Heterogeneous chromosomal radiosensitivity of phytohaemagglutinin-stimulated human blood lymphocytes in culture

Influence of endogenous glutathione level on X-ray induced cell cycle delay in human lymphocytes

Here, induction of chromosomal aberration after X-irradiation and the pattern of cell cycle kinetics have been investigated in human lymphocytes, after exogenous addition of reduced glutathione or by depleting levels of reduced glutathione endogenously. Involvement of cell cycle regulator proteins such as p53 and p21 has been investigated to elucidate their role in induction of delay in cell cycle progression after irradiation.

Current status of cytogenetic procedures to detect and quantify previous exposures to radiation

The estimation of the magnitude of a dose of ionizing radiation to which an individual has been exposed (or of the plausibility of an alleged exposure) from chromosomal aberration frequencies determined in peripheral blood lymphocyte cultures is a well-established methodology, having first been employed over 25 years ago. The cytogenetics working group has reviewed the accumulated data and the possible applicability of the technique to the determination of radiation doses to which American veterans might have been exposed as participants in nuclear weapons tests in the continental U.S.A. or the Pacific Atolls during the late 1940s and 1950s or as members of the Occupation Forces entering Hiroshima or Nagasaki shortly after the nuclear detonations there. The working group believes that with prompt peripheral blood sampling, external doses to individuals of the order of about 10 rad (or less if the exposure was to high-LET radiation) can accurately be detected and measured. It also believes that exposures of populations to doses of the order of maximum permissible occupational exposures can also be detected (but only in populations; not in an individual). Large exposures of populations can also be detected even several decades after their exposure, but only in the case of populations, and of large doses (of the order of 100 to several hundred rad). The working group does not believe that cytogenetic measurements can detect internal doses from fallout radionuclides in individuals unless these are very large. The working group has approached the problem of detection of small doses (less than or equal to 10 or so rad) sampled decades after the exposure of individuals by using a Bayesian statistical approach. Only a preliminary evaluation of this approach was possible, but it is clear that it could provide a formal statement of the likelihood that any given observation of a particular number of chromosomal aberrations in a sample of any particular number of lymphocytes actually indicates an exposure to any given dose of radiation. It is also clear that aberration frequencies (and consequently doses) would have to be quite high before much confidence could be given to either exposure or dose estimation by this method, given the approximately 3 decades of elapsed time between the exposures and any future blood sampling.(ABSTRACT TRUNCATED AT 400 WORDS)

Transmission of radiation-induced rearrangements through cell divisions

A BrdU incorporation technique associated with heat denaturation, acridine orange staining and UV irradiation, was applied to G0-irradiated lymphocyte cultures. This made it possible both to obtain an R-banding, and to estimate the number of divisions undergone by each cell in mitosis irradiation. Cell survival and slowing down of the cell cycle could be distinguished. The frequency of various types of rearrangements, and their association was studied at each cell division. It is shown that the loss of cells carrying chromosomal rearrangements is determined by several parameters such as the presence of dicentric or multicentric chromosomes and above all the association of several rearrangements in the same cell.

Factors that influence formation of sister chromatid exchanges in human blood lymphocytes

Sister chromatid exchange (SCE) reflects an interchange of DNA sequences between helices in a replicating chromosome. This was initially accomplished by Taylor and colleagues (1957) using tritiated thymidine incorporation followed by autoradiography. The development of an elegant technique for differential staining of sister chromatids by incorporating a thymidine analog, 5-bromodeoxyuridine (BrdU) has greatly simplified the detection of SCEs in metaphase chromosomes. In recent years, the analysis of SCE has been considered to be a highly sensitive and additional (i.e., with chromosome aberrations) end point for measuring mutagenic/carcinogenic potential of various environmental agents and is increasingly being used to detect and differentiate among chromosome fragility human diseases that predispose to neoplasia. Attention has been focused to see if the induction of SCEs in lymphocyte cultures can be used as a reliable "biological dosimeter" for genetic risk assessment and to monitor the exposed populations. Several physical or preparatory as well as biological factors that modify the response and formation of SCEs make the monitoring difficult. The purpose of this article is to review and analyze these factors to facilitate an effective development of a standard protocol for SCE testing and for appropriate evaluation of test results. This may also provide clues to understand the yet unknown molecular mechanism(s) and biological significance of SCE formation.

