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PubMed1998

Subcellular localization of prostaglandin endoperoxide H synthases-1 and -2 by immunoelectron microscopy.

Prostaglandin endoperoxide H synthases-1 and -2 (PGHS-1 and -2) are the major targets of nonsteroidal anti-inflammatory drugs like aspirin and ibuprofen. These enzymes catalyze the committed step in the formation of prostanoids from arachidonic acid. Although PGHS-1 and -2 are similar biochemically, a number of studies suggest that PGHS-1 and PGHS-2 function independently to form prostanoids that subserve different cellular functions. We have hypothesized that these isozymes may reside, at least in part, in different subcellular compartments and that their compartmentation may affect their access to arachidonic acid and serve to separate the functions of the enzymes. To obtain high resolution data on the subcellular locations of PGHS-1 and -2, we employed immunoelectron microscopy with multiple antibodies specific to each isozyme. Both PGHS-1 and -2 were found on the lumenal surfaces of the endoplasmic reticulum (ER) and nuclear envelope of human monocytes, murine NIH 3T3 cells, and human umbilical vein endothelial cells. Within the nuclear envelope, PGHS-1 and -2 were present on both the inner and outer nuclear membranes and in similar proportions. Western blotting data showed a similar distribution of PGHS-1 and -2 in subcellular fractions, and product analysis using isozyme-specific inhibitors suggested that both enzymes generate the same products in NIH 3T3 cells. Thus, we are unable to attribute the independent functioning of PGHS-1 and PGHS-2 to differences in their subcellular locations. Instead, the independent operation of these isozymes may be attributable to subtle kinetic differences (e.g. negative allosteric regulation of PGHS-1 at low concentrations of arachidonate (500-1000 nM)). A further conclusion of importance from a cell biological perspective is that membrane proteins such as PGHS-1 and -2, which are located on the lumenal surface of the ER, are able to diffuse freely among the ER and the inner and outer membranes of the nuclear envelope.

3T3 Cells↗

PubMed1996

Reconstitution of ionic channels from inner and outer membranes of mammalian cardiac nuclei.

Recent reports suggest that the nuclear envelope possesses specific ion transport mechanisms that regulate the electrolyte concentrations within the nucleoplasm and perinuclear space. In this work, intact nuclei were isolated from sheep cardiac cells. After chromatin digestion, the nuclear envelopes were sonicated and four nuclear vesicle populations were separated by sucrose step gradients (SF1-SF4). These fractions were compared by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and their protein content was analyzed by Western blot, using lamin and SEC 61 antibodies. The lamins, which are associated with the inner nuclear membrane, were present in three fractions, SF2, SF3, and SF4, with a lower amount in SF2. The SEC 61 protein, a marker of the rough endoplasmic reticulum, was detected in small amounts in SF1 and SF2. Upon fusion of vesicles into bilayers, the activities of nuclear ionic channels were recorded in 50 mM trans/250 mM cis KCl or CsCl, pH 7.2. Two types of Cl- selective channels were recorded: a large conducting 150-180-pS channel displaying substates, and a low conducting channel of 30 pS. They were both spontaneously active into bilayers, and their open probability was poorly voltage dependent at negative voltages. Retinoic acid (10(-8) M) increases the po of the large Cl- conducting channel, whereas ATP modifies the kinetics of the low conductance anion selective channel. Our data also suggest that this anionic channel is mainly present in the SF3 and SF4 population. The presence of a 181 +/- 10 pS cation-selective channel was consistently observed in the SF2 population. The behavior of this channel was voltage dependent in the voltage range -80 to +60 mV. Furthermore, we report for the first time the activity of a channel exclusively present in the SF3 and SF4 fractions, shown to contain mainly inner membrane vesicles. This cation selective channel displays a 75-pS conductance in 50 mM trans/250 mM cis K-gluconate. It is concluded that the bilayer reconstitution technique is an attractive approach to studying the electrophysiological properties of the inner and outer membranes of the nuclear envelope.

Animals↗

PubMedDate not supplied

Nuclear pore complex ion channels (review).

