Final Division Research Articles

Sequential Auctions of Endogenously Valued Objects

Two completely informed but possibly asymmetric bidders buy or sell identical “claims” in sequential auctions. They subsequently receive monetary prizes that depend upon the final allocation of claims. Iterated elimination of weakly dominated strategies leaves a unique Nash equilibrium. For any prize schedule, prices weakly decline as the auctions progress, and points of strict decline have a simple characterization. For one class of prize schedules, which arises naturally if duopolists bid for a scarce input, the equilibrium is completely characterized; many initial allocations generate the same final (unequal) division of claims, which may be interpreted as the natural market structure. Journal of Economic Literature Classification Numbers: D43, D44, L11.

Party Cohesion in the Danish Parliament

This article formulates a comprehensive and systematic taxonomy of micro level explanations of party cohesion; party cohesion being understood as party group members acting in unity externally. This apparatus is used in an analysis of party cohesion in the final divisions in the Danish Parliament, where cohesion figures are among the highest in the liberal democratic world. The investigation is based on interviews, survey data and data on voting behaviour. The main explanations of the high level of cohesion are the absence of disagreement in the party groups and a moral commitment to the party. Variation among MPs, parties and topics is also documented and discussed. The cohesion of each party and the compliant behaviour of individual MPs are related to the importance MPs ascribe to representing their party. Furthermore, divisions on moral issues, EU integration and local matters show lower than normal degrees of cohesion.

Radial migration in the cerebral cortex is enhanced by signals from thalamus.

The six layered cerebral cortex derives from cells that divide in the ventricular zone and migrate to their final destination in the cortical plate (future cortex). In the mouse, cortical layer III and IV neurons undergo their final mitotic division at around E16, at which time thalamic axons are beginning to enter the cortex. We used bromodeoxyuridine-birth dating of cells in cortical slice cultures to show that the thalamus enhances the migration out of the ventricular zone of future layer III/IV cells. When cortical slices were cultured alone, less than 35% of cells born in vitro on E16 were present in the pial half of the slice after 48 h in culture. In contrast, when cortical slices were cocultured with thalamus, 69% of these cells were found in the pial half of the slice. Explants of other developing tissues did not mimic the effect of the thalamus. The thalamus had no obvious effect on cortical radial glial cells, cortical cell viability or maintenance of cortical slice structure. We found that most precursors born at a similar age but in vivo, shortly before cortical slices were isolated, migrated to the pial half of the cultured slices in the absence of a cocultured thalamic explant. Thus, E16 cortical slices cultured without thalamus permit migration of cells born in vivo and therefore already exposed to the thalamus. Our results indicate that the thalamus provides factors to E16-born cortical precursors that enhance their directed migration out of the ventricular zone to the cortical plate.

Expression and regulation of the LIM‐class homeobox gene rlim1 in neuronal progenitors of the rat cerebellum

To investigate LIM gene function in the rat cerebellar system, we analyzed expression and regulation of the rat homologue of frog Xlim-1 (rlim-1) in vivo and in cultured cells. In developing cerebellum, peak levels of rlim-1 mRNA at postnatal day 8 (p8) are coincident with the peak period of granule cell proliferation. Analysis of rlim-1 protein with a specific antibody showed that expression was also maximal at p8. In situ hybridization showed that at p8 rlim-1 mRNA was expressed in Purkinje and granule cells. Both the proliferative and the premigratory granule cells in the external germinal zone displayed high levels of rlim-1 mRNA expression. Immunocytochemical staining demonstrated that at p8 rlim-1 protein was also present in proliferative and premigratory granule cells. In adult cerebellum (p30), rlim-1 mRNA and protein expression in granule cells was strongly attenuated. The down-regulation of rlim-1 mRNA occurred in granule cells just after the time of final division, coinciding with the onset of their migration. rlim-1 protein was detected in migratory granule neurons. The developmental decrease in rlim-1 mRNA and protein found in vivo was reproduced in pure cerebellar granule cell cultures. In these cultures, granule neurons were postmitotic 1 day after plating but still displayed high levels of rlim-1 protein expression up to 3 days in vitro. Our findings indicate that 1) rlim-1 is likely to act in concert with other genes to specify granule cell fate, 2) rlim-1 expression in granule neurons is regulated autonomously, and 3) rlim-1 protein may also play an important role in granule neuron differentiation and survival. J. Neurosci. Res. 63:237–251, 2001. Published 2001 Wiley-Liss, Inc.