Influence of various mitogens on the yield of sister-chromatid exchanges, induced by chemicals, in human lymphocytes

The fluorochrome-plus-Giemsa (FPG) method of Perry and Wolff was used to compare the frequencies of sister-chromatid exchanges (SCEs) induced by cyclophosphamide (CP) or mitomycin C (MMC) in human lymphocytes stimulated by phytohaemagglutinin (PHA), concanavalin A (Con A), Wistaria floribunda (WFA), or lentil lectin (LcH-A) extracts. These 4 mitogens, differing in lectin valency and/or sugar specificity, are considered as activating primarily thymus-derived (T) lymphocytes. Regardless of the mitogen used, control cultures displayed a mean yield of about 8 SCEs/cell. A contact, of 1 h, with mitomycin alone or with cyclophosphamide and enzymatic activation, resulted in a significant augmentation of SCEs dependent on the mitogen used. An approximately 2-fold, 4-fold, or 6-fold increase in SCEs was observed for the cultures stimulated by PHA, Con A, and WFA or LcH-A respectively. Furthermore, there were mitogen-dependent differences in mitotic indices and cell-cycle kinetics in human lymphocytes harvested 72 h after stimulation.

Blood lymphocyte culture system: quantitative analysis of X-ray-induced chromosome aberrations in man, muntjac and cattle

Peripheral blood lymphocytes of 3 mammalian species, man, muntjac and cattle, which have various amounts of DNA and divergent karyotypes, were exposed to 100-400 rad of X-rays, and frequencies of dicentrics and other aberrations were analysed at first post-irradiation metaphases. During experiments, various preparative or physical and biological factors that could influence the yield of chromosome aberrations were taken into account. The frequency of dicentrics scored at first post-irradiation metaphases showed best fit to both linear and quadratic dose-response curves, y = a + bD and y = bD + cD2 with a high correlation coefficient of 0.98 (P less than 0.001). The frequency of dicentrics obtained at different post-irradiation fixation times did not show significant variation, indicating a homogeneous sensitivity of peripheral lymphocytes to X-irradiation. BrdU incorporation following X-irradiation showed no increase in the frequency of chromosome aberrations. The frequency of dicentrics in man, muntjac and cattle showed a close correlation with their DNA content, but no meaningful correlation was found between the yield of dicentrics and the chromosome arm number or the nuclear volume. The ratio of dicentric yields, 1.00:0.67:1.04 obtained in man, muntjac and cattle were comparable to the ratio of their DNA contents, 1.00: 0.65: 1.07. The base-line frequency of SCEs was similar in the 3 species and no significant variation in SCE frequency was noticed even after administration of 400 rad of X-rays.

Survival and PHA-stimulation of gamma-irradiated human peripheral blood T lymphocyte subpopulations

Human peripheral blood T lymphocyte subpopulations were identified and isolated on the basis of their ability to bind IgG (T-G), IgM (T-M), or neither immunoglobulin class (T-null). Lymphocytes were exposed to 0, 0.5, 1.0, 2.5 or 5.0 Gy of 60Co gamma-rays either as a T-cell suspension or as separated T cell subsets. Survival curves, determined 5 days after irradiation, revealed that each subset has radiosensitive and radioresistant portions, and that the T-G cell is the most sensitive subset. Mitotic indices of 48-h cultures showed that the response of unirradiated T lymphocytes to PHA varied greatly among the subsets, the highest indices being obtained for the T-M and the lowest for the T-G cells. With the possible exception of the T-G cells, the subsets are relatively resistant to mitotic effects of gamma-rays. T-G cells suppress the PHA-induced mitotic response of the other T lymphocyte subsets, and this suppressor effect is radiosensitive, being abolished by 1.0 Gy. It is concluded that lymphocytes exposed to greater than or equal to 1 Gy of gamma-rays will have very few dividing B lymphocytes or T-G cells. This together with radiation-induced loss of T-G suppressor action means that the predominant lymphocyte types in mitosis after greater than or equal to 1 Gy are the radioresistant T-M and T-null cells.

