Search PubMed⌕ Search

SEARCH · Search PubMed

Results for “Prophase”

Search indexed PubMed citations on genomics, clinical trials, systematic reviews and public health. Explore titles, authors and supplied subject terms, then open the PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 469 records · Page 26Linked to original sources
PubMed1978

Surface functions during Mitosis I: phagocytosis, pinocytosis and mobility of surface-bound Con A.

Phagocytosis, pinocytosis and the surface distribution of concanavalin A (ConA) have been analyzed during mitosis in several mammalian cell lines. Use of the bisbenzimidazole dye, Hoechst 33258, for chromosome staining after gentle fixation made possible the rapid identification and correlation of mitotic phase with surface properties. Phagocytosis of both opsonized and nonopsonized particles is markedly depressed in mitotic cells of the mouse macrophage cell line J774.1. The uptake of opsonized particles (IgG-coated erythrocytes) is impaired from early prophase through early G1, whereas phagocytosis of nonopsonized particles (latex beads) is restored by telophase. Fluid pinocytosis, determined by the uptake of soluble horseradish peroxidase, is also inhibited during mitosis. Thus peroxidase-containing cytoplasmic vesicles were virtually absent from mid-prophase through telophase in both J774 and Chinese hamster ovary (CHO) cells. Adsorptive pinocytosis of ConA was determined from the different distributions of fluorescence in single cells incubated at 37 degrees C with rhodamine-conjugated ConA (surface and cytoplasmic label), then fixed and further incubated with fluorescein-conjugated anti-ConA (surface only). The separate fluorescence of Hoechst, fluorescein and rhodamine could be optically isolated. In interphase J774 cells, ConA is rapidly internalized into cytoplasmic vesicles. In contrast, ConA is restricted to the plasma membrane from mid-prophase through telophase. In CHO, the depressed pattern of internalization is not fully established until metaphse. The surface distribution of ConA also varied dramatically as a function of mitotic phase. Between mid-prophase and early anaphase, the pattern of surface ConA-receptor complexes is diffuse. Once the cleavage furrow begins to develop, however, ConA moves into the region of the furrow. This was shown in J774, CHO and 3T3 mouse embryonic fibroblasts, and is probably universal. ConA movement into the membrane that overlies the microfilaments of the contractile ring is analogous to similar movements that occur in interphase cells during ConA cap formation and during the development of phagocytic pseudopods. The analogy emphasizes the common functional consequences of microfilament-membrane organization. It is evident that membrane processes which depend upon endocytosis--for example, certain hormone-induced signals--may be interrupted during mitosis. Inhibition of endocytosis thus may be a significant element in the control of cellular activities during mitosis and a strong influence on the properties of the emergent post-mitotic cell.

Cell Cycle↗
PubMed2001

A model for chromosome structure during the mitotic and meiotic cell cycles.

The chromosome scaffold model in which loops of chromatin are attached to a central, coiled chromosome core (scaffold) is the current paradigm for chromosome structure. Here we present a modified version of the chromosome scaffold model to describe chromosome structure and behavior through the mitotic and meiotic cell cycles. We suggest that a salient feature of chromosome structure is established during DNA replication when sister loops of DNA extend in opposite directions from replication sites on nuclear matrix strands. This orientation is maintained into prophase when the nuclear matrix strand is converted into two closely associated sister chromatid cores with sister DNA loops extending in opposite directions. We propose that chromatid cores are contractile and show, using a physical model, that contraction of cores during late prophase can result in coiled chromatids. Coiling accounts for the majority of chromosome shortening that is needed to separate sister chromatids within the confines of a cell. In early prophase I of meiosis, the orientation of sister DNA loops in opposite directions from axial elements assures that DNA loops interact preferentially with homologous DNA loops rather than with sister DNA loops. In this context, we propose a bar code model for homologous presynaptic chromosome alignment that involves weak paranemic interactions of homologous DNA loops. Opposite orientation of sister loops also suppresses crossing over between sister chromatids in favor of crossing over between homologous non-sister chromatids. After crossing over is completed in pachytene and the synaptonemal complex breaks down in early diplotene (= diffuse stage), new contractile cores are laid down along each chromatid. These chromatid cores are comparable to the chromatid cores in mitotic prophase chromosomes. As an aside, we propose that leptotene through early diplotene represent the 'missing' G2 period of the premeiotic interphase. The new chromosome cores, along with sister chromatid cohesion, stabilize chiasmata. Contraction of cores in late diplotene causes chromosomes to coil in a configuration that encourages subsequent syntelic orientation of sister kinetochores and amphitelic orientation of homologous kinetochore pairs on the spindle at metaphase I.

