Period Of Mitosis Research Articles

Maintaining Transcriptional Specificity Through Mitosis.

Virtually all cell types have the same DNA, yet each type exhibits its own cell-specific pattern of gene expression. During the brief period of mitosis, the chromosomes exhibit changes in protein composition and modifications, a marked condensation, and a consequent reduction in transcription. Yet as cells exit mitosis, they reactivate their cell-specific programs with high fidelity. Initially, the field focused on the subset of transcription factors that are selectively retained in, and hence bookmark, chromatin in mitosis. However, recent studies show that many transcription factors can be retained in mitotic chromatin and that, surprisingly, such retention can be due to nonspecific chromatin binding. Here, we review the latest studies focusing on low-level transcription via promoters, rather than enhancers, as contributing to mitotic memory, as well as new insights into chromosome structure dynamics, histone modifications, cell cycle signaling, and nuclear envelope proteins that together ensure the fidelity of gene expression through a round of mitosis.

S. Ramon Cajal. Nouvelles contributions à l’étude histologique de la rétine. Journal de l’Anatomie et de la Physiologie. XXXII année, 1896, № 5. Septembre—octobre. P. p. 481—543

In the development of rods and cones, the author notes 4 common and other main periods: the embryonic period, the period of unipolarity, the period of bipolarity and the period of the young state. The first period corresponds to the period of the germinal bodies of His and, to some extent, the period of mitosis, described, for example, by Koganeem and Shevich. The shape of the visual cells at this time is irregular, spherical. In newborn cats, rabbits and dogs, all these cells, apparently, have already passed this period, at least it is impossible to see mitosis at this time. In the second period, located in the beginning along the neighborhood with m. limit. externa, the cell stretches out and gives a long outgrowth, the end of which is the cell itself, gradually descending downward to the point at which it should be in adulthood. The body is ellipsoidal with a vertically standing long axis; but sometimes this form is changed through compression by the neighboring elements.

Regional abnormalities in retinal development are associated with local ocular hypopigmentation

The retinal pigment epithelium (RPE) plays a key role in regulating retinal development. The critical enzyme in pigment production is tyrosinase. Transgenic mice with a tyrosinase construct where the locus control region was deleted (YRT4) display a variegated phenotype of tyrosinase expression. Their central retina is largely pigment free, whereas more peripheral regions are heavily pigmented. We have used this model to ask whether the influence of pigmented RPE over the retina during development is fundamentally governed by local interactions or is global. Our data show that YRT4 eyes have intermediate melanin content and relatively low tyrosinase activity compared with wild-type and albino animals. Rod counts are comparable to those in pigmented mice in peripheral regions but similar to those in albinos centrally. Anterograde labelling of retinal pathways demonstrates the presence of relatively normal ipsilateral chiasmatic projection in YRT4 mice, comparable with that in pigmented animals and consistent with the peripheral pigmented origin of this pathway. Examination of cellular proliferation levels during retinal development reveals that YRT4 mice display an extended period of mitosis, similar to that found in albinos. Hence, our results show that the regulatory influence of the RPE over the developing retina depends on localized interactions between these tissues.

A propagated wave of MPF activation accompanies surface contraction waves at first mitosis in Xenopus.

During the period of mitosis, two surface contraction waves (SCWs) progress from the animal to vegetal poles of the Xenopus egg. It has been shown that these SCWs occur in parallel with the activation of MPF and with its subsequent inactivation in the animal and vegetal hemispheres, suggesting that they are responses to propagated waves of MPF activity across the egg. We have analysed the mechanism of MPF regulation in different regions of the egg in detail in relation to SCW progression. The distributions of histone HI kinase activity and of Cdc2 and cyclin B (the catalytic and regulatory subunits of MPF) were followed by dissection of intact eggs following freezing and in cultured fragments separated by ligation. Cdc2 was found to be distributed evenly throughout the egg cytoplasm. Loss of phosphorylated (inactive) forms of Cdc2 coincided spatially with the wave of MPF activation, while cyclin B2 accumulation occurred in parallel in animal and vegetal regions. In ligated vegetal pole fragments no MPF activation or Cdc2 dephosphorylation were detectable. A wave of cyclin B destruction that occurred in concert with the second SCW was also blocked. Taken together these results indicate that the triggering mechanism for MPF activation requires components specific to the animal cytoplasm, acting via Cdc2 dephosphorylation, and that MPF activation subsequently propagates autocatalytically across the egg. SCW progression in the vegetal hemisphere was followed directly by time-lapse videomicroscopy of subcortical mitochondrial islands. The first SCW traversed the vegetal pole at the time of MPF activation in this region. Like MPF activation and inactivation, SCWs were blocked in the vegetal region by ligation. These observations reinforce the hypothesis that the first SCW is a direct consequence of the MPF activation wave. It may reflect depolymerisation of the subcortical microtubule network since it coincided exactly with the arrest of the microtubule-dependent movement of 'cortical rotation' and was related in direction in most eggs. The cyclin B destruction wave and associated cortical contraction of the second SCW may be localised downstream consequences of the MPF activation wave, or they may propagate independently from the animal cytoplasm.