Cell kinetics and radiosensitivity of human lymphocytes stimulated by phytohemagglutinin, Wistaria floribunda or Lentil lectin

The cell kinetics and the radiosensitivities of human lymphocytes (four donors) exposed to 200 rads of X-rays and stimulated with phytohemagglutinin (PHA), Wistaria floribunda (WFA) or Lens culinaris (LcH-A) extracts have been compared after cultivation times from 42 up to 54 h. All these mitogens are considered activating primarily T lymphocytes. PHA is a tetrameric molecule, whereas WFA as well as LcH-A are dimeric structures having only two reactive sites. PHA displays a higher mitogenic activity than WFA, while LcH-A is much less active than PHA and WFA. After 42 h of culturing, only metaphases of the first mitosis are found, irrespective of the mitogen used. With increasing cultivation times, however, differences in the cell kinetics have been observed for the different mitogens. Furthermore, no differences in radiosensitivity of lymphocytes stimulated by these mitogens were observed when cells are analyzed exclusively in their first mitosis.

Bleomycin-induced chromosomal aberrations in Down's syndrome lymphocytes

Blood samples from 4 Down's syndrome (DS) patients with a 47,XY,21 + karyotype and from 4 normal male probands were cultured for 72 h in the presence of BrdU and lymphocytes analysed at their first mitosis for chromosomal aberrations. The frequencies of spontaneous aberrations and the proportions of cells in the first or later mitoses in culture were not different between the groups. Treatment with various doses of bleomycin in vitro resulted in similar delays in cell development for both DS and normal lymphocytes and dose-dependent increases in the incidence of chromosome-type aberrations. However, the induction of both dicentric aberrations and acentric fragments was significantly enhanced in DS cells relative to cells of normal karyotype.

Enhanced frequency of chromosome aberrations in lymphocytes of male compared with female muntjacs after X-ray irradiation in vitro

Recently, there has been considerable interest in the study of radiation-induced dicentrics and other chromosome aberrations in phytohaemagglutinin (PHA)-stimulated blood lymphocytes of various mammals, including man, with the aim of establishing a proper biological dosimetry for the assessment of genetic radiation hazards in human. These studies have revealed that the radiosensitivity of lymphocytes differs between species and even between individuals of the same species, but the cause(s) of these variations still remains unclear. The life-shortening response to whole-body X-ray irradiation is influenced by both age and sex of the individuals, but although age has been shown to have effects on chromosome aberration yields, in vitro, the influence of sex on the sensitivity of lymphocyte chromosomes to radiation-induced damage has not been reported. It is generally thought that sex does not influence the aberration yield within the limitation of measurements used in such studies. However, we report here a study of X-ray induced chromosome aberration in lymphocytes of Indian muntjac, in which the yield of dicentrics was found to be consistently greater in male than in female lymphocytes.

Chromosomal radiosensitivity of pig leucocytes in relation to sampling time

Pig blood cultures were used to analyse the sensitivity to X-rays (measured as frequency of induced dicentrics) of lymphocytes sampled at variable times. By using the BrdU-Giemsa method it was possible to identify the lymphocytes that were performing their first division at early (less than 30% of cells in second division), intermediate (30-50% of cells in second or subsequent divisions) and late stages (more than 50% of cells in second or subsequent divisions). No difference was found in the radiosensitivity of these 3 varieties of lymphocyte. It was also observed that: (a) the combination of radiation followed by BrdU treatment did not increase the clastogenic action of X-rays, (b) X-rays in the dose used in our cultures did not increase the frequency of SCEs, and (c) minor changes in culture conditions probably influence the basal frequency of SCEs.

The yield of radiation-induced chromosomal aberrations in lymphocytes as related to the time of arrival at first post-stimulation mitosis

Blood from 3 donors of each species, man, rabbit and pig, were irradiated with a dose of 2.5 Gy 60Co gamma-rays. Micro-cultures of lymphocytes, established in presence of BrdUrd, were harvested at 6 different times after stimulation by PHA. The preparations containing metaphase figures were stained according to Perry and Wolff to permit differentiation of the cells in first and later mitoses. In all individuals and species studied there was a highly significant negative correlation between dicentric yield and time from stimulation to harvest. The decline of the yield with time of harvest varied in 3 species between 1.0 and 3.6% per hour. Implications for biological dosimetry are discussed.

Effects of washing on phytohemagglutinin responsiveness of lymphocytes from irradiated patients

The peripheral lymphocytes from irradiated patients generally have a reduced capability to respond to phytohemagglutinin (PHA). Whether a relationship exists between PHA-responsiveness and chromosome aberration frequencies was examined by washing the lymphocytes with culture medium. The results indicate that the defect in lymphocyte activation in patients receiving radiation therapy was caused by some reversible changes in the lymphocyte membrane directly associated with radiation exposure rather than by a radiation induced suppressor substance secondarily acting on the lymphocyte membrane.