Chromosomes↗
PubMed2002

Thr-161 phosphorylation of monomeric Cdc2. Regulation by protein phosphatase 2C in Xenopus oocytes.

Fully grown Xenopus oocyte is arrested at prophase I of meiosis. Re-entry into meiosis depends on the activation of MPF (M-phase promoting factor or cyclin B.Cdc2 complex), triggered by progesterone. The prophase-arrested oocyte contains a store of Cdc2. Most of the protein is present as a monomer whereas a minor fraction, called pre-MPF, is found to be associated with cyclin B. Activation of Cdc2 depends on two key events: cyclin binding and an activating phosphorylation on Thr-161 residue located in the T-loop. To get new insights into the regulation of Thr-161 phosphorylation of Cdc2, monomeric Cdc2 was isolated from prophase oocytes. Based on its activation upon cyclin addition and detection by an antibody directed specifically against Cdc2 phosphorylated on Thr-161, we show for the first time that the prophase oocyte contains a significant amount of monomeric Cdc2 phosphorylated on Thr-161. PP2C, a Mg2+-dependent phosphatase, negatively controls Thr-161 phosphorylation of Cdc2. The unexpected presence of a population of free Cdc2 already phosphorylated on Thr-161 could contribute to the generation of the Cdc2 kinase activity threshold required to initiate MPF amplification.

Animals↗
PubMed1998

Activation of the MKK/ERK pathway during somatic cell mitosis: direct interactions of active ERK with kinetochores and regulation of the mitotic 3F3/2 phosphoantigen.

The mitogen-activated protein (MAP) kinase pathway, which includes extracellular signal-regulated protein kinases 1 and 2 (ERK1, ERK2) and MAP kinase kinases 1 and 2 (MKK1, MKK2), is well-known to be required for cell cycle progression from G1 to S phase, but its role in somatic cell mitosis has not been clearly established. We have examined the regulation of ERK and MKK in mammalian cells during mitosis using antibodies selective for active phosphorylated forms of these enzymes. In NIH 3T3 cells, both ERK and MKK are activated within the nucleus during early prophase; they localize to spindle poles between prophase and anaphase, and to the midbody during cytokinesis. During metaphase, active ERK is localized in the chromosome periphery, in contrast to active MKK, which shows clear chromosome exclusion. Prophase activation and spindle pole localization of active ERK and MKK are also observed in PtK1 cells. Discrete localization of active ERK at kinetochores is apparent by early prophase and during prometaphase with decreased staining on chromosomes aligned at the metaphase plate. The kinetochores of chromosomes displaced from the metaphase plate, or in microtubule-disrupted cells, still react strongly with the active ERK antibody. This pattern resembles that reported for the 3F3/2 monoclonal antibody, which recognizes a phosphoepitope that disappears with kinetochore attachment to the spindles, and has been implicated in the mitotic checkpoint for anaphase onset (Gorbsky and Ricketts, 1993. J. Cell Biol. 122:1311-1321). The 3F3/2 reactivity of kinetochores on isolated chromosomes decreases after dephosphorylation with protein phosphatase, and then increases after subsequent phosphorylation by purified active ERK or active MKK. These results suggest that the MAP kinase pathway has multiple functions during mitosis, helping to promote mitotic entry as well as targeting proteins that mediate mitotic progression in response to kinetochore attachment.