Diurnal and circadian periodicity of mitosis and growth in marine macroalgae. III. The red alga Porphyra umbilicalis

In Porphyra umbilicalis, circadian rhythms of nuclear division and growth activity persisted for at least 7 cycles in continuous white fluorescent light, with a period of 21 h for the free-running growth rhythm at 15 µmol m−2 s−1 and 10 °C. Growth rhythmicity was lost at irradiances above 20 µmol m−2 −1. The growth and mitotic rhythms seem to be driven in parallel by circadian rhythmicity, and the details of growth kinetics must be due mainly to the growth behaviour of non-dividing cells. This was inferred from the finding that individual cells continued to grow from one cell division to the next, with generation times of 2–6 days. Transfer from continuous light to 12:12 h light : dark synchronized the free-running growth rhythm, with a high growth peak appearing every 24 h at the start of the light phase and an increase in growth rate during the dark phase. The ascending portion of the free-running growth curve was thus shifted into the night phase of the diurnal regime and the descending portion into the light phase. This behaviour of decreasing growth activity after the morning hours, previously found in a brown and a green marine macroalga, ensures that cell activity is available primarily for photosynthesis during the day.

Retinoic acid stimulates regeneration of mammalian auditory hair cells.

Sensorineural hearing loss resulting from the loss of auditory hair cells is thought to be irreversible in mammals. This study provides evidence that retinoic acid can stimulate the regeneration in vitro of mammalian auditory hair cells in ototoxic-poisoned organ of Corti explants in the rat. In contrast, treatment with retinoic acid does not stimulate the formation of extra hair cells in control cultures of Corti's organ. Retinoic acid-stimulated hair cell regeneration can be blocked by cytosine arabinoside, which suggests that a period of mitosis is required for the regeneration of auditory hair cells in this system. These results provide hope for a recovery of hearing function in mammals after auditory hair cell damage.

The cell cycle dependence of the secretory pathway in developing Xenopus laevis

The cell cycle during the cleavage period of the amphibian Xenopus laevis is about 30 min long and oscillates between equal periods of mitosis and interphase. At the midblastula transition (MBT) the length of interphase begins to elongate and brings about corresponding changes in the activities of cell cycle-dependent processes. In this study protein secretion and Golgi processing during embryonic Xenopus development were examined. The elongation of interphase, either during normal development or experimentally induced, resulted in an increase in the secretion of both endogenous and exogenous proteins. Secretion was found to increase linearly with the increase in interphase length, indicating that the rate of secretion was constant and was regulated by the length of interphase. M-phase arrest in embryos and oocytes produced an inhibition of protein secretion that was reversible if the cell cycle was returned to interphase. This M-phase block of the secretory pathway was found to take place between the trans Golgi compartment and the plasma membrane. The developmental increase in the function of this pathway after the MBT may affect the expression of surface and secreted proteins important for the cell-cell interactions necessary for subsequent development through gastrulation.

Regulation of cell division in the subapical shoot meristem of dwarf watermelon seedlings by gibberellic acid and polyethylene glycol 4000