It is currently thought that nuclear pore complexes (NPCs) primarily govern nucleocytoplasmic interactions via selective recognition and active transport of macromolecules. However, in various nuclear preparations, patch-clamp and fluorescence, luminiscence and ion microscopy support classical microelectrode measurements indicating that monoatomic ion flow across the nuclear envelope (NE) is strictly regulated. Gating of large conductance nuclear envelope ion channels (NICs) somewhat resembles that of gap junctional channels. In other respects, NICs are distinct in that they require cytosolic factors, are blocked by wheat germ agglutinin and are blocked and/or modified by antibodies to epitopes of NPC glycoproteins. Therefore, NIC activity, recorded as electrical current/conductance is likely to be intrinsic to NPCs. This observation suggests a potential use for the patch-clamp technique in establishing the mechanisms underlying nuclear pore gating in response to cytosolic and nucleosolic factors such as transcription and growth factors, oncogene and proto-oncogene products and receptors for retinoids, steroids and thyroid hormone. NIC activity may also be useful in evaluating the mechanisms of nuclear import of foreign nucleic acid material such as that contained in virons and viroids. Finally, in consideration to the electrophysiological data accumulated so far, the study of nuclear pore ion channel activity may help our understanding of other important issues such as cell suicide, programmed cell death or apoptosis.

Animals↗

PubMed1991

Pentraxin binding to isolated rat liver nuclei.

The interaction of human C-reactive protein (CRP) and serum amyloid P-component (SAP) with isolated rat liver nuclei was studied to identify nuclear ligands for each pentraxin using the iodinatable heterobifunctional thiol-cleavable cross-linking reagent sulphosuccinimidyl-2-(p-azidosalicylamido)-1,3'-dithiopropio nate (SASD). Nuclei (100 micrograms of DNA) bound 21 pmol of 125I-labelled CRP Ca(2+)-dependently at saturation with half-saturation occurring at 200 pmol of 125I-CRP. By contrast, only 2.7 pmol of 125I-labelled SAP was bound at saturation, with half-saturation at 50 pmol. The binding of pentraxins to nuclei is, in addition to putative chromatin binding, due to nuclear-envelope binding, where 3.2 pmol 125I-labelled CRP binds Ca2+ dependently to nuclear envelopes (25 micrograms) at saturation, but only 0.62 pmol SAP is required to saturate. Specific photocross-linking of 125I-2-(p-azidosalicylamido)-1,3'-dithiopropionate (125I-ASD)-CRP and 125I-ASD-SAP to nuclei revealed transfer of 125I-photoreactive azides to nuclear-envelope proteins of 43, 46, 52 and 70 kDa. In addition, SAP binding to histones H2A, H2B, H3 and H4 was detected, whereas CRP bound only to H4. Neither pentraxin cross-linked to histone H1.

Animals↗

PubMed1998

Nucleocytoplasmic transport: the soluble phase.

Active transport between the nucleus and cytoplasm involves primarily three classes of macromolecules: substrates, adaptors, and receptors. Some transport substrates bind directly to an import or an export receptor while others require one or more adaptors to mediate formation of a receptor-substrate complex. Once assembled, these transport complexes are transferred in one direction across the nuclear envelope through aqueous channels that are part of the nuclear pore complexes (NPCs). Dissociation of the transport complex must then take place, and both adaptors and receptors must be recycled through the NPC to allow another round of transport to occur. Directionality of either import or export therefore depends on association between a substrate and its receptor on one side of the nuclear envelope and dissociation on the other. The Ran GTPase is critical in generating this asymmetry. Regulation of nucleocytoplasmic transport generally involves specific inhibition of the formation of a transport complex; however, more global forms of regulation also occur.

Biological Transport, Active↗

PubMed1987

A monoclonal antibody against a family of nuclear pore proteins (nucleoporins): O-linked N-acetylglucosamine is part of the immunodeterminant.

Using nuclear envelopes from Chinese hamster ovary (CHO) cells as an antigen, a mouse monoclonal antibody (IgM; designated mAb CHON211) that specifically binds to components of the nuclear pore complex (nucleoporins) was isolated. Immunofluorescence localization of the antigen recognized by mAb CHON211 revealed a punctate pattern restricted to the nuclear envelope; this pattern changed dramatically during the cell cycle. When examined by electron microscopy, the antigen was largely restricted to the nucleoplasmic and cytoplasmic faces of the nuclear pore complex. Immunoblots showed that mAb CHON211 bound to a series of polypeptides enriched in the nuclear envelope fraction with apparent molecular masses ranging from 110 to 35 kDa. Using galactosyltransferase and the lectin wheat germ agglutinin as probes, we have previously shown that proteins bearing O-linked N-acetylglucosamine (GlcNAc) are restricted to the cytoplasmic and nucleoplasmic faces of the nuclear pore complex. The nuclear membrane proteins recognized by mAb CHON211 had properties very similar to those identified by galactosyltransferase and wheat germ agglutinin labeling. Removal of O-linked GlcNAc residues with beta-hexosaminidase greatly reduced the binding of mAb CHON211, strongly suggesting that O-linked GlcNAc moieties are part of the immunodeterminant. This monoclonal antibody defines a new family of antigens that are restricted to the nuclear pore and bear a common modification: O-linked GlcNAc. mAb CHON211 will be useful for defining the role of the nucleoporins in nucleo-cytoplasmic exchange.