3T3 Cells↗
PubMed1978

Analysis of the distribution of spindle microtubules in the diatom Fragilaria.

The spindle of the colonial diatom Fragilaria contains two distinct sets of spindle microtubules (MTs): (a) MTs comprising the central spindle, which is composed of two half-spindles interdigitated to form a region of "overlap"; (b) MTs which radiate laterally from the poles. The central spindles from 28 cells are reconstructed by tracking each MT of the central spindle through consecutive serial sections. Because the colonies of Fragilaria are flat ribbons of contiguous cells (clones), it is possible, by using single ribbons of cells, to compare reconstructed spindles at different mitotic stages with minimal intercellular variability. From these reconstructions we have determined: (a) the changes in distribution of MTs along the spindle during mitosis; (b) the change in the total number of MTs during mitosis; (c) the length of each MT (measured by the number of sections each traverses) at different mitotic stages; (d) the frequency of different classes of MTs (i.e., free, continuous, etc.); (e) the spatial arrangement of MTs from opposite poles in the overlap; (f) the approximate number of MTs, separate from the central spindle, which radiate from each spindle pole. From longitudinal sections of the central spindle, the lengths of the whole spindle, half-spindle, and overlap were measured from 80 cells at different mitotic stages. Numerous sources of error may create inaccuracies in these measurements; these problems are discussed. The central spindle at prophase consists predominantly of continuous MTs (pole to pole). Between late prophase and prometaphase, spindle length increases, and the spindle is transformed into two half-spindles (mainly polar MTs) interdigitated to form the overlap. At late anaphase-telophase, the overlap decreases concurrent with spindle elongation. Our interpretation is that the MTs of the central spindle slide past one another at both late prophase and late anaphase. These changes in MT distribution have the effect of elongating the spindle and are not involved in the poleward movement of the chromosomes. Some aspects of tracking spindle MTs, the interaction of MTs in the overlap, formation of the prophase spindle, and our interpretation of rearrangements of MTs, are discussed.

Cell Cycle↗
PubMed2004

CDK1/cyclin B regulation during oocyte maturation in two closely related lugworm species, Arenicola marina and Arenicola defodiens.

The molecular mechanisms underlying oocyte maturation in the annelid polychaetes Arenicola marina and Arenicola defodiens were investigated. In both species, a hitherto unidentified hormone triggers synchronous and rapid transition from prophase to metaphase, a maturation process which can be easily reproduced in vitro. Activation of a roscovitine- and olomoucine-sensitive M-phase-specific histone, H1 kinase, occurs during oocyte maturation. Using affinity chromatography on immobilized p9CKShs1, we purified CDK1 and cyclin B from oocyte extracts prepared from both phases and both species. In prophase, CDK1 is present both as an inactive, but Thr161-phosphorylated monomer, and as an inactive (Tyr15-phosphorylated) heterodimer with cyclin B. Prophase to metaphase transition is associated with complete tyrosine dephosphorylation of the cyclin B-associated CDK1, with phosphorylation of cyclin B, and with dramatic activation of the kinase activity of the CDK1/cyclin B complex. We propose that Arenicola oocytes may provide an ideal model system to investigate the acquisition of the ability of oocytes to be fertilized that occurs as oocyte shift from prophase to metaphase, an important physiological event, probably regulated by active CDK1/cyclin B.

Animals↗
PubMed1996

The pattern of histone H4 acetylation on the X chromosome during spermatogenesis of the desert locust Schistocerca gregaria.