Immunohistochemical analyses of DNA topoisomerase II isoforms in developing rat cerebellum.

In mammalian cells, two isoforms of DNA topoisomerase II (topo IIalpha and topo IIbeta) have been identified. Topo IIalpha is essential in mitotic cells, whereas the function of topo IIbeta remains unclear. In the present study, we investigated the developmental control of topo II isoforms in two different neuronal lineages, cerebellar Purkinje cells and granule cells, by immunohistochemical analysis with isoform-specific monoclonal antibodies. As expected, proliferating cells in the neuroepithelium and in the external germinal layer (EGL) were topo IIalpha immunopositive. The migrating as well as differentiating Purkinje cells and granule cells showed an enhanced topo IIbeta immunoreactivity. The postmitotic granule cells in the postnatal EGL showed an abrupt transition of expressed topo II isoforms from IIalpha to IIbeta. The transition was clearly coincident with the completion of final cell division and the initiation of terminal differentiation because no increase of the topo IIbeta immunoreactivity was observed in the spreading EGL cells that are still in the cell division cycle. The topo IIbeta signal was detected in both nucleoplasm and nucleolus of differentiating cells. However, the nucleoplasmic signal decreased significantly as the cells reached terminal differentiation. The residual topo IIbeta in nucleoli was shown to occupy an unique location with respect to other nucleolar proteins, nucleolin and DNA topoisomerase I. Our findings indicate that both Purkinje cells and granule cells express the topo II isoforms in a similar timing during the cerebellar development and also suggest that topo IIbeta localized in nucleoplasm is the functional entity involved in neuronal differentiation.

Expression and regulation of the LIM-class homeobox gene rlim-1 in neuronal progenitors of the rat cerebellum.

To investigate LIM gene function in the rat cerebellar system, we analyzed expression and regulation of the rat homologue of frog Xlim-1 (rlim-1) in vivo and in cultured cells. In developing cerebellum, peak levels of rlim-1 mRNA at postnatal day 8 (p8) are coincident with the peak period of granule cell proliferation. Analysis of rlim-1 protein with a specific antibody showed that expression was also maximal at p8. In situ hybridization showed that at p8 rlim-1 mRNA was expressed in Purkinje and granule cells. Both the proliferative and the premigratory granule cells in the external germinal zone displayed high levels of rlim-1 mRNA expression. Immunocytochemical staining demonstrated that at p8 rlim-1 protein was also present in proliferative and premigratory granule cells. In adult cerebellum (p30), rlim-1 mRNA and protein expression in granule cells was strongly attenuated. The down-regulation of rlim-1 mRNA occurred in granule cells just after the time of final division, coinciding with the onset of their migration. rlim-1 protein was detected in migratory granule neurons. The developmental decrease in rlim-1 mRNA and protein found in vivo was reproduced in pure cerebellar granule cell cultures. In these cultures, granule neurons were postmitotic 1 day after plating but still displayed high levels of rlim-1 protein expression up to 3 days in vitro. Our findings indicate that 1) rlim-1 is likely to act in concert with other genes to specify granule cell fate, 2) rlim-1 expression in granule neurons is regulated autonomously, and 3) rlim-1 protein may also play an important role in granule neuron differentiation and survival. Published 2001 Wiley-Liss, Inc.

Two male‐sterile mutants of Zea Mays (Poaceae) with an extra cell division in the anther wall

Two recessive male-sterile mutants of maize with similar patterns of pollen abortion were studied. Genetic studies showed that one of the two mutations was allelic with a previously identified male-sterility locus (ms23) and the other mutation was in a newly identified male-sterility locus (ms32). Cytological characterization of homozygous mutants and fertile heterozygous control siblings was performed using brightfield, fluorescence, and electron microscopy. During normal anther development, the final anther wall periclinal division divides the secondary parietal anther wall layer into the middle layer and tapetum, forming an anther with four wall layers. This is followed by differentiation of the tapetal cells into protoplastic binucleate, secretory tissue. In both the ms23 and ms32 mutants, the prospective tapetal layer divided into two layers, termed t1 and t2, forming an anther with five wall layers. Neither the t1 nor the t2 layers differentiated normally into tapetal layers, as determined by examination of cell walls, nucleus number, and cytoplasmic organization. Pollen mother cells aborted after the onset of prophase I of meiosis, suggesting that an early developmental coordination may exist between tapetum and pollen mother cells.

