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R R Klevecz

Publications and source records attributed to R R Klevecz.

At least 19 recordsLinked to original sources
PubMed1991

Autogenous formation of spiral waves by coupled chaotic attractors.

When large arrays of strange attractors are coupled diffusively through one of the variables, chaotic systems become periodic and form large archimedean spirals or concentric bands. This observation may have importance for many applications in the field of deterministic chaos and seems particularly relevant to the question of the formal temporal structure of the biological clock in metazoan organisms. In particular, although individual cellular oscillators, as manifested in the cell cycle, may have deep basins of attraction and appear to be more or less periodic, we suggest that cells oscillate with chaotic dynamics in the ultradian domain. Only when large aggregates of these cells are tightly coupled can a precise circadian clock emerge. For changing coupling strength or parameter values, period increase occurs through quantal or integral multiple increments of the fundamental. All calculations were implemented on a 386AT, using a Mercury MC6400 floating point processor.

Animals↗
PubMed1987

Circadian gating of S phase in human ovarian cancer.

Cell proliferation in 30 patients with ovarian cancer was analyzed using flow cytometry to determine changes in the percentage of cells in S phase. By this measure, proliferation in tumor cells appears to follow a cyclical pattern of peaks and troughs that is out of phase with the circadian rhythm in proliferation of normal tissues. In round-the-clock monitoring of replication stages in tumor cells recovered from i.p. lavage fluid in postsurgery patients, peaks of tumor and nontumor cell DNA synthesis commonly occurred at different times of day. When patients were grouped so that only tumor cell proliferation was being measured, a highly significant 24-h rhythm nearly 12 h out of phase with nontumor cell proliferation was found. This peak in the percentage of S-phase cells occurs most commonly in mid- to late morning and appears to offer an opportunity for timing chemotherapy to coincide with high tumor cell vulnerability and low toxicity to normal tissue.

Cell Division↗
PubMed1987

Isolation of viable mouse primordial germ cells by antibody-directed flow sorting.

The germ line represents a cell type of unique interest in mammals because it retains complete genotypic totipotency while undergoing significant phenotypic differentiation. Analysis of the mechanism that underlies the maintenance of this totipotency requires the ability to isolate and study all stages of the germ cell lineage. The primordial germ cells (PGC) are the earliest identifiable germ cells in the embryo. It has not previously been possible to isolate PGC in sufficient numbers and purity to facilitate biochemical and/or molecular analysis. We report here that the use of a monoclonal antibody in combination with flow cytometry does permit the isolation of reasonably large and pure yields of viable mouse PGC.

Animals↗
PubMed1987

Tumor DNA content as a prognostic feature in advanced epithelial ovarian carcinoma.

Tumor DNA content (ploidy) was determined in 84 patients with epithelial ovarian carcinoma. Stage II-IV. A total of 251 DNA histograms generated by flow cytometry on cells derived from paraffin-embedded specimens were analyzed retrospectively. Of the 84 patients, 44 had tumors which were aneuploid, whereas 33 had diploid, and 7 had tetraploid tumors. Cox regression analysis revealed that age (P less than 0.001), stage (P less than 0.001), and ploidy (P less than 0.001) were independent prognostic features. The median survival time was 19 months and 48 months, respectively, in aneuploid and euploid tumors (P less than 0.001). The size of residual after surgery lost its significance when corrected for stage. Multivariate analysis in Stage III tumors revealed that ploidy was the most important prognostic factor (P less than 0.001) followed by age (P less than 0.025). A remarkable stability of cellular DNA content was found when the primary tumor was compared to the following groups: (1) various metastatic specimens from the primary operation in the same patient; (2) specimens analyzed sequentially from primary, secondary, and tertiary exploratory laparotomy; and (3) peritoneal washings before and after intraperitoneal chemotherapy.

Adult↗
PubMed1987

Circadian and ultradian rhythms of proliferation in human ovarian cancer.

Synchronous waves of proliferation in tumor cells taken from patients with ovarian cancer were observed using flow cytometry to measure the fraction of cells undergoing DNA replication and displaying tumor-cell-specific immunofluorescence. When saline washings of the abdominal cavity were analyzed at 2-4 hr intervals round-the-clock, the percentage of cells in the chromosome replication cycle (S + G2 percentage) showed 12-hr and often higher frequency rhythms in proliferation. These higher frequency rhythms in DNA replication show a relatively constant phase relationship to the patient's circadian clock with peak proliferation occurring most commonly at 10 a.m. to 12 noon and again at 10 p.m. This proliferation rhythm is therefore partially out of phase with the 24-hr rhythms in proliferation seen in normal cells. The findings on human cancer reveal a fundamental difference in the temporal organization of normal and tumor cell growth that should be exploited for therapeutic benefit.

Activity Cycles↗
PubMed1986

Automatic digital image analysis for identification of mitotic cells in synchronous mammalian cell cultures.

Mitotic frequency in a synchronous culture of mammalian cells was determined fully automatically and in real time using low-intensity phase-contrast microscopy and a newvicon video camera connected to an EyeCom III image processor. Image samples, at a frequency of one per minute for 50 hours, were analyzed by first extracting the high-frequency picture components, then thresholding and probing for annular objects indicative of putative mitotic cells. Both the extraction of high-frequency components and the recognition of rings of varying radii and discontinuities employed novel algorithms. Spatial and temporal relationships between annuli were examined to discern the occurrences of mitoses, and such events were recorded in a computer data file. At present, the automatic analysis is suited for random cell proliferation rate measurements or cell cycle studies. The automatic identification of mitotic cells as described here provides a measure of the average proliferative activity of the cell population as a whole and eliminates more than eight hours of manual review per time-lapse video recording.