We have used antibodies directed against histone H4 acetylated at lysine residue 5, 8, 12, or 16 and indirect immunofluorescence microscopy to probe chromosomes from spermatogonia and spermatocytes of the desert locust, Schistocerca gregaria. The autosomes showed bright overall fluorescence, indicative of high levels of H4 acetylation. In contrast, the X chromosome, which is facultatively heterochromatic during spermatogenesis of the locust, remained completely unstained in spermatogonia and secondary spermatocytes and showed only a small terminal fluorescent band in primary spermatocytes. This band probably corresponds to centromere associated constitutive heterochromatin. Thus, underacetylation is a cytogenetic marker for facultative heterochromatin, but not necessarily constitutive heterochromatin, during spermatogenesis of the locust. Scanning electron microscopy of chromosomes from prophase spermatogonia and prophase I spermatocytes revealed that underacetylation of histone H4 in the X chromosome was not accompanied by a chromatin organization visibly different from that of the autosomes. Transmission electron microscopy of mitotic spermatogonia showed that the X chromosome is separated from the autosomes in a small nuclear compartment of its own in prophase and telophase and associated with membranes in metaphase. In prophase I spermatocytes, autosomes and the sex univalent were in the same compartment. This compartmentalization may be responsible for the underacetylation and (or) transcriptional silencing of the X chromosome in spermatogonia mitosis.

Acetylation↗
PubMed1976

The blepharoplast of Marsilea: its de novo formation and spindle association.

Blepharoplast formation and function has been investigated by light and electron microscopy in developing microspores of the water fern Marsilea vestita. When single-celled microspores are hydrated they undergo a series of 9 cell divisions producing 7 sterile cells plus 32 spermatids. The blepharoplast, a densely staining, spherical inclusion (0-5-1-0 mum in diameter) interpenetrated by numerous lightly stained channels, is a precursor organelle for the production of the 100-150 basal bodies of the motile sperm. During development it arises twice in the dividing spermatogenous cells; first in telophase of the second to last or 7th division, and again in telophase of the next to last or 8th division. The blepharoplast first forms on the distal side of the nucleus during telophase of the 7th division but soon degenerates as the cell enters prophase and metaphase of the 8th division. The blepharoplast first forms on the distal side of the nucleus during telophase of the 7th division but soon degenerates as the cell enters prophase and metaphase of the 8th division. During telophase of the 8th division the blepharoplast again arises in the cytoplasm near a small indentation of the nuclear envelope. The forming blepharoplast consists of a sphere of lightly stained flocculent material within which 2 plaques separated by 40-50 nm arise. Each plaque possesses 2 dense layers (20 nm) separated by a light layer (10 nm). During subsequent development the distal layer of each plaque becomes progressively more densely stained. Light-staining channels appear within the accumulating dense material a-d 2 hemispherical blepharoplasts emerge. The blepharoplasts become spherical through continued growth and finally separate from each other as they move to the spindle poles during prophase of the last or 9th division. The blepharoplast appears to act as a microtubule organizing centre during formation of the spindle apparatus in prophase of the 9th division. While it remains at the pole throughout mitosis it does not continue to serve as the focal point for the spindle tubules. During metaphase-anaphase of the 9th division the blepharoplast swells, and the channels at its surface begin transforming into procentrioles and finally basal bodies. The results support the contention that basal bodies in Marsilea arise de novo, since no preexisting template such as a centriolar pinwheel is observed and sine the intermediates which initially occur are structurally dissimilar from a procentriole.

Microtubules↗
PubMed1983

Events in the cytoplasm during male meiosis in Lilium.

An electron microscopic investigation of the events associated with meiosis in Lilium has revealed a number of changes in both the organellar population and the other cytoplasmic components. Ribosome numbers decrease significantly in early prophase and are later replenished in the tetrads, a process most likely involving the newly arising cytoplasmic nucleoloids. The organelles show a cycle of de- and redifferentiation and later in meiosis unusual internal structures can be seen before these organelles enter a division phase resulting in increased numbers. The localization of acid phosphatase during these changes has also been studied using electron microscopic cytochemical methods. In early prophase, considerable amounts of acid phosphatase are found in vesicles scattered through the cytoplasm; activity is also found in association with most membranous surfaces and often markedly associated with condensing mitochondria. Later in prophase the enzyme activity decreases to normal levels. Electron microscopic autoradiography revealed that DNA is synthesized in both plastids and mitochondria during meiotic prophase with activity reaching a peak during zygotene and ceasing by diakinesis and tetrad formation. These changes point to a certain independence of organelles from nuclear control during meiosis. The events are also evaluated in relation to a cytoplasmic clearing mechanism, which may occur in preparation for the changeover from sporophytic to gametophytic control and the development of gametes.