The stimulatory effects of gibberellic acid (GA3) and the inhibitory effects of polyethylene glycol 4000 (PEG) on hypocotyl elongation and cell cycle kinetics in subapical pith cells of dwarf watermelon seedlings (Citrullus lanatus [Thunb.] Matsu and Nakai) were investigated. Mitotic indices (MI) were determined from direct counts of pith cells stained by a modified Feulgen technique. Labeling indices (LI) were determined from direct counts of labeled pith cells sampled 1.5 h after apical applications of3H-thymidine. Root application of 0.32 mM GA3 at 96, 120, or 144 h after sowing resulted in significant increases in both mitotic and labeling indices within 4.5 to 7.5 h following treatment. A single mitotic peak at 13.5 h occurred in all three treatment periods. Labeling peaks were often less defined than mitotic peaks; however, a relatively high proportion of labeled nuclei were usually observed between 7.5 and 9 h after GA3 treatment and at 16.5 h, the latter period coinciding with progression of cells into S phase from the peak period of mitosis. The results suggest that GA3 increases the proportion of rapidly dividing cells in the subapical meristem by increasing the probability that slowly cycling or nonproliferative cells in both 2C and 4C DNA states will enter the proliferative pool. The addition of PEG (200 g/l, ψ = 1.5 mPA) to the rooting medium of dwarf watermelon seedlings inhibited hypocotyl elongation and reduced both mitotic and labeling indices simultaneously within 4.5 h after treatment. Within 24–28 h after PEG treatment, mitotic and labeling indices approached 0. Seedlings transferred from PEG to either water or GA3 exhibited rapid recovery of cell division and hypocotyl elongation. Mitotic and labeling indices increased within 4.5–7.5 h into the recovery period in either water or GA3 and reached control values within 10.5 h. GA3 hastened the recovery from PEG-induced stress. It is concluded that water stress imposed by PEG 4000 causes arrest of cell division in meristematic cells of watermelon seedlings in both G1 and G2 periods. PEG and GA treatments resulted in only a partial and transitory synchronization of the cell cycle.

Cell differentiation in microsporangia of Pinus sylvestris. II. Early pachytene

Tapetal cells of Pinus sylvesiris L. underwent one hypersecretory cycle during early pachytene and entered a second cycle by the time of mid pachytene in microspore mother cells. Initially in each cycle tapetal cells became papillaform then dome‐shaped and thereafter, in conjunction with radial narrowing of the cells, dilation of the ER system was intensive throughout the cytoplasm. Recovery from hyperactivity culminated with the presence of plasmodesma‐like connections between tapetal cells and a period of mitosis in tapetal cells. The differentiation of tapetal cells occurred asynchronously within a microsporangium and their dedifferentiation began nonuni‐formly, becoming synchronous coincidental with the presence of “plasmodesmata”. Microsporangia increased in size and tapetal cells were laterally wide before “plasmodesmata” receded and tapetal cells again differentiated asynchronously.

Cell cycle regulation of ribonucleoside diphosphate reductase activity in permeable mouse L cells and in extracts.

Ribonucleoside diphosphate reductase (EC1.17.4.1) was previously characterized in exponentially growing mouse L cells selectively permeabilized to small molecules by treatment with dextran sulfate (Kucera and Paulus, 1982b). This characterization has now been extended to cells in specific phases of the cell cycle and in transition between cell cycle phases, with activity studied both in situ (permeabilized cells) and in cell extracts. Cells at various stages in the cell cycle were obtained by unit-gravity sedimentation employing a commercially available reorienting chamber device, by G1 arrest induced by isoleucine limitation, and by metaphase arrest induced by Colcemid. G1 cells from both cycling and noncycling populations had negligible levels of ribonucleotide reductase activity as measured by CDP reduction both in situ and in extracts. When G1 arrested cells were allowed to progress to S phase, ribonucleotide reductase activity increased in parallel with [3H]thymidine incorporation into DNA. Ribonucleotide reductase activity in extracts increased at a somewhat greater rate than in situ activity. S phase ribonucleotide reductase activity measured in situ resembled the previously characterized activity in exponentially growing cells with respect to an absolute dependence on ATP or its analogs as positive allosteric effector, sensitivity to the negative allosteric effector dATP, and low susceptibility to stimulation by NADPH, dithiothreitol, and FeCl3. Disruption of permeabilized cells caused reductase activity to become highly dependent on the presence of both dithiothreitol and FeCl3. As synchronized cultures progressed from S into G2/M phase, no significant change in ribonucleotide reductase activity was seen. On the other hand, when cells that had been arrested in metaphase by Colcemid were allowed to resume cell cycle traversal by removing the drug, in situ ribonucleotide reductase activity decreased by 75% within 2.5 h. This decrease seemed to be a late mitotic event, since it was not correlated with the percentage of cells entering G1 phase. The cause of a subsequent slight increase of in situ ribonucleotide reductase activity is not clear. Parallel measurements of ribonucleotide reductase activity in cell extracts indicated also an initial decline accompanied by increasing dependence on added dithiols and FeCl3, followed by complete activity loss. Our results suggest a cell cycle pattern of ribonucleotide reductase activity that involves negligible levels in G1 phase, a progressive increase of activity upon entry into S phase paralleling overall DNA synthesis, continued retention of significant ribonucleotide reductase activity well into the metaphase period of mitosis, and a very rapid decline in activity during the later phases of mitosis. The periods of increase and decrease of ribonucleotide reductase activity were accompanied by modulation of the properties of the enzyme as indicated by differential changes in enzyme activity measured in situ and in extracts.