Acetylglucosamine↗

PubMed1989

Chinese hamster ovary cell mitosis and its response to ionizing radiation: a morphological analysis of the living cell.

Repeated microscopic observations of exponentially growing Chinese hamster ovary cells were made and the times and mitotic stages were recorded in control and irradiated cultures at 37 degrees C. As determined by autoradiography, the time from the end of S phase to early prophase (the G2 phase) was 46 min, to breakdown of the nuclear envelope was 91 min, and to restoration of the nuclear envelope was 116 min. The time spent in morphologically distinguishable phases of mitosis and the effects of 0.5, 1.0, 1.5, 2.0, and 4.0 Gy of gamma or X radiation on cells at each phase were determined. Affected cells were found to be delayed without or with reversion to an earlier mitotic stage before recovering and advancing through mitosis. Cells were timed in the five steps comprising delay with reversion: inertia, cessation I, regression, cessation II, and reprogression. No cells treated in late prophase, i.e., within 8-10 min of nuclear envelope breakdown, were delayed by the doses used; therefore the critical or transition point must be situated in middle prophase. Cells irradiated in this stage were not delayed by 0.5 or 1.0 Gy, but suffered a dose-dependent delay with or without reversion after 1.5, 2.0, and 4.0 Gy. Cells irradiated in early prophase and very late interphase responded similarly, but a greater percentage of the latter reverted.

Animals↗

PubMed1979

Effects of hyperthermia (41.5 degrees) on Chinese hamster ovary cells analyzed in motisis.

The mitotic cells from an asynchronous population of Chinese hamster ovary cells exposed to 41.5 degrees for 7 hr were examined by light and electron microscopy to determine if there were any morphological abnormalities related to cell death or lengthening of metaphase induced by hyperthermia. All components of the mitotic apparatus were formed during exposure to heat, and the mitotic apparatus was functional as demonstrated by eventual cell division. However, heat caused the nuclear envelope to reform precociously around the chromosomes except in the region of the kinetochores, and the nuclear envelope remained associated with the chromatids during segregation. The precocious reformation of the nuclear envelope may be responsible for the lengthening of metaphase. Cells undergoing mitosis during the heat treatment possessed large evaginations of the plasma membrane, and the ubiquitous cortical microfilaments were absent in the region of these evaginations. Possibly related to the membrane damage were osmotic changes resulting in swollen mitochondria observed in heated cells entering mitosis. Since hyperthermic damage to the plasma membrane-microfilament complex was not observed in interphase cells or in cells completing division but was morphologically expressed during mitosis, the thermal lability of the plasma membrane must increase as the cells enter mitosis.

Animals↗

PubMed2002

Role of endothelin-1 receptors in the sarcolemma membrane and the nuclear membrane in the modulation of basal cytosolic and nuclear calcium levels in heart cells.

Our previous work in heart cells showed that the nuclear envelope membranes possess receptors, such as those for angiotensin II. Using non-working single cells from the hearts of 10-day chick embryos and the confocal microscopy technique, our present results show that stimulation of endothelin-1 (ET-1) receptors at the sarcolemma membrane induced a dose-dependent sustained increase in basal cytosolic and nuclear calcium levels that was insensitive to the ET(A) and ET(B) receptor antagonists BQ123 and BQ788. The cytosolic application of ET-1 induced a dose-dependent increase in the sustained basal nuclear Ca(2+) concentration that was insensitive to BQ123 and BQ788. The ET-1 receptors at the nuclear envelope membrane were more sensitive to ET-1 than those located at the sarcolemma membrane of heart cells. Cytosolic application of ET-1 prevented sustained basal cytosolic Ca(2+) uptake by the nuclei. The use of an ET-1 fluorescent probe demonstrated the presence of ET-1 receptors at both the sarcolemma membrane and the nuclear envelope membrane. Thus our results suggest that ET-1 receptors that are insensitive to BQ123 and BQ788 are present at both the sarcolemma and nuclear envelope membranes. Extracellular and cytosolic ET-1 may play a role in regulating basal cytosolic and nuclear Ca(2+) levels in heart cells.