Acid Phosphatase↗
PubMed1987

Levels of microtubules during the meiotic maturation of the Xenopus oocyte.

The total level of tubulin and the ratio of polymeric tubulin to tubulin dimer were measured by a colchicine filter-binding assay during meiotic maturation of the Xenopus oocyte. Although the total level of tubulin remains unchanged (0.12 +/- 0.03 micrograms/oocyte), the level of polymeric tubulin decreases during maturation (25% in prophase oocytes versus 20% in metaphase oocytes). The percentage of polymerized tubulin was estimated after drug (nocodazole and taxol) treatments and cold treatment in prophase and progesterone-matured oocytes; in all cases the microtubules present in mature oocyte are less stable than prophase microtubules. The presence of the nucleus modifies neither the level nor the stability of prophase microtubules. Our quantitative results as well as cytological arguments suggest that full-grown Xenopus oocytes may contain a cortical microtubular array.

Animals↗
PubMed1975

Studies of oogenesis and follicular development in the golden hamster. 2. Initiation and control of meiosis in vitro.

The onset of meiotic prophase in the hamster occurs after birth and therefore it is possible to obtain ovaries which are at a relatively early stage of differentiation. Ovaries were maintained in vitro where the factors which might influence the onset of meiotic prophase could be studied in isolation. It was found that if ovaries were explanted prior to day 15 p.c. meiotic prophase began, but was subsequently blocked at the leptotene stage: the remaining oogonia and the oocytes at leptotene degenerated around day 5 p.p. By day 7 p.p. the few remaining oocytes were found at an abnormal stage which resembled leptotene. The addition of gonadotrophic hormones or maternal serum to the culture medium had no effect on the initiation of meiosis, neither had culture with the intact oviduct. By contrast, removal of the ovaries on or after the 15th day of gestation permitted meiotic prophase to proceed normally and oocytes at the diplotene stage were observed in the explants 8 days later. Culture of ovaries removed before day 15 p.c. resulted in abnormal development of the ovarian stroma: only when ovaries were explanted on day 15 or after was the development of 'nests' of oocytes normal. The evidence for endocrine control of oogenesis is discussed in the light of work on intersex animals and artificial chimaeras. These studies strongly suggest that oogonia enter meiosis spontaneously at the time which is specific for the species.

Animals↗
PubMed2006

[Abnormal meiosis in bisporic mutants of white button mushroom Agaricus bisporus (Lange) Imbach].

A formerly developed method of obtaining spread preparations of mushroom basidial nuclei was applied to study of meiotic prophase I in bisporic white button mushroom (Agaricus bisporus) strains. Meiotic recombination and assemblage of axial structures (axial elements and synaptonemal complexes) of chromosomes in meiotic prophase I are interrelated. It is known that the frequency of meiotic recombination is reduced in the bisporic A. bisporus variety. We showed that formation of axial structures of meiotic chromosomes in bisporic strains of this mushroom was disrupted. The phenotypes of disruptions in spread prophase nuclei are diverse. In leptotene and early zygotene, many nuclei contain abnormal, often short, and, as a rule, few chromosomal axial elements. The abnormalities in the formation of synaptonemal complexes at the zygotene-diplotene stage are of the same kind and even more pronounced. We discovered an important feature of meiosis in A. bisporus associated with fruit-body morphogenesis. Meiosis starting in basidia (meiocytes) of young closed fruit bodies is accompanied by disruption of chromatin condensation in prophase I and, probably, is arrested. After indusium breakage, the course of meiosis normalizes. Preparations with clearly observable chromosomal axial structures can be obtained only at this stage of fruit-body development.