Regeneration of rat tracheal epithelium: changes in gap junctions during specific phases of the cell cycle.

Uninjured mitotically inactive tracheal epithelium has virtually no gap junctions. When the epithelium is stimulated to proliferate synchronously through a single wave of DNA synthesis and cell division, study of thin sections and freeze-fracture replicas by conventional transmission electron microscopy reveals that gap junctions are formed by the end of the DNA synthetic (S) phase. During the period of mitosis (M), the gap junctions disappear and are not again observed as the superficial cells undergo mucous and ciliary differentiation and normal pseudostratified architecture is restored. If the regenerative cells are continuously stimulated so they go through at least one additional S phase, gap junctions begin to reappear in G1 and reach peak numbers at the end of the second S phase. The correlation of the appearance of gap junctions in regenerating tracheal epithelium with a specific phase of the cell cycle and the absence of these junctions in cells in other phases of the cycle and in the differentiated progeny cells suggests that this surface feature plays a role in control of mitotic activity.

Mitosis and mitotic activity in Codium fragile (Suringar) Hariot (Chlorophyceae)

Abstract The mitotic cycle and periodicity of mitosis in Codium fragile in culture were investigated, primarily using germlings. In most respects mitosis resembles “normal” mitosis. However, a persistent nucleolus-like body was observed during all stages of division using acetocarmine stain. Attempts to ascertain the chemical nature of this body were unsuccessful. Experiments on the periodicity of mitosis disclosed a series of peaks of mitotic activity which appear unrelated to the time of onset of the light or dark period. Mitosis within any one germling is asynchronous.

HELA CELLS: EFFECTS OF TEMPERATURE ON THE LIFE CYCLE.

After a shift in temperature, the population kinetics of HeLa cells does not immediately reach a steady state characteristic of the new temperature. During the transition period the duration of each period of the life cycle (period before DNA synthesis, of DNA synthesis, after DNA synthesis, and the period of mitosis) increases toward its steady-state value. Of the four periods, the period before DNA synthesis reaches its steady-state value fastest. Exponential growth can be maintained only from 33 degrees C through 40 degrees C. The period of mitosis is the most temperature-sensitive period of the life cycle of the cell. At subnormal temperatures (26 degrees C through 31 degrees C) there is an accumulation of cells in mitosis, and mitotic indices as high as 0.44 can be obtained. The duration of mitosis is a function both of the temperature and of the time which the cell has spent at this temperature.

Diurnal periodicity of mitosis in mice after irradiation with ?-rays

A study was made of the effect produced by ionizing radiation (190 and 400 r) on the 24-hour periodicity of mitotic activity of the mouse corneal epithelium. As noted, the 24-hour mitotic rhythm was retained on the 5th postirradiation day (doses of 190 and 400 r being used). The time of the maximal (8 A.M.) and the minimal (8 P.M.) number of mitosis in the corneal epithelium of experimental mice coincided with control indices. The difference between the maximal and minimal mitotic activity in the two experiments was highly significant. The range of the 24-hour variations of the number of mitoses showed no significant change in the experimental animals. After irradiation of mice in a dose of 400 r there was a tendency to a rise of the mean 24-hour intensity of cellular multiplication.

The character of the 24-hour periodicity of mitosis in the corneal epithelium of various laboratory animals

The 24-hour periodicity of mitosis was studied in the corneal epithelium of white mice, white rats and guinea pigs. There were species-specific differences both in the intensity of the cellular multiplication, and in its 24-hour rhythm. The 24-hour periodicity of mitotic activity is clearly manifested in mice and rats, the maximal amount of mitoses being observed during morning hours, and the minimal—in the evening. The curve, charaterizing the 24-hour periodicity of mitoses is uniform during all the seasons of the year. The amplitude of the 24-hour variation of mitotic indices in mice is on the average 3 to 4 times greater than in rats. In guinea pigs the 24-hour mitotic periodicity of the corneal epithelium is not manifested so clearly.