Dopamine cells in nigral grafts differentiate prior to implantation.

The yield of surviving dopamine cells in nigral grafts is typically low. It is unclear whether the dopamine neurons that do survive are postmitotic at the time of implantation, or are precursor cells that differentiate into dopamine neurons following transplantation in the host brain. We have therefore compared the survival of dopamine neurons in grafts that have been labelled with BrdU at different times prior to or following implantation in order to identify those cells that undergo final cell division at each stage of the procedure. Seven groups of rats were prepared with unilateral nigrostriatal lesions. Three groups received nigral grafts derived from E14 embryos labelled with BrdU in utero on either E12, E13 or E14 days of embryonic age (the E14 injection made 2 h prior to preparation of the graft cell suspension). Three further groups received nigral grafts from untreated E14 embryos, and then dividing cells within the grafts were labelled by injection of BrdU into the host lateral ventricle, 2 h, 1 day or 2 days after implantation (equivalent to E14, E15 and E16 days of embryonic age). The control group received standard (unlabelled) E14 grafts. Five weeks after the transplantation surgery, the host brains were processed using double immunohistochemical techniques to detect tyrosine hydroxylase (TH)-positive neurons which had incorporated BrdU. In the grafts labelled with BrdU prior to implantation, there was an increasing proportion of double-labelled cells (out of the total TH-positive cells surviving in the grafts) with birth dates on E12, E13 and E14 (1%, 12% and 10% per day, respectively). By contrast, grafts labelled following implantation, although containing many dividing neurons, had very few of these BrdU-labelled cells expressing a dopaminergic phenotype; < 1% surviving TH-positive cells were double-labelled from the 2 h post-transplant injection, and < 0.1% from each subsequent injection. This suggests not only that the great majority of TH-positive neurons in nigral grafts were already differentiated at the time of implantation, but also that transplantation of E14 ventral mesencephalic tissue either kills dopaminergic precursors or (more likely in our opinion) prevents their differentiation into a dopaminergic phenotype. Precursor cells that would differentiate into dopaminergic neurons beyond E14 if left in situ in the intact ventral mesencephalon do not readily differentiate into mature dopamine neurons following transplantation. If we are to enhance yields of functional dopamine-rich transplants, then we must identify strategies both to protect predifferentiated dopamine neurons in the grafts and to promote differentiation of a dopaminergic phenotype in precursor cells that continue to divide within the grafts following transplantation into an adult host environment.

The distinctive roles of five different ARC genes in the chloroplast division process in Arabidopsis.

ARC (accumulation and replication of chloroplasts) genes control different aspects of the chloroplast division process in higher plants. In order to establish the hierarchy of the ARC genes in the chloroplast division process and to provide evidence for their specific roles, double mutants were constructed between arc11, arc6, arc5, arc3 and arc1 in all combinations and phenotypically analysed. arc11 is a new nuclear recessive mutant with 29 chloroplasts compared with 120 in wild type. All the phenotypes of the double mutants are unambiguous. ARC1 down-regulates proplastid division but is on a separate pathway from ARC3, ARC5, ARC6 and ARC11. ARC6 initiates both proplastid and chloroplast division. ARC3 controls the rate of chloroplast expansion and ARC11 the central positioning of the final division plane in chloroplast division. ARC5 facilitates separation of the two daughter chloroplasts. ARC5 maps to chromosome 3 and ARC11 and ARC6 map approximately 60 cM apart on chromosome 5.