Animals↗

PubMed1990

Identification of a Golgi-associated protein that undergoes mitosis dependent phosphorylation and relocation.

By means of a monoclonal antibody (BH3), we have identified a 57-kD protein (p57) that in interphase is restricted largely to the perinuclear region of the cell. Double label immunofluorescence microscopy suggests localization of p57 to the Golgi complex and associated membranous structures. Protease protection experiments and chemical extractability indicate that p57 is a peripheral membrane protein exposed to the cytoplasm. p57 displays unique behavior during mitosis. At the end of G2 or in early prophase, p57 leaves the perinuclear region and accumulates very rapidly within the nucleus, at a time when the nuclear envelope is still intact and before nuclear lamina disassembly. This relocation of p57 coincides with its hyperphosphorylation on serine and threonine residues. After nuclear envelope breakdown p57 becomes uniformly distributed throughout the mitotic cytoplasm until in late telophase when it returns to its perinuclear location and is once again excluded from the nucleus. The behavior of p57 during mitosis suggests that it may play a role in the cellular reorganization evident during mitotic prophase.

Animals↗

PubMed1999

Hormonal influences on tryptophan binding to rat hepatic nuclei.

This study evaluated whether selected hormones, 3,5,3'-triiodothyronine (T3), hydrocortisone (HC), or insulin, would influence the binding of L-tryptophan to rat hepatic nuclei or nuclear envelopes. The first two hormones have nuclear receptors that belong to the same superfamily, while insulin belongs to a different unrelated superfamily of receptors. Previous reports have indicated that the binding of L-tryptophan to hepatic nuclear proteins was saturable, stereospecific, and of high affinity. Also, previous studies showed that administration of L-tryptophan rapidly stimulated hepatic protein synthesis. In this study, we investigated whether each hormone alone or together with unlabeled tryptophan would influence tryptophan binding to rat hepatic nuclei or nuclear envelopes as assayed by in vitro L-5-(3)H-tryptophan binding. Our results indicate that T3 10(-14) to 1(-10) mol/L appreciably inhibited in vitro 3H-tryptophan binding to hepatic nuclei and T3 10(-16) to 10(-4) mol/L appreciably ameliorated the inhibitory effect of unlabeled tryptophan (10(-4) mol/L) on in vitro 3H-tryptophan binding. In vivo administration (1 hour) of tryptophan alone stimulated hepatic protein synthesis, but addition of T3 negated such stimulation. Addition of HC 10(-12) to 10(-4) mol/L had no effect and addition of insulin 10(-16) to 10(-4) mol/L had only a small inhibitory effect on in vitro 3H-tryptophan binding to rat hepatic nuclei, but each (10(-12) to 10(-4) mol/L), when added to unlabeled tryptophan (10(-4) mol/L), diminished the inhibitory binding effect of unlabeled tryptophan alone. Our study indicates that T3 competes with tryptophan for hepatic nuclear tryptophan binding, and it also appears to negate tryptophan's stimulatory effect on hepatic protein synthesis.

Animals↗

PubMed2000

Nuclear pores collapse in response to CO2 imaged with atomic force microscopy.

Nuclear pore complexes (NPCs) are the rate-limiting barriers for the exchange of macromolecules (e.g. transcription factors or mRNA) between the nuclear and cytosolic compartments. NPC conformation determines movement of cargo in either direction and thus controls gene expression. ATP and calcium are known to induce an NPC shape change (increase in height and decrease in diameter) indicating pore contraction. Here we report a CO2-induced shape change which is different to the ATP/calcium response. Experiments were performed on the isolated nuclear envelope of Xenopus laevis oocytes. The nuclear envelope was spread on glass and the native cytoplasmic surface was imaged with atomic force microscopy (AFM). The preparation was scanned in a water-saturated 100% O2 atmosphere at room temperature. Exposure to 5% CO2 (95%O2) led over a time course of minutes to a dramatic NPC shape change (decrease in height and decrease in diameter) indicating pore closure. NPCs turned flat and central channel openings virtually disappeared. The CO2 response was only slowly reversible. We conclude that NPCs apparently collapse in response to CO2, a structural change that could lead to the functional isolation of the cell nucleus.