PERIODICITY OF MITOSIS AND CELL DIVISION IN THE EUGLENINEAE

1. The periodicity of mitosis and cell division has been investigated in 15 green and 5 colorless species of the Euglenineae.2. Green species in biphasic culture under natural light conditions have mitosis confined to the dark period. Mitosis begins one to two hours after the onset of darkness, each species having a predictable percentage of cells dividing each night. There is a threshold period at the beginning of the dark period after which mitosis cannot be inhibited by light. The mitotic rhythm is exogenous, being removed by growth in artificial light or darkness (resulting in no mitosis), or in a rich organic medium (resulting in continuous mitosis at a constant rate).3. Colorless species in biphasic culture under any light conditions have an irregular mitotic periodicity, bursts of mitosis occurring at any time of the clock and alternating with periods in which mitosis is almost absent.4. It is suggested that the presence or absence of regular or irregular mitotic periodicity is related to the different modes and rates of nutrition of green and colorless species in various conditions of light and darkness, and in various media.

The development of the malpighian tubules of schistocerca gregaria (orthoptera)

ABSTRACT 1. Schistocerca gregaria Forsk. possesses about 250 Malpighian tubules arranged in twelve groups, situated dorso-laterally, laterally, ventro-laterally, and in the six intervening positions around the posterior end of the mid-gut. The gut forms an expanded ampulla at the point of entry of each group. 2. Each tubule is composed of five rows of cells, spirally arranged. Upper and lower segments are present, as in Rhodnius (Wigglesworth, 1931), the former very short. There is also a spiral trachea. 3. Tubules arise in seven generations, two in the embryo and one in each of the five nymphal instars. The first embryonal generation is composed of six primary tubules, the second of twelve embryonal secondaries. In the other five a variable number of nymphal secondaries is added, with means of 25, 51, 88, 63, and 6 respectively. The six primary tubules are attached distally to the rectum, the embryonal and first and second instar secondaries pass forwards, the fourth and fifth instar secondaries pass backwards, and the third in both directions. 4. The development of any one tubule comprises a period of initiation, a period of mitosis and elongation, and a period of differentiation. The first is restricted to the first few days of the particular stadium, the second takes place throughout the stadium, and the last throughout the life of the tubule. 5. The secondary tubules are arranged in an arc round the periphery of the ampulla. These arcs are perfectly symmetrical, the tubules arising alternately left and right of the mid-line during all the nymphal stages. 6. Tubule buds formed late in the period of initiation may cease to develop until the corresponding phase of the succeeding instar is reached. 7. The origin of tubules from a posterior interstitial ring and its relation to the interpretation of insect gastrulation and the nature of nymphal developmental cycles is discussed.

Cytology and Life History of Vaucheria geminata

1. Vaucheria geminata is found most abundantly in the spring and autumn. Sexual reproductive organs are most abundant in September and October; they are also found about April 15. 2. There are one or two periods of general mitosis in the oogonium. The egg nucleus undergoes mitosis just in advance of the vegetative nuclei. 3. The nuclear membrane disappears in both the vegetative and reproductive nuclei during mitosis. Spindle fibers are visible in the vegetative nuclei in the metaphase. 4. The cross wall begins to form about the time the sperm enters the oogonium, and is of vacuolar origin. The vegetative nuclei begin disintegration at the time of fertilization. 5. There is no migration of nuclei either to or from the oogonium of Kaucheria geminata. 6. A swelling forms from the oogonium in the "receptive region," which comes in contact with the antheridium at the time of fertilization. The male element enters through this process. 7. Fertilization occurs during the latter part of the night. Maturation divisions in the egg nucleus occur in the afternoon. The formation of the cross wall, the disintegration of the vegetative nuclei, and the fusion of the gamete nuclei occur in the latter part of the night and early morning. 8. The number of vegetative nuclei in the oogonium varies in specimens examined from 55 to 141. They are increased considerably in number during the periods of mitosis. 9. The fusion of the gamete nuclei does not occur immediately when the male element enters the oogonium. The male nucleus lies beside the egg nucleus and there enlarges to approximately the size of the female gamete nucleus. 10. In the center of the oogonium a vacuolar structure forms after fertilization. The zygote nucleus takes its position in the midst of this structure.