The rate of progression of radiation-transformed mammary epithelial cells is enhanced after low-dose-rate neutron irradiation

Studies in this laboratory have shown enhancement of the mammary tumorigenic effects of neutron irradiation after low-dose-rate neutron exposures. To investigate possible reasons, a mammary cell system was used which allows quantitation of initiated mammary epithelial cells and examination of the progression of these radiation-altered cells toward the neoplastic phenotype. Female BALB/c mice were irradiated with fission-spectrum neutrons at dose rates of 1 rad/min or 1 rad/day. Twenty-four hours or 16 weeks after irradiation, mammary cells were obtained by enzymatic dissociation. Mammary outgrowths were derived by injection of 10(4) cells into gland-free fat pads of 3-week-old female BALB/c mice. The frequency of ductal dysplasias in outgrowths from cells irradiated at high or low dose rates was similar. Persistence of dysplasias differed markedly. Few of the dysplasias in outgrowths derived from cells irradiated at the high dose rate persisted, while a large fraction of the dysplasias in outgrowths derived from cells irradiated at low dose rate persisted. When cells remained in situ for 16 weeks prior to dissociation a higher frequency of persistent altered cells was also observed in outgrowths derived from cells irradiated at low neutron dose rates. These data suggest that low-dose-rate neutron exposures enhance the probability of progression of carcinogen-altered cells rather than increase the numbers of initiated cells.     

The role of electrical signals in murine corneal wound re-epithelialization

Ion flow from intact tissue into epithelial wound sites results in lateral electric currents that may represent a major driver of wound healing cell migration. Use of applied electric fields (EF) to promote wound healing is the basis of Medicare-approved electric stimulation therapy. This study investigated the roles for EFs in wound re-epithelialization, using the Pax6(+/-) mouse model of the human ocular surface abnormality aniridic keratopathy (in which wound healing and corneal epithelial cell migration are disrupted). Both wild-type (WT) and Pax6(+/-) corneal epithelial cells showed increased migration speeds in response to applied EFs in vitro. However, only Pax6(+/+) cells demonstrated consistent directional galvanotaxis towards the cathode, with activation of pSrc signaling, polarized to the leading edges of cells. In vivo, the epithelial wound site normally represents a cathode, but 43% of Pax6(+/-) corneas exhibited reversed endogenous wound-induced currents (the wound was an anode). These corneas healed at the same rate as WT. Surprisingly, epithelial migration did not correlate with direction or magnitude of endogenous currents for WT or mutant corneas. Furthermore, during healing in vivo, no polarization of pSrc was observed. We found little evidence that Src-dependent mechanisms of cell migration, observed in response to applied EFs in vitro, normally exist in vivo. It is concluded that endogenous EFs do not drive long-term directionality of sustained healing migration in this mouse corneal epithelial model. Ion flow from wounds may nevertheless represent an important component of wound signaling initiation.     

In vivo growth stimulation of collagen gel embedded normal human and mouse primary mammary epithelial cells

A new system for studying growth of normal human mammary epithelial cells in an in vivo environment using athymic nude mice is described. Human mammary epithelial cells dissociated from reduction mammoplasty specimens were embedded within collagen gels and subsequently transplanted subcutaneously into nude mice. Histological sections of recovered collagen gels showed epithelial cells arranged as short tubules with some branching. Proliferation of mammary epithelial cells was quantitated in vivo by 3 days' continuous infusion with 5 bromo-2′-deoxy-uridine followed by immunostaining of sections from recovered gels. Ovarian steroids administered to the host animals, resulting in blood serum levels normally found in the human female, had little or no effect on the proliferation of human mammary epithelial cells. Collagen gel embedded mouse mammary epithelial cells, mouse mammary explants, and host mammary glands all responded similarly to ovarian steroids, suggesting that the unresponsiveness of the human mammary epithelial cells under these conditions was not due to dissociation per se. However, an increased dose of 17β-estradiol or a growth factor combination containing epidermal growth factor, cholera toxin, and cortisol significantly stimulated the proliferation of human outgrowths. The growth factor response was dependent on the location of the cells, with the greatest response seen in the part of the gel proximal to the osmotic pump delivering the growth factors and the effect gradually waning in area more distal to the pump. The effect was especially striking since the mitotic figures could be easily identified and the labeling index was as high as 75%. The host mouse mammary gland also responded to growth factors, resulting in ductal hyperplasia. The proliferative and morphogenetic effects of various agents on normal human mammary epithelial cells embedded in collagen gel can be studied in vivo in nude mice. © 1995 Wiley-Liss, Inc.     