Adenosine Triphosphate↗

PubMed1992

Assembly and cell cycle dynamics of the nuclear lamina.

The nuclear lamina is a karyoskeletal structure composed of intermediate filament type proteins. It underlies the inner nuclear membrane and confers mechanical stability to the nuclear envelope. In addition, it interacts with chromatin and may thereby participate in determining the three-dimensional organization of the interphase nucleus. During mitosis, the nuclear lamina is transiently disassembled, most probably through hyperphosphorylation of lamin proteins by the protein kinase p34cdc2, a key regulator of the eukaryotic cell cycle. Mitotic disassembly of the lamina is necessary but not sufficient for nuclear envelope breakdown. Electron microscopic analyses have begun to provide insights into the principles that govern lamina assembly in vitro, and sequence motifs required for targeting newly synthesized lamins to the nuclear envelope have been identified. Of particular interest, lamins were shown to undergo a type of hydrophobic modification known as isoprenylation. Finally, recent studies addressing the nature of lamin-chromatin interactions may provide the basis for elucidating the role of lamins in organizing the distribution of interphase chromatin.

Amino Acid Sequence↗

PubMed1998

Assembly of nuclear pore complexes and annulate lamellae promotes normal pronuclear development in fertilized mammalian oocytes.

In addition to functional nuclear pore complexes engaged in nucleo-cytoplasmic transport, the cytoplasmic stacks of pore complexes, called annulate lamellae, exist in numerous cell types. Although both annulate lamellae and nuclear pore complexes are present in fertilized mammalian oocytes, their relative roles in the process of fertilization and preimplantation development are not known. Using epifluorescence and electron microscopy, we explored their fate during bovine fertilization. The assembly of annulate lamellae in bovine oocytes was triggered by sperm-oocyte binding and continued concomitantly with the incorporation of the nuclear pores in the nuclear envelopes of the developing male and female pronuclei. This process was also induced by the parthenogenetic activation of metaphase-II-arrested oocytes. Depletion of Ca2+, previously implicated in oocyte activation and in the insertion of pore complexes into the nuclear envelope, prevented the formation of nuclear pore complexes, but not the assembly of annulate lamellae in oocyte cytoplasm. Injection of the nuclear pore antagonist, wheat germ agglutinin, into the cytoplasm of mature oocytes that were subsequently fertilized caused the arrest of pronuclear development, indicating the requirement of nuclear pore complexes for normal pronuclear development. Treatment of the fertilized oocytes with the microtubule inhibitor, nocodazole, prevented gathering of annulate lamellae around the developing pronuclei, insertion of nuclear pores into their nuclear envelopes, and further pronuclear development. The formation of the male pronuclei was reconstituted in Xenopus egg extracts and reflected the behavior of nuclear pores during natural fertilization. These data suggest that nuclear pore complexes are required for normal pronuclear development from its beginning up until pronuclear apposition. Annulate lamellae may be involved in the turnover of nuclear pore complexes during fertilization, which is in turn facilitated by the reorganization of oocyte microtubules and influx of Ca2+ into oocyte cytoplasm.

Animals↗

PubMed1988

The fates of chicken nuclear lamin proteins during mitosis: evidence for a reversible redistribution of lamin B2 between inner nuclear membrane and elements of the endoplasmic reticulum.

In chicken, three structurally distinct nuclear lamin proteins have been described. According to their migration on two-dimensional gels, these proteins have been designated as lamins A, B1, and B2. To investigate the functional relationship between chicken lamins and their mammalian counterparts, we have examined here the state of individual chicken lamin proteins during mitosis. Current models proposing functional specializations of mammalian lamin subtypes are in fact largely based on the observation that during mitosis mammalian lamin B remains associated with membrane vesicles, whereas lamins A and C become freely soluble. Cell fractionation experiments combined with immunoblotting show that during mitosis both chicken lamins B1 and B2 remain associated with membranes, whereas lamin A exists in a soluble form. In situ immunoelectron microscopy carried out on mitotic cells also reveals membrane association of lamin B2, whereas the distribution of lamin A is random. From these results we conclude that both chicken lamins B1 and B2 may functionally resemble mammalian lamin B. Interestingly, immunolabeling of mitotic cells revealed an association of lamin B2 with extended membrane cisternae that resembled elements of the endoplasmic reticulum. Quantitatively, we found that all large endoplasmic reticulum-like membranes present in metaphase cells were decorated with lamin B2-specific antibodies. Given that labeling of these mitotic membranes was lower than labeling of interphase nuclear envelopes, it appears likely that during mitotic disassembly and reassembly of the nuclear envelope lamin B2 may reversibly distribute between the inner nuclear membrane and the endoplasmic reticulum.