Agaricus↗
PubMed1991

Stage-specific damage to synaptonemal complexes and metaphase chromosomes induced by X rays in male mouse germ cells.

Synaptonemal complexes reveal mutagen-induced effects in germ cell meiotic chromosomes. This study was aimed at characterizing relationships between damage to synaptonemal complexes and metaphase I chromosomes following radiation exposure at various stages of spermatogenesis. Male mice were irradiated with doses of 0, 2, or 4 Gy, and spermatocytes were harvested at times consistent with earlier exposures as spermatogonial stem cells, preleptotene cells (premeiotic DNA synthesis), or meiotic prophase cells. After stem-cell exposure, twice as many rearrangements were observed in synaptonemal complexes as in metaphase I chromosomes. Irradiation during premeiotic DNA synthesis resulted in dose-related increases in synaptonemal complex breakage and rearrangements (including novel forms) and in metaphase chromosomal aberrations. Following prophase exposure, various types and levels of damage to synaptonemal complexes and metaphase chromosomes were observed. Irradiation of zygotene cells led to high frequencies of chromosome multivalents in metaphase I without a correspondingly high level of damage in preceding prophase synaptonemal complexes. Thus irradiation of premeiotic and meiotic cells results in variable relationships between damage to synaptonemal complexes and metaphase chromosomes. Interpretations of these relationships are based upon what is known about both radiation clastogenesis and the structural/temporal relationships between synaptonemal complexes at prophase and chromosomes at metaphase I of meiosis.

Animals↗
PubMed1990

Mitosis arrested by deuterium oxide. Light microscopic, immunofluorescence and ultrastructural characterization.

Dynamics and correctness of mitotic division of PtK1 cells grown in vitro in the presence of deuterium oxide were assessed by morphological criteria. Incubation of PtK1 monolayers for 2 h in media supplemented with 25% to 75% D2O retarded in a dose-dependent manner the interphase/prophase transition, thus substantially decreasing the fraction of prophase cells. The progression of mitosis was also arrested at the metaphase/anaphase transition resulting in an accumulation of prometaphase and metaphase cells. After a prolonged incubation (12-24 h) in 75% D2O, prophase cells reappeared, often displaying a peculiar, "overmatured" pattern of chromosome condensation. In the presence of D2O an "overmatured" prophase cell is able to transform into a prometaphase cell and then directly into a multinucleate/micronucleate cell. Arrested prometaphase and metaphase cells developed restitution nuclei when incubated up to 24 h in the media containing 25% to 75% of heavy water. Immunofluorescence and electron microscopic studies revealed that the treatment with deuterium oxide profoundly disturbed development of the mitotic spindle. On the basis of these observations, a possible mechanism of the antimitotic action of deuterium oxide is discussed. We postulate that centrosomally controlled transformation of the microtubular cytoskeleton into the mitotic spindle is impaired by D2O. This delays the progression of prophase and disturbs congression of chromosomes into a regular metaphase plate.

Animals↗
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↗
PubMed1997

Frailty of two cell cycle checkpoints which prevent entry into mitosis and progression through early mitotic stages in higher plant cells.

Allium cepa L. root meristems were given two short caffeine treatments spaced by 15 hours, the time which roughly corresponds to the duration of one cell cycle. In this way two subsequent cytokineses were prevented, and multinucleate cells with their in complement distributed into two, three or four nuclei were formed. Though all nuclei started to replicate synchronously in these cells, some of them (fast nuclei) completed their replication earlier than others (slow nuclei). The present report shows that two successive checkpoints operate before prometaphase in these cells. The first one prevents the entry of the fast nuclei into prophase until the slow ones have completed their replication. The second checkpoint ensures the synchronous entry into prometaphase after all nuclei have reached and finished prophase. By treating the multinucleate cells with an inhibitor of DNA synthesis at that time when fast but not slow nuclei had finished their replication, it was observed that both checkpoint mechanisms became leaky with time. Under these conditions the fast nuclei entered prophase in the presence of nuclei which were prevented from finishing the replication of their DNA. Subsequently, even prometaphase was triggered after a prolonged prophase. Finally, as expected from the presence of mitotic stages in these cells, nuclei with incompletely replicated DNA endured premature chromosome condensation. The prematurely condensed chromosomes either remained in a prometaphase-like stage until reconstitution nuclei formed or they followed the progression of the fast nuclei into metaphase and anaphase leading to the appearance of acentric chromosomal segments which after reconstitution gave rise to aneuploid nuclei containing unstable and broken DNA.