Tumor induction in BALB/c female mice after fission neutron or gamma irradiation

This study was designed to examine the dose-response relationships for tumor induction after neutron irradiation in female BALB/c mice, with emphasis on the response in the dose range 0 to 50 rad. Tumors induced after radiation exposure included ovarian tumors, lung adenocarcinomas, and mammary adenocarcinomas. For comparison the dose responses for induction of these tumors after 137Cs gamma irradiation were also examined. As previously described for the female RFM mouse, the data for ovarian tumor induction after neutron and gamma irradiation were consistent with a threshold model. For lung and mammary tumors the dose-response curve after neutron irradiation appeared to "bend over" in the dose range 10 to 20 rad. The factors responsible for this bend-over and their relative contributions to the overall form of the dose-response relationship are not presently known. However, these data strongly indicate that extrapolation from data above 50 rad could result in a significant underestimate of risks. Further, it is clear that current models of neutron carcinogenesis are inadequate, since such a bend-over is not predicted at these low dose levels.     

Sensitivity and specificity of bioassay of estrogenicity on mammary gland and uterus of female mice

Young intact (18 days of age) and adult ovariectomized (OV-X, ovariectomized between 21 to 24 days of age) C3H/Di mice were used to measure the estrogenicity on the basis of the growth response of mammary epithelial structures and weight of the uterus. The percentage area of the mammary fat pad occupied by mammary epithelial structures was progressively increased by 17beta estradiol from dose 0.001 microg.d(-1). The maximum effective dose of estradiol was 0.01 microg.d(-1) and the dose 10 microg.d(-1) of estradiol decreased mammary size to control levels (inverted-U-shaped dose-response curve). Progesterone alone progressively stimulated mammary growth in young intact females from dose 125 microg.d(-1), in adult OV-X animals from dose 1000 microg.d(-1). Both in young intact and adult OV-X animals, uterine weight progressively increased during estradiol treatment. Progesterone alone had no effect on uterine weight in young intact animals; in adult OV-X animals, uterine weight was increased starting from dose 250 microg.d(-1). Progesterone acted synergistically with estradiol to produce higher mammary growth than that in females treated with estradiol alone. The effects of a combination of estradiol plus progesterone in the mammary gland were mimicked by norethindrone acetate and inhibited by cortisol in both young intact and adult OV-X animals. Testosterone inhibited estradiol plus progesterone stimulated growth of mammary gland only in OV-X animals, but stimulated uterine weights in both young intact and adult OV-X animals. Spleen weight and size of mammary lymph nodes were not affected by estradiol, progesterone, norethindrone acetate or testosterone, but were decreased by cortisol. Cortisol also decreased the percent area of the mammary fat pad occupied by mammary epithelial structures, but had no effect on weight of the uterus. These results show that bioassay of estrogenicity in females is not specific. Mammary and uterine growth is stimulated not only by estrogens but also by progesterone and testosterone, respectively.     

A model of cytokine shedding induced by low doses of gamma radiation

A model for sustained shedding of epidermal growth factor (EGF) in response to low doses of gamma radiation was developed based on a time delay in the feedback from mitogen-activated protein kinase (MAPK) activation to metalloprotease activity in an autocrine signaling process. We determined the kinetic parameters of our model using the data available for MAPK activation by gamma irradiation in the 1-2-Gy dose range and then showed that predictions of the model were consistent with experimental results for the kinetics of EGF shedding into the growth medium after exposure of human mammary epithelial cells to 1-5 cGy of gamma radiation in the presence of antibodies that block ligand binding to EGF receptors. The model allowed us to estimate the rate of radiation-induced cytokine release per cell from measurements of EGF concentration in the growth medium and to assess the effectiveness of EGF shedding and subsequent diffusion through the medium as a mechanism for signal transmission between hit cells and bystanders.     

