoli. At late diplotene, diakinesis, metaphase I and metaphase II of meiosis, pKi-67 preferentially bound to the perichromosomal layer and was almost absent from the heterochromatic centromeric regions of the chromosomes. After the second division of male meiosis, the protein reappeared at the centromeric heterochromatin and an adjacent region in the earliest spermatid stage and then faded out. The general patterns of pKi-67 distribution were comparable to those in mitotic cell cycles. With respect to the timing, it is interesting to note that relocation from the nucleolus to the perichromosomal layer takes place at the G2/M-phase transition in the mitotic cell cycle but at late diplotene of prophase I in meiosis, suggesting physiological similarity of these stages.

Animals↗
PubMed2005

Kiss and break up--a safe passage to anaphase in mitosis and meiosis.

Eukaryotic chromosomes have many challenges to overcome between DNA replication and sister chromatid segregation. If these challenges are not met, cell death or unregulated cell division (cancer) may result. During prophase, chromosomes condense, the nuclear membrane breaks down and cohesins are removed from chromosome arms. In prometaphase, initial spindle attachments are made by sister kinetochores followed by correction of erroneous attachments, centromere oscillation between spindle poles and congression towards the cell's equator. In metaphase, all chromosomes attain stable bipolar spindle attachments and align at the metaphase plate, ready for the metaphase-anaphase transition when all ties between sister chromatids are broken. This review concentrates on recent developments that have revealed the intricacies of these processes. We now know more about how the mechanisms of cohesin removal differ between prophase and the metaphase-anaphase transition, the processes for detection and correction of improper spindle-kinetochore attachments and the concept that tension between sister kinetochores is the driving factor for satisfying the spindle checkpoint. We are also beginning to gain some understanding of the mechanisms behind the co-segregation of sister chromatids at the first meiotic division.

Anaphase↗
PubMed2006

S. pombe linear elements: the modest cousins of synaptonemal complexes.

Synaptonemal complexes (SCs) are not formed during meiotic prophase in the fission yeast, Schizosaccharomyces pombe. Instead, so-called linear elements (LinEs) are formed at the corresponding stages. LinEs are remarkable in that their number does not correspond to the number of chromosomes or bivalents and that the changes in their organisation during prophase do not evidently reflect the pairing of chromosomes. Yet, LinEs are necessary for full meiotic pairing levels and for meiotic recombination. In this review, the composition of LinEs, their evolutionary relationship to SCs and their possible functions are discussed.

Cell Cycle Proteins↗
PubMed2006

From early homologue recognition to synaptonemal complex formation.

This review focuses on various aspects of chromosome homology searching and their relationship to meiotic and vegetative pairing and to the silencing of unpaired copies of genes. Chromosome recognition and pairing is a prominent characteristic of meiosis; however, for some organisms, this association (complete or partial) is also a normal part of nuclear organization. The multiple mechanisms suggested to contribute to homologous pairing are analyzed. Recognition of DNA/DNA homology also plays an important role in detecting DNA segments that are present in inappropriate number of copies before and during meiosis. In this context, the mechanisms of methylation induced premeiotically, repeat-induced point mutation, meiotic silencing by unpaired DNA, and meiotic sex chromosome inactivation will be discussed. Homologue juxtaposition during meiotic prophase can be divided into three mechanistically distinct steps, namely, recognition, presynaptic alignment, and synapsis by the synaptonemal complex (SC). In most organisms, these three steps are distinguished by their dependence on DNA double-strand breaks (DSBs). The coupling of SC initiation to (and downstream effects of) DSB formation and the exceptions to this dependency are discussed. Finally, this review addresses the specific factors that appear to promote chromosome movement at various stages of meiotic prophase, most particularly at the bouquet stage, and on their significance for homologue pairing and/or achieving a final pachytene configuration.

Animals↗
PubMed1997

XY body formation during rat spermatogenesis: an immunocytochemical study using antibodies against XY body-associated proteins.

The process of formation of the XY body during meiotic prophase was investigated by immunocytochemistry on cryosections of pubertal rat testes using antibodies against three different XY body-associated proteins. Here we show that these proteins are detectable at only partially overlapping temporal windows. These findings provide the first evidence that the previously described morphological changes in the structure of the XY body that occur during meiotic prophase are accompanied by considerable changes in its protein composition.

Animals↗
PubMed1999

The association of ATR protein with mouse meiotic chromosome cores.