The relative contributions of B and T lymphocytes in the human peripheral blood mutagen test system as determined by cell survival, mitogenic stimulation, and induction of chromosome aberrations by radiation

Ficoll-Hypaque-separated subpopulations of human peripheral blood T and B lymphocytes were exposed to 0, 0.5, 1.0, 2.5, or 5.0 Gy of gamma rays. Three parameters were examined: Survival, as measured by trypan blue dye exclusion in unstimulated cultures five days after irradiation; mitotic index, measured in phytohemagglutinin (PHA)-stimulated cultures 48 and 72 hours after irradiation; and chromosome aberration frequency, measured 48 or 60 hours after irradiation. Survival curves of T, B, and null cells are biphasic; the Do values for the radiosensitive populations of all three cell types are close to 0.6 Gy but are different for the radioresistant populations: 2.7 Gy for B cells, 4.77 Gy for T cells, and 6.03 Gy for null cells. B cells, as well as T cells, are stimulated to divide by PHA, and B cells comprise at least 10% of the mitotic figures seen in unirradiated cultures at 48 hours. The proportion of B lymphocytes in mitosis at any particular time after PHA stimulation decreases with increasing radiation dose, which reflects a higher mitotic radiosensitivity of B than of T cells. No significant difference, however, in chromosome aberration frequency was found between T and B cells.

Relation between cell cycle and yield of aberrations observed in irradiated human lymphocytes

The bromodeoxyuridine-Giemsa technique has been used to study systematically the incidence of cells in first or subsequent mitoses at different fixation times of human lymphocyte control cultures as well as the influence of ionizing radiations on cell kinetics. Second divisions appear (3%) in cultures harvested 48 h after initiation. In 72 h cultures 40% of the dividing cells are in second and 33% in third division. Administration of 200 rads of X-rays before PHA stimulation results in a mitotic delay but does not increase the incidence of SCE. The yield of dicentrics after an exposure to 200 rads was the same for all cells in first mitosis regardless of fixation time. These results demonstrate that there is no evidence for the existence of sensitive subpopulations that could be distinguished by the time of the first mitotic division following stimulation.

Kinetics of human lymphocyte division and chromosomal radiosensitivity

Human blood from normal donors was irradiated with 200 R during the Go phase, and the X-ray sensitivity of early and late dividing lymphocytes in culture was expressed as percentage of induced dicentrics. Cells in first or subsequent divisions were individualized by BrdU-Giemsa techniques. Lymphocytes in the first division at 40, 44 and 72 h after the start of culture had a lower sensitivity to radiation than lymphocytes making their first division at 48, 52 and 56 h. It was observed that: (a) the combination of radiation followed by BrdU did not increase the clastogenic action of X-rays, (b) X-rays in the dose and duration used in our cultures did not increase the frequency of SCEs, and (c) minor changes in culture conditions probably influenced the frequency of SCEs.

Sister chromatid exchanges in human leukocyte chromosomes: spontaneous and induced frequencies in early- and late-proliferating cells in vitro

Human leukocyte cultures were pulse-treated with the trifunctional alkylating mutagen trenimon in a final concentration of 10(-7) M for 15--20 h after culture start, i.e., in the G1 phase of the cell cycle. At 24 h after culture start bromodeoxyuridine (BUdR) was added to the trenimon-treated cultures and to several untreated cultures running in parallel. The series treated with BUdR only and the series treated with BUdR+trenimon were each used to prepare two cultures at different culture times. Mitoses were collected during consecutive intervals of 12 h from 30 h up to 102 h after culture initiation by colcemid. For all preparation times (42 h, 54 h, 66 h, 78 h, 90 h, and 102 h) the frequencies of first, second, and third and further mitoses were determined in the BUdR- and in the BUdR+trenimon-treated series. In the trenimon-treated series a clear cell cycle delay was detected as compared with the normal distribution of different types of mitoses found in series treated with BUdR only. Spontaneous and trenimon-induced sister chromatid exchange (SCE) frequencies were determined in second mitoses occurring at 66 h, 78 h, 90 h, and 102 h after culture start. For all these preparation times about six SCE per metaphase were consistently found in BUdR-treated, and about 19 SCE per metaphase in BUdR+-trenimon-treated series, indicating a homogeneous sensitivity of early- and late-proliferating cells with respect to the induction of SCE.