Allium↗
PubMed1998

Germ cell kinetics during early ovarian differentiation: an analysis of the oogonial cell cycle and the subsequent changes in oocyte development during the onset of meiosis in the rat.

The aim of this study was the comparison between the mitoses of oogonia and the initial stages of oocyte meiosis. The structural alterations that the germ cell chromatin undergoes during the oogonial mitosis have been compared with those occurring during the G1- and S-phase just before meiosis. Using plastic embedded 1-microm sections of fetal rat ovaries (embryonic days = ED 14-20) labeled with 3H-thymidine and re-embedded for electron microscopy, a study of the structural conditions of the nuclear chromatin has been combined with a kinetic analysis of the oogonial cell cycle and the transitional period into the meiotic prophase. After ovarian differentiation (ED 14) the oogonia show a non-clonal, but strong proliferation. On ED 16, proliferation changes to a clonal pattern and decreases during ED 17. A final increase in 3H-thymidine incorporation on ED 18 characterizes the meiotic S-phase. On ED 19 the nuclear labeling drops to zero. The mitotic cycle of the oogonia lasts 16.5 hr and can be divided into 11 stages according to the concept of El-Alfy and Leblond [(1988) Am. J. Anat., 183:45-56] on the basis of the chromatin pattern. The S-phase (10.0 hours) extends from the telophase-interphase transition through the interphase to early prophase. The postmitotic G1- and S-phases show a more extensive duration, respectively 10 and 11.5 hours, and differ from their oogonial counterparts by the spherical shape of the nuclei from the very beginning. The chromatin pattern is similar until the end of the S-phase and lacks any prophase-like, preleptotenal chromatin condensation before the oocytes exhibit (pre-) leptotenal structures. Once the germ cell has completed a sequence of clonal mitotic divisions, it irrevocably progresses into meiosis. During an extended postmitotic period, the structural characteristics of meiosis emerge stepwise.

Animals↗
PubMed1996

The relationship of HsEg5 and the actin cytoskeleton to centrosome separation.

Although centrosome separation is essential to the formation of a bipolar spindle, it can proceed along several different pathways. This raises questions as to the similarity between the mechanism(s) underlying these various forms of separation. To address this question we reinvestigated centrosome separation in HeLa cells using a variety of techniques. We present a refined description of the two major pathways of centrosome separation found in HeLa cells and demonstrate that each of these pathways has its own timing, protein requirements, morphological characteristics, and relationship to spindle assembly. The first pathway, which occurs in prophase cells, is dependent on an intact actin cytoskeleton, and when this pathway is completed prior to nuclear envelope breakdown, the microtubules associated with this process do not become part of the spindle. Thus, centrosome separation and spindle pole organization can occur as two separate events. The second centrosome separation pathway is found in cells in which separation occurs concurrent with prometaphase. In this case, centrosome separation and the formation of the mitotic spindle are integrated together and an intact actin cytoskeleton is not required. The relationship between these multiple pathways of centrosome separation and the distribution of the human kinesin-like protein HsEg5 was also investigated. This protein was found associated with all centrosomal microtubules present during both prophase and prometaphase centrosome separation, as well as with prophase centrosomes displaying independent movement in Cytochalasin-D treated cells. In addition, we demonstrate that this protein is associated with post-mitotic centrosome movement which involves a single centrosome. Thus, HsEg5 is a feature of individual centrosome function and does not require anti-parallel microtubule arrays.

Actins↗