ELT-3: ACaenorhabditis elegansGATA Factor Expressed in the Embryonic Epidermis during Morphogenesis

We have identified a gene encoding a new member of theCaenorhabditis elegansGATA transcription factor family,elt-3.The predicted ELT-3 polypeptide contains a single GATA-type zinc finger (C-X2-C-X17-C-X2-C) along with a conserved adjacent basic region.elt-3mRNA is present in all stages ofC. elegansdevelopment but is most abundant in embryos. Reporter gene analysis and antibody staining show thatelt-3is first expressed in the dorsal and ventral hypodermal cells, and in hypodermal cells of the head and tail, immediately after the final embryonic cell division that gives rise to these cells. No expression is seen in the lateral hypodermal (seam) cells.elt-3expression is maintained at a constant level in the epidermis until the 212-fold stage of development, after which reporter gene expression declines to a low level and endogenous protein can no longer be detected by specific antibody. A second phase ofelt-3expression in cells immediately anterior and posterior to the gut begins in pretzel-stage embryos.elt-1andlin-26are two genes known to be important in specification and maintenance of hypodermal cell fates. We have found thatelt-1is required for the formation of most, but not all,elt-3-expressing cells. In contrast,lin-26function does not appear necessary forelt-3expression. Finally, we have characterised the candidate homologue ofelt-3in the nematodeCaenorhabditis briggsae.Many features of theelt-3genomic and transcript structure are conserved between the two species, suggesting thatelt-3is likely to perform an evolutionarily significant function during development.

The Neuronal Progenitor Cells of the Forebrain Subventricular Zone: Intrinsic Propertiesin Vitroand Following Transplantation

During the development of the central nervous system, progenitor cells, located within distinct germinal zones, produce presumptive neurons that migrate to their destinations and differentiate. Recent studies have demonstrated that a discrete region of the anterior part of the postnatal subventricular zone (SVZa) comprises neuronal progenitor cells whose progeny are fated to become the interneurons of the olfactory bulb. The SVZa is of particular interest because it is one of few germinal zones to persist postnatally and may be the only postnatal germinal zone to give rise exclusively to neurons. To the extent that the SVZa is unique among proliferative zones, the SVZa progeny are unique among neurons. First, unlike most cortical neurons, the SVZa-derived cells do not rely on radial glia-assisted migration when traveling to their target region. Second, the SVZa progeny continue to proliferate as they migrate to their target region. And third, the SVZa progeny express early neuron-specific antigens prior to their final division and, therefore, prior to reaching their destination where they will terminally differentiate. To better understand the capacity of the SVZa progeny to concurrently proliferate, migrate, and differentiate, we studied the cellsin vitroand following transplantation into the neonatal SVZa and adult striatum. In each setting, we found that the SVZa cells continue both to proliferate and to differentiate into neurons. In addition, after homotopic and heterotopic transplantation, we found that the SVZa cells maintain their ability to migrate. These results suggest that the unique features of the SVZa progeny are specified intrinsically rather than by their extrinsic environment.

Hair Cells and Supporting Cells Share a Common Progenitor in the Avian Inner Ear

Sensory organs of the vertebrate inner ear contain two major cell types: hair cells (HCs) and supporting cells (SCs). To study the lineage relationships between these two populations, replication-defective retroviral vectors encoding marker genes were delivered to the otic vesicle of the chicken embryo. The resulting labeled clones were analyzed in the hearing organ of the chicken, called the basilar papilla (BP), after cellular differentiation. BPs were allowed to develop for 2 weeks after delivery of the retrovirus, were removed, and were processed histochemically as whole mounts to identify clones of cells. Clusters of labeled cells were evident in the sensory epithelium, the nonsensory epithelium, and in adjacent tissues. Labeled cell types included HCs, two morphologically distinct types of SCs, homogene cells, border cells, hyaline cells, ganglion cells, and connective tissue cells. Each clone was sectioned and cell-type identification was performed on sensory clones expressing retrovirally transduced beta-galactosidase. Cell composition was determined for 41 sensory clones, most of which contained both HCs and SCs. Clones containing one HC and one SC were observed, suggesting that a common progenitor exists that can remain bipotential up to its final mitotic division. The possibility that these two cell types may also arise from a mitotic precursor during HC regeneration in the mature basilar papilla is consistent with their developmental history.

Brassinolide affects the rate of cell division in isolated leaf protoplasts of Petunia hybrida.