Animals↗
PubMed1985

Cell-cycle-dependent expression of human melanoma membrane antigen analyzed by flow cytometry.

Knowledge of tumor antigenic expression is crucial to the design of therapeutic strategy. A murine monoclonal antibody (BE4) against a human melanoma membrane antigen, was used to study the in vitro expression of this antigen. By membrane immunofluorescence, BE4 reacted against 5 of 8 melanoma lines as compared to zero of 13 other cell populations. Using flow cytometry, the antigenic M14 CEM melanoma cells consisted of 40% to 60% of the total cell population. Dual-parameter measurements of DNA content and membrane antigen demonstrated that the nonantigenic cells were predominantly in G0/G1 phase, whereas the antigenic cells were distributed throughout the cell cycle. Within one passage, the sorted and recultured nonantigenic population demonstrated a similar proportion of antigenic cells as the unsorted original population. It was concluded that the expression of human melanoma antigen was cell-cycle-dependent. Understanding factors that turn off the expression of antigen in G0/G1 phase may lead to better immunotherapeutic strategies.

Antibodies, Monoclonal↗
PubMed1985

Quasi-exponential generation time distributions from a limit cycle oscillator.

In spite of the apparently random behaviour and the often exponential distribution of generation times expressed in cell populations, there is evidence for rather precise timekeeping in the cell cycle. In experiments using time-lapse video-tape microscopy, we have noted that cell generation times are often not distributed smoothly but in many cases seem to cluster at roughly 4 hr intervals. Phase shift responses following application of heat shock, ionizing radiation or serum pulses in each case show a pattern which is repeated twice in cells with an 8-9 hr modal generation time. We describe here a cell cycle model with an independent cellular clock controlling cell cycle events which accounts for the phase response data, while also reconciling the stochastic and periodic behaviour characteristic of animal cells.

Animals↗
PubMed1978

Tyrosyltubulin ligase and colchicine binding activity in synchronized Chinese hamster cells.

Tyrosyltubulin ligase (TTL) was found to be present in CHO and V79 Chinese hamster cells grown in tissue culture. The enzyme is soluble and requires potassium, magnesium, and ATP for maximum activity and requires tubulin as a substrate. TTL was analyzed through the cell cycle of V79 and CHO Chinese hamster cells. The enzyme showed two peaks of activity in V79 cells at 4 h and 7 h after mitotic selection, corresponding to the early S and mid to late S phases of the cell cycle. In CHO cells the enzyme displayed a major peak of activity at mid S and a minor peak or plateau during early S. Tubulin, as measured by (3H)colchicine binding, was shown to increase through S phase and reach a maximum late in the cycle during G2 approx. 3 h after maximum TTL activity.

Adenosine Triphosphate↗
PubMed1978

Cell cycle and the concept of physiological age with special reference to pyruvate kinase activity in WI-38 cells.

WI-38 cells were synchronized by mitotic collection and periodically assayed for pyruvate kinase activity. The kinetics of the synchronous cohort were determined by continuous labelling index and by mitotic index. The experimental data were analysed by computer using a state vector model to yield the probability density functions for phase transit times and for cell physiological ages. Pyruvate kinase activity for these cells as a function of physiological age was then examined using the computer model. Considering DNA synthesis, pyruvate kinase activity and mitosis to be markers of physiological age, it was found that a model which assumes that a cohort of synchronized cells desynchronizes irreversibly and uniformly from one age marker to the next is incompatible with the experimental data. For example, the times over which cells entered the S phase were too widely distributed to be consistent with the mitotic index data. Also, for pyruvate kinase activity to be a function of physiological age alone, the cell ages were probably too dispersed to be compatible with the experimental enzyme data. Alternative models for cell physiological ageing are presented, which are compatible with the experimental data.

Cell Cycle↗
PubMed1976

Quantized generation time in mammalian cells as an expression of the cellular clock.

The distribution of possible generation times in mammalian cells does not appear to be continous within the limits of range for each cell type; rather, generation time is quantized in multiples of 3-4 hr. Synchronous cultures of Chinese hamster V79 cells were prepared using manual and automated methods to select and stage mitotic cells. Using synchronous cultures and time-lapse video tape microscopy, it was possible to show that generation times within a population of mitotically selected cells normally disperse in a quantized fashion, with intervals of 3-4 hr occurring between bursts in division. In addition, at temperatures above 37 degrees, V79 cells have a 7.5-8.5 hr modal cell cycle, while at temperatures from 36.5 degrees to 33.5 degrees the modal cell cycle is 11-12 hr long. A survey of the synchrony literature reveals that the tendency to preferred generation times holds between cell lines. The distribution of modal generation times from a variety of different cell types forms a series with a similar interval but with a greater range of values than that observed here for V79 cells. To satisfy the published data and the work presented here, I propose a subcycle, Gq, which has a traverse time equal to the period of the clock. The period appears to be fixed at close to the same value in all mammalian somatic cells. The timekeeping mechanism appears to be temperature compensated, since the time required to traverse Gq is constant at temperatures between 34 degrees and 39 degrees. It is suggested that cell cycle time increases at lower temperatures, lower serum concentration, and high cell densitite because the number of rounds of traverse through Gq increases.

Biological Clocks↗