Animals↗

PubMed2002

The expression, lamin-dependent localization and RNAi depletion phenotype for emerin in C. elegans.

Emerin belongs to the LEM-domain family of nuclear membrane proteins, which are conserved in metazoans from C. elegans to humans. Loss of emerin in humans causes the X-linked form of Emery-Dreifuss muscular dystrophy (EDMD), but the disease mechanism is not understood. We have begun to address the function of emerin in C. elegans, a genetically tractable nematode. The emerin gene (emr-1) is conserved in C. elegans. We detect Ce-emerin protein in the nuclear envelopes of all cell types except sperm, and find that Ce-emerin co-immunoprecipitates with Ce-lamin from embryo lysates. We show for the first time in any organism that nuclear lamins are essential for the nuclear envelope localization of emerin during early development. We further show that four other types of nuclear envelope proteins, including fellow LEM-domain protein Ce-MAN1, as well as Ce-lamin, UNC-84 and nucleoporins do not depend on Ce-emerin for their localization. This result suggests that emerin is not essential to organize or localize the only lamin (B-type) expressed in C. elegans. We also analyzed the RNAi phenotype resulting from the loss of emerin function in C. elegans under laboratory growth conditions, and found no detectable phenotype throughout development. We propose that C. elegans is an appropriate system in which to study the molecular mechanisms of emerin function in vivo.

Animals↗

PubMed2004

NT agonist regulates expression of nuclear high-affinity neurotensin receptors.

Neurotensin (NT) exerts multiple functions in the central nervous system and peripheral tissues. Its actions are mainly mediated by a high-affinity G-protein-coupled receptor, the NT-1 receptor. In this study we demonstrated a nuclear NT binding site in different cellular models. We first noted that a large percentage of NT-1 receptor cell body immunoreactivity was located in the nuclear soma and nuclear envelope of rat substantia nigra, a brain area rich in NT-containing axon terminals. The NT-1 receptor was also visualized in purified nuclei from CHO cells stably transfected with NT-1 receptor coupled to the enhanced green fluorescence protein by immunocytochemistry. We observed that both the nuclear envelope and the nuclear soma were labeled, and the labeling intensity significantly increased after NT agonist treatment. These results suggested that NT-1 receptors, present in both the nuclear soma and the nuclear envelope, can be modulated by the ligand. Lastly, [(125)I]-NT binding experiments performed on isolated nuclei from a human lung cancer cell line endogenously expressing NT-1 receptor and NT, LNM35, revealed the existence of nuclear Gpp(NHp)-sensitive binding sites. These binding sites markedly decreased when cells were chronically treated with an NT-1 receptor antagonist, SR 48692. Taken together, these data suggest that the agonist regulates the expression of nuclear NT-1 receptors.

Animals↗

PubMed1975

The synaptic behaviour of the X and Y chromosomes in the marsupial Monodelphis dimidiata.

The pairing behaviour of the X and Y chromosomes of Monodelphis dimidiata was studied with light and electron microscopy. Pairing of the sex chromosomes is delayed with respect to autosome synapsis. Both the X and the minute Y chromosome show an axis attached by its two ends to the nuclear envelope. Synapsis of the sex chromosomes occurs by the joining of the chromatin sheaths that surround the axes and by a small, three-layered structure close to the nuclear envelope. The X and Y chromosomes remain joined to each other during the diffuse stage and diplotene-diakinesis but they do not show a synaptonemal complex. During the diffuse stage a dense plate is formed at the boundary between the X-Y body and the nuclear envelope. During early metaphase a folded sheet is attached to the periphery of the X-Y body. This sheet is formed by a piece of the nuclear envelope carrying the dense plate and it shows transverse fibrils and a central element similar to synaptonemal-complex remains. No evidence of a non-chiasmate segregation mechanism was observed. Polarization of the axial ends of the sex chromosomes is observed after X-Y synapsis. These important departures from the X-Y pairing pattern of eutherian mammals are discussed and assumed to present a special mechanism for holding the minute Y joined to the X chromosome in this marsupial.

Animals↗