The effect of X-rays and neutrons on lymphocyte death and transformation

The effects of X-rays and neutrons on human lymphocytes in vitro has been tested. Radiation sensitivity of untransformed lymphocytes was assessed by the appearance of pyknotic cells, and the response of cells after stimulation by phyto-haemagglutinin was tested (a) morphologically and (b) by changes in DNA synthesis, using a labelled thymidine analogue. The data obtained for interphase cells suggest that lymphocytes are a mixed cell population with an insensitive component forming about 20 per cent of the population. The percentage of normal cells observed after both X-ray and neutron irradiation lie on the same dose--effect curve giving an r.b.e. of one. A biphasic response is seen after PHA stimulation with both tests of damage indicating at least two sub-populations of lymphocytes and these give r.b.e. values in the range 1.95 to 2.45. Providing the in vivo response is similar to that in vitro the r.b.e. for damage to circulating lymphocytes will be small and the reduction in white cell count will not therefore be a major factor limiting dose in neutron therapy.

Dicentric yields induced by gamma-radiation and chromosome arm number in primates

To evaluate the effect of the chromosome arm number on the yield of dicentric chromosomes, frequencies of gamma-ray-induced chromosome aberrations were examined with peripheral lymphocytes from three different primate species, Saimiri sciureus (arm number, 77), Macaca fascicularis (arm number, 83) and Nycticebus coucang (arm number, 99). Irradiated blood samples were cultured by the same standard technique as that commonly used for human lymphocytes. The yields of dicentrics and dicentrics plus rings at doses of 100, 200 and 300 rad of gamma-irradiation were not significantly different among the three species, in spite of the difference in the chromosome arm number. Furthermore, dose-response relationships for these species were consistent with that for man. Statistical analysis indicated that the expected dicentric yields calculated from the arm number model were significantly different from the observed yields at 200 and 300 rad doses (P less than 0.01). From these results it can be pointed out that there is no correlation between the yield of dicentrics and the effective chromosome arm number, and that the chromosomal radiosensitivity of these primates is essentially the same as that of man, at least in the lymphocyte system.

Chromosome aberrations in the leukocytes of pigs after half-body or whole-body irradiation

Chromosome aberrations were scored in 48-h leukocyte cultures from pigs subjected to whole-body or half-body gamma irradiation with 100, 150, 200, 300, or 400 R. Half-body irradiation resulted in the recovery of approximately half as many aberrations as did equivalent whole-body exposures at levels of 200 R or less. Higher exposures yielded proportionally fewer anomalies in half-body irradiated subjects. These lower levels apparently resulted from the selective disadvantage of irradiated cells in coming to mitosis but did not seem to be related to the amount of chromosome damage sustained by the cell. When adjustments were made for effective dose to the in vivo cells, the dose-response pattern showed good agreement with published values for mixtures of normal and in vitro-irradiated human lymphocytes.

The effect of x-ray induced mitotic delay on chromosome aberration yields in human lymphocytes

The extent to which X-ray induced mitotic delay at 150 and 400 rad influences chromosome aberration yields was examined in human peripheral blood lymphocytes. The dicentric was used as a marker and aberrations yields were obtained for mixed cultures prepared from equal numbers of normal and irradiated cells. The cultures were terminated following incubation times of 36-120 h. Greater mitotic delay of the order of a few hours was observed at the higher dose. However most reduction in the number of lymphocytes arriving at metaphase by 48 h may be ascribed to interphase death or failure to transform. Analysis of the dicentric distributions which were expected to follow Poisson statistics indicated that cells containing dicentrics were delayed relative to irradiated but aberration-free cells. Cells with one dicentric moved more easily through the first cell cycle than cells containing two dicentrics. Following accidental partial body irradiation, selection in culture favouring the unirradiated lymphocytes does not distort the aberration yield sufficiently to warrant incubation times in excess of the standard 48-52 h.

Differential chromosomal radiosensitivity within the first G1-phase of the cell cycle of early-dividing human leukocytes in vitro after stimulation with PHA

Human leukocyte cultures were irradiated with 200 RX-rays before the addition of phytohemagglutinin (PHA) in the Go-stage and at different times up to 25 h within the first G1-phase of the cell cyle after the addition of PHA. The results of the analysis of chromosomal aberrations show that the frequencies of dicentric chromosomes increase significantly when leukocytes leave the Go-stage, reaching a miximum yield of aberrations about halfway through the first G1-phase. After that, toward the end of the G1-phase, the frequencies of dicentric chromosomes decrease again to a level similar to that found in the Go-stage. Different possible explanations for the differential chromosomal radiosensitivity of human leukocytes within the first post-stimulation G1-phase are discussed.