The ATR (ataxia telangiectasia- and RAD3-related) protein is present on meiotic prophase chromosome cores and paired cores (synaptonemal complexes, SCs). Its striking characteristic is that the protein forms dense aggregates on the cores and SCs of the last chromosomes to pair at the zygotene-pachytene transition. It would appear that the ATR protein either signals delays in pairing or it is directly involved in the completion of the pairing phase. Atm-deficient spermatocytes, which are defective in the chromosome pairing phase, accumulate large amounts of ATR. The behaviour of ATR at meiotic prophase sets it apart from the distribution of the RAD51/DMC1 recombinase complex and our electron microscope observations confirm that they do not co-localize. We failed to detect ATM in association with cores/SCs and we have reported elsewhere that RAD1 protein does not co-localize with DMC1 foci. The expectation that putative DNA-damage checkpoint proteins. ATR, ATM and RAD1, are associated with RAD51/DMC1 recombination sites where DNA breaks are expected to be present, is therefore not supported by our observations.

Adenosine Triphosphatases↗
PubMed2000

Localisation of RAD50 and MRE11 in spermatocyte nuclei of mouse and rat.

Synaptonemal complexes (SCs) are zipperlike structures that are assembled between homologous chromosomes during meiotic prophase. They consist of two axial elements (AEs) (one along each of the two homologous chromosomes), which, in mature SCs, are connected by numerous transverse filaments along their length. Several proteins involved in the later steps of meiotic recombination most probably function in close association with the AEs of SCs, because the proteins involved in these steps have all been localised along AEs or SCs by immunocytochemical methods. It is not known at which step in meiotic recombination this association with the AEs is established. In order to shed some light on this issue, we analysed the localisation of two proteins that are involved in early steps of meiotic recombination, RAD50 and MRE11, relative to AEs and SCs by immunofluorescence labelling of paraffin sections of the mouse testis, using affinity-purified polyclonal antibodies against RAD50 and MRE11, and monoclonal and polyclonal antibodies against SC components. The localisation patterns of MRE11 and RAD50 within spermatocytes were very similar. MRE11 and RAD50 appeared in high abundance in preleptotene spermatocytes, just before SC components could be detected. From preleptotene until early zygotene they were present throughout the nucleus. In mid and late zygotene, MRE11 and RAD50 concentrated in distinct areas; in early pachytene the two proteins had almost disappeared from the nucleus, except from the sex vesicle (the chromatin of the XY bivalent), where they persisted in high abundance until diplotene. We propose that MRE11 and RAD50, together with other proteins, prepare chromatin throughout the early meiotic prophase nucleus for the initiation of meiotic recombination. Possibly, only a small fraction of the RAD50- and MRE11-containing (pre)recombination complexes associates transiently with AEs, where further steps in meiotic recombination can take place.

Animals↗
PubMed2000

Distribution of Atr protein in primary spermatocytes of a mouse chromosomal mutant: a comparison of preparation techniques.

In this study, we examined the suitability of a three dimensional preparation technique for studying chromosome behaviour in the first meiotic prophase in the mouse chromosomal mutant T(1;13)H/T(1;13)Wa. To preserve cellular shape, primary spermatocytes were encapsulated in a fibrin clot. Conventionally sedimented prophase nuclei served as controls. Axial elements and lateral synaptonemal complex components were subsequently stained by immunofluorescence and the presence of axial elements at the pachytene stage was highlighted with indirect immunofluorescence against the Atr protein. We compared the distribution of Atr signal in the fibrin-embedded spermatocytes with surface-spread preparations and immunohistochemically stained histological sections of seminiferous tubules. Furthermore, fluorescence in situ hybridisation of the mouse minor satellite DNA was done on fibrin-embedded spermatocytes. The Atr signal is most conspicuous in fibrin-embedded nuclei on unpaired axial elements during pachytene, both for sex chromosomal and for autosomal segments, and expanding from these elements into the surrounding chromatin. Both spread and encapsulated zygotene nuclei with extended axial element formation proved to be positive for Atr. Mid- to late zygotene nuclei were devoid of 3,3'-diaminodibenzene deposition in the histological sections. Highlighting the unpaired axial elements in the small heteromorphic 1(13)H;1(13)Wa bivalent with an Atr signal enabled meiotic analysis of this bivalent to be carried out in a three-dimensional context. Thus, proximity of this bivalent with the sex chromosomes is found more often in three-dimensional preparations than in spread preparations. Furthermore, the development of the Atr signal over the sex chromosomes as pachytene proceeds helps in substaging of this long and heterogeneous meiotic phase, in sedimented but especially in fibrin-encapsulated nuclei.

Animals↗