Brassinosteroids are known to promote cell elongation in a wide range of plant species but their effect on cell division has not been as extensively studied. We examined the effect of brassinolide on the kinetics and final division frequencies of regenerating leaf mesophyll protoplasts of Petunia hybrida Vilm v. Comanche. Under optimal auxin and cytokinin conditions, 10-100 nM brassinolide accelerated the time of first cell division by 12 h but had little effect on the final division frequencies after 72-120 h of culture. One micromolar brassinolide showed the same acceleration of first cell division but inhibited the final division frequency by approximately 20%. Under sub-optimal auxin conditions, 10-100 nM brassinolide both accelerated the time of first cell division and dramatically increased the 72- to 120-h final division frequencies. Isolated protoplasts may provide a useful model system to investigate the molecular mechanisms of brassinosteroid action on cell proliferation.

The C. elegans MDL-1 and MXL-1 proteins can functionally substitute for vertebrate MAD and MAX.

The genes of the myc/max/mad family play an important role in controlling cell proliferation and differentiation. We have identified the first homologues of the mad and max genes in the nematode C. elegans, which we have named mdl-1 and mxl-1 respectively. Like the vertebrate MAD proteins, MDL-1 binds an E-box DNA sequence (CACGTG) when dimerized with MXL-1. However, unlike vertebrate MAX, MXL-1 can not form homodimers and bind to DNA alone. Promoter fusions to a GFP reporter suggest that these genes are coexpressed in posterior intestinal and post-mitotic neuronal cells during larval development. The coexpression in the posterior intestinal cells occurs before their final division at the end of the L1 stage and persists afterwards, demonstrating that mad and max expression can be correlated directly to the cell cycle state of an individual cell type. These data also show that mxl-1 is an obligate partner for mdl-1 in vivo and in vitro and indicate that these genes may play an important role in post-embryonic development. Finally, MDL-1 can suppress activated c-MYC/RAS-induced focus formation in a rat embryo fibroblast transformation assay. Like the vertebrate MAD protein, MDL-1 activity in suppressing transformation is dependent on a functional SIN3 interaction domain.

Cyclin A, cyclin B and stringlike are regulated separately in cell cycle arrested trochoblasts of Patella vulgata embryos

Trochoblasts are the first cells to differentiate during the development of spiralian embryos. Differentiation is accompanied by a cell division arrest. In embryos of the limpet Patella vulgata, the participation of cell cycle-regulating factors in trochoblast arrest was analysed as a first step to unravel its cause. We determined the cell cycle phase in which the trochoblasts are arrested by analysing the subcellular locations of mitotic cyclins. The results show that the trochoblasts are most likely arrested in the G2 phase. This was supported by measurement of the DNA content in trochoblast nuclei after the last division. Trochoblasts complete their final division at the sixth mitotic cycle. This mitotic cycle resembles the first postblastoderm cell cycle of Drosophila, in which mitotic activity is controlled by expression of the string gene. As failure of string expression results in cell cycle arrest in the G2 phase, negative regulation of a Patella string homolog could be responsible for trochoblast arrest. Although Stl messengers disappeared from trochoblasts during their final division, expression was observed again 20 min later. Messengers remained present in all trochoblasts at low levels during further development. Thus, expression of the stringlike gene allows the cell cycle arrest of these cells, whereas in Drosophila cells arrested in division lack string messengers.

Vegetative and reproductive morphology of Dudresnaya canariensis sp. nov. (Dumontiaceae, Rhodophyta)

The marine red alga Dudresnaya canariensis sp. nov. (Dumontiaceae, Gigartinales) from the Canary Islands is described. Gametophytes are terete and radially branched, young branches are nonannulate, outer cortical cells are cylindrical, axial cells contain hexagonal crystals, cells of primary axes are conspicuous, and rhizoids reach 95 μm in diameter. Spermatangial mother cells occur in corymbose clusters on the outer cortex, and carpogonial and auxiliary-cell filaments, both of which lack a thick mucilage coat, arise in the mid-cortex. Carpogonial filaments are reflexed, the carpogonium and trichogyne differentiating after final oblique division. The gonimoblast initials do not recurve around the auxiliary-cell filament, and the cystocarps are distinctly cleft. Tetrasporophytes are unknown. The only unique feature of D. canariensis is the arrangement of its spermatangia in corymbose terminal and subterminal fascicles, a feature unreported in the few species of Dudresnaya where male gametophytes are known. However, D. canariensis differs from the other Dudresnaya species by a unique combination of significant attributes. It is postulated that the Australian D. capricornica and the Japanese D. kuroshioensis are the closest relatives of the new species.