The human leukocyte test system. X. higher sensitivity to X-irradiation in the G0 stage of the cell cycle of early as compared to late replicating cells

Leukocyte cultures were set up with X-irradiated whole blood (200 R). Cells starting with their DNA synthesis between 25 and 35 h after cultureinitiation ("early replicating cells") were pulse-labeled with tritiated thymidine ([3H]TdR). Mitoses were collected with colcemid in adjacent intervals from 36 up to 72 h after culture initiation. At fixation times of 50, 56, 62, and 72 h enough mitoses for a determination of the frequencies of chromosomal aberrations (dicentric and ring chromosomes) were found. After that the preparations were processed for autoradiography. All mitoses analyzed for chromosomal aberrations were re-analyzed for labeling, and the frequencies of chromosomal aberrations in labeled (="late replicating cells") mitoses were compared. At all fixation times, higher frequencies of dicentric chromosomes were found in labeled as compared to unlabeled mitoses, indicating a higher sensitivity of early replicating cells to X-irradiation in the G0 stage of the cell cycle.

Survival of human T and B lymphocytes after X-irradiation

The survival of unstimulated human T and B lymphocytes after X-irradiation in vitro was measured by Trypan Blue dye exclusion over a period of four days. B cell numbers were observed to decline rapidly even after relatively low doses, but T cell numbers fell much more slowly. A comparison of the percentage survival 96 hours after irradiation shows that in this system T cells are between approximately 2 and 5 times more resistant than B cells. Data for interphase death after 48 hours are compared with cytogenetic data for interphase loss of PHA-stimulated human lymphocytes and are shown to be in broad agreement at radiation doses below 400 rad. It is suggested that at higher doses mitotic delay may be increasingly important leading to selection of non-irradiated cells at 48 hours.

The effect of sodium fluoride on DNA synthesis, mitotic indices and chromosomal aberrations in human leukocytes treated with trenimon in vitro

Human leukocyte cultures were set up with Ham's F-10 medium and stimulated with PHA-M. Treatment of the cells in G1 from 15-20 h with 0.5 X 10(-6) M Trenimon resulted in a considerable cell cycle delay, as measured by [3H]-TdR autoradiography and determination of mitotic indices. Under these conditions only few cells incorporated the tracer at the same time as most cells did in untreated cultures. However, this did not lead to a mitotic activity at the same time as obtained in controls. Most of the treated cells started their DNA synthesis and mitotic activities with a delay of around 20 h, as compared with the controls. Continuous treatment of the cells with 10(-3) M NaF had no effect on [3H]TdR labelling or mitotic indices in otherwise untreated cultures, but led to an impressive effect on DNA synthesis in Trenimon-treated cultures, without a considerable effect on the mitotic indices. This finding could be explained as due to a lower alkylation in cellular DNA in the presence of NaF. More cells can start with their DNA synthesis, although they are, like Trenimon-treated cultures, incapable of completing it normally. Analysis of the effect of NaF on chromosome aberrations induced by Trenimon revealed that pre-, simultaneous and post-treatments significantly enhanced the frequency of undamaged mitoses. Continuous fluoride treatment also protected the cells from Trenimon-induced damage, but the effect was not significant, possibly because of heavily damaged mitoses which appeared under these conditions. We interpret our findings as an indication of a real anti-mutagenic activity of NaF.

The human leukocyte test system. IX. DNA synthesis and mitoses in PHA-stimulated 2-day cultures set up with Eagle's mimimal essential medium (MEM)

In human leukocyte cultures set up with Eagle's MEM and stimulated with Difco's PHA M, DNA synthesis and mitotic indices were analyzed by means of 3(H)-thymidine autoradiography and cell counting from 23 up to 52 h after culture initiation. Considerable amounts of DNA synthesis and mitoses were found in this time span. This resembles the results found with Ham's F-10 medium. However, the DNA synthesis pattern and the distribution of mitotic indices a higher yield of asynchrony in Eagle's MEM as compared with Ham's F-10 cultures. Proportions of first, second, and third mitoses at 72 h culture time were determined with different methods.