Failed Cell Migration and Death of Purkinje Cells and Deep Nuclear Neurons in theweaverCerebellum

The mouse neurological mutant weaver has an atrophic cerebellar cortex with deficits in both Purkinje and granule cell number. Although granule cells are known to die postnatally shortly after their final cell division, the cause of the Purkinje cell deficit (cell death vs lack of production) is unknown. We report here a quantitative analysis of large cerebellar neurons of the weaver mutant during postnatal development. We explored the hypothesis that the cells of the entire cerebellar anlage were affected by the mutation by including in our study the neurons of the deep cerebellar nuclei (DCN). Our analysis reveals that in homozygous weaver mutants (1) the DCN are displaced laterally, display an abnormal anatomy, and suffer a 20-25% decrease in neuron number; (2) this numerical deficit is located in medial regions, similar to the localization of cortical deficits in both Purkinje and granule cells; (3) pyknotic figures are present in the juvenile DCN and in the Purkinje cell layer; and (4) the majority of cell death in these populations occurs not in medial regions where the numerical deficits are observed, but rather laterally where adult cell number is nearly normal. These results lead us to propose that the complete weaver phenotype includes a failure of the cell movements that lead to the fusion of the bilateral cerebellar anlage, and that this failure to migrate properly leaves some of the Purkinje cells and DCN neurons in a position where they are unable to make appropriate connections, leading to their death. In addition to implications for normal development, these observations suggest that weaver effects on the cerebellum can be unified into one consolidated model in which failure of cell movement affects all major cerebellar neurons.

Predominant expression of Brn-2 in the postmitotic neurons of the developing mouse neocortex

The expression of Brn-2, a central nervous system (CNS)-specific POU domain transcription factor, in the developing mouse neocortex was examined with an anti-Brn-2 antibody. Brn-2 protein was first detected in CNS on embryonic day (E) 11.5, and remained strong until E15.5. From E11.5 to postnatal day (P) 0, a high level of Brn-2 expression was observed in the subventricular zone, the intermediate zone, and the outer layer of the neocortex, but not in the ventricular zone. In the double-staining experiments, most of the Brn-2 positive cells were also positive for NCAM-H, an adhesion molecule specific to post mitotic neurons. Furthermore, BrdU-labeling experiments demonstrated the presence of Brn-2 protein exclusively in postmitotic cells. These results indicated that, in the developing neocortex. Brn-2 expression is up-regulated after the final cell division. Therefore, this transcription factor may be involved in the migration and/or maturation process of the immature neuronal cells.

Two Critical Periods of Sonic Hedgehog Signaling Required for the Specification of Motor Neuron Identity

Antibodies that block Sonic Hedgehog (SHH) signaling have been used to show that SHH activity is required for the induction of floor plate differentiation by the notochord and independently for the induction of motor neurons by both the notochord and midline neural cells. Motor neuron generation depends on two critical periods of SHH signaling: an early period during which naive neural plate cells are converted into ventralized progenitors and a late period that extends well into S phase of the final progenitor cell division, during which SHH drives the differentiation of ventralized progenitors into motor neurons. The ambient SHH concentration during the late period determines whether ventralized progenitors differentiate into motor neurons or interneurons, thus defining the pattern of neuronal cell types generated in the neural tube.

Hedgehog, Transmitted along Retinal Axons, Triggers Neurogenesis in the Developing Visual Centers of the Drosophila Brain

The development of the visual centers of the Drosophila brain is tightly regulated by the ingrowth of retinal axons from the developing eye. In the first optic ganglion, the lamina, arriving retinal axons trigger the precursors of their synaptic partners to complete a final cell division and commence neural differentiation. The secreted product of the hedgehog gene regulates the temporal assembly of photoreceptor precursor cells into ommatidial clusters in the compound eye. Here, we show that Hedgehog is transmitted along the retinal axons to serve as the inductive signal in the brain. Hedgehog acts in the first of two retinal axon–mediated steps in the assembly of lamina synaptic cartridges. These observations provide a novel insight into the molecular interactions that orchestrate the assembly of neural precursor cells into precise synaptic circuits.