Evaluation and re-evaluation of genetic radiation hazards in man. II. The arm number hypothesis and the induction of reciprocal translocations in man

The arm number hypothesis proposed by Brewen and collagues in 1973 has been examined in the light of information thus far available from mammalian studies. In experiments with peripheral blood lymphocytes (radiation in vitro), a linear relationship between dicentric yield and the effective chromosome arm number of the species was obtained in the mouse, Chinese hamster, goat, sheep, pig, wallaby and man. However, the data are not consistent with such a relationship in several primate species (marmoset, rhesus monkey, cynomolgus monkey, squirrel monkey and the slow loris), the cat and the dog. In the rabbit, the data are conflicting. In the mouse and Chinese hamster the frequencies of reciprocal translocations recorded in spermatocytes descended from irradiated spermatogonia are in line with the expectation based on the arm number hypothesis, whereas in the golden hamster, rabbit and the rhesus monkey they are not. In man and the marmoset, the limited data are not inconsistent with a 2-fold higher sensitivity of these species relative to the mouse although they do not rule out a difference as high as 4-fold. In the guinea-pig, the situation is unclear. New data on the transmission of reciprocal translocations in mice suggest that the frequency in the F1 progeny may be close to one-quarter of that recorded in the spermatocytes of the irradiated fathers (spermatogonial irradiation) at an exposure level of 150 R, whereas at higher exposures, the reduction factor is about one-eighth, the latter being in line with the earlier finding. All these results taken together suggest that inter-specific extrapolation from the radiosensitivity of somatic cells (to dicentric induction) to that of germ cells (to translocation induction) is fraught with uncertainity at present. Certain aspects that need to be studied in more detail in the context of induced chromosome aberrations are discussed.

A comparison of chromosomal radiosensitivities of somatic cells of mouse and man

A cell culture technique for quantitative analysis of radiation-induced chromosome aberrations in somatic cells has been developed and used for the comparison of chromosomal sensitivity of skin cells of mouse and man to 60Co-gamma-rays. This includes culture of irradiated tissues or cells in culture in arginine and isoleucine-deficient medium and subsequent refeeding with complete medium (CM). With this technique, radiation-induced chromosome aberrations can be analyzed selectively in the cells exposed in G1 phase and recovered at their first post-irradiation mitosis. When tested on the human embryonic cells, the dicentric yield was essentially the same whether they were skin cells irradiated in silu or cultured cells at various in vitro passages irradiated in vitro. In contrast, when studied in the skin cells irradiated in silu, mouse embryos and newborns were insensitive to the induction of dicentrics. In young mice on day II however, the sensitivity was at a level comparable to that in human embryonic cells and it was intermediate on day 4. Such embryonic insensitivity of the mouse cells was rapidly lost during serial transfer in vitro; and, when tested at 4th or later subculture generations, mouse and human cells were equally sensitive to the induction of dicentrics. These results suggest that the chromosomal radiosensitivity is essentially the same for mouse and human cells but can be modified by some biological factors, possibly DNA repair mechanisms, which differ between species as well as among the states of differentiation of particular cell types. Special attention was paid to the parellelism between the age-dependent changes in the chromosomal, mutational and carcinogenic radiosensitivities in the mouse. If this parallelism can be carried over to man, human pre-natal irradiation will not present any reduced genetic hazards.

The relationship between chromosome aberrations and low LET radiation dose to human lymphocytes

In vitro dose-response curves of unstable chromosome aberrations in human lymphocytes have been obtained for 250 kV X-rays and cobalt-60gamma-radiation. The aberration yields have been fitted to the quadratic function Y = alphaD +betaD2, which is consistent with the single-track and two-track model for aberration formation. The values of the coefficients alpha and beta support the hypothesis that the dose-rate effect is limited to the D2 term. The main difference between the coefficients for X- and gamma-radiation is in the alpha values, indicating that X-rays are slightly more efficient, at lower doses, in producing two lesions with a single ionizing track. The lower limits of dose estimate, with 500 cells analysed, are 4 rad for X-rays and 10 rad gamma-radiation. Further evidence is presented confirming that, for cytogenetic dosimetry, in vitro dose-response curves should be prepared by irradiating whole blood maintained at 37 degrees C and prior to PHA stimulation. Curves were plotted showing the variation of the number of cells without aberrations with radiation dose and the shape of these curves were compared with those from human cell survival experiments.