Effect of the extracellular matrix molecules fibronectin and laminin on the adhesion and growth of primary renal cortical epithelial cells

Primary tubular epithelial cells were isolated from renal cortex following enzymatic dissociation with collagenase. These cells were then grown in chemically defined media containing insulin, transferrin, selenium, tri-iodothyronine and either fibronectin or laminin. The tubular epithelial cells were studied ultrastructurally and compared to another epithelial cell type present in the renal cortex, the glomerular epithelial cell. In contrast to the constant morphology of glomerular epithelial cells grown in chemically defined media, tubular epithelial cell morphology depended on whether the cells were placed in fibronectin or laminin and on the age of the donor animal used for culture. Primary tubular cells grown in laminin formed colonies; cells grown from young animals were rounded, whereas cells grown from adult animals were flattened. Primary tubular cells grown in fibronectin were flattened regardless of age, but cells from young animals formed colonies while those from adult animals formed a monolayer. Despite these differences in gross morphology, scanning and transmission electron microscopy revealed similar ultrastructural features in primary tubular cells from young and adult animals grown in fibronectin or laminin. Quantitative adhesion studies demonstrated that secondary subcultured tubular cells adhered equally well to dimeric and multimeric forms of fibronectin, but not to laminin. Quantitative colony growth studies of subcultured secondary tubular cells showed that laminin supports colony formation of trypsinized tubular cells, while previous work has demonstrated that fibronectin supports colony formation of glomerular cells. These results are consistent with the hypothesis that different extracellular matrix molecules are involved in colony formation of different cell types, with fibronectin stimulating growth of glomerular cells and laminin supporting growth of tubular cells.     

Estimation of mutation rates induced by large doses of gamma, proton and neutron irradiation of the X-chromosome of the nematode Panagrellus redivivus

The radiation-resistant free-living nematode Panagrellus redivivus was used to study mutation rates in oocytes, following gamma, proton and neutron irradiation in the dose range 45-225 grays. gamma-Radiation produced approximately 0.001 lethal X-chromosomes per gray over the range tested. Proton or neutron irradiation produced approximately 0.003 lethal X-chromosomes per gray at lower doses, with the mutation rate dropping to 0.001 lethal X-chromosome per gray at the higher doses. These results suggest a dose-dependent mutation-repair system. Cell lethality was also examined. gamma-Radiation produced the greatest amount of cell lethality at all doses, while neutron irradiation had no cell lethal effect at any of the doses examined.     

Cripto: roles in mammary cell growth, survival, differentiation and transformation

Cripto-1 (Cr-1) protein, encoded by the teratocarcinoma-derived growth factor gene (TDGF-1), is highly correlated with transformation in breast cancer. Eighty-two percent of breast carcinomas express Cr-1 whereas it is undetected in normal human breast tissue. We confirmed and extended findings that Cr-1 protein is expressed during the pregnancy and lactating stages of normal murine mammary glands but is barely detectable in glands from virgin animals and is undetectable in involuted glands. Cr-1 was found to be expressed in CID 9 cells, a line of mammary epithelial cells derived from 14.5 day pregnant mice and we have used these cells to investigate the roles of this gene. Exogenous mouse Cr-1 expression from a retroviral vector caused CID 9 cells to grow at an increased rate and to increased cell densities compared to parental and control cells. CID 9 cells overexpressing Cr-1 did not differentiate efficiently. Infection of CID 9 cells with a Cr-1 antisense vector caused these cells to change in morphology, to grow slowly, to undergo apoptosis at a higher rate and to achieve lower saturation densities but the cells were still capable of differentiating. We concluded that Cr-1 is an autocrine growth factor for normal breast cells, that when over-expressed stimulates excessive cell proliferation at the expense of differentiation. In transplantation studies, Cr-1 over-expression stimulated the growth and survival of mammary cells, but did not stimulate tumorigenesis in vivo.     

A comparison of gamma and neutron irradiation on Raji cells: effects on DNA damage, repair, cell cycle distribution and lethality.

The Comet assay (microgel electrophoresis) was used to study DNA damage in Raji cells, a B-lymphoblastoid cell line, after treatment with different doses of neutrons (0.5 to 16 Gy) or gamma rays (1.4 to 44.8 Gy). A better growth recovery was observed in cells after gamma-ray treatments compared with neutron treatments. The relative biological effectiveness (RBE) of neutron in cell killing was determined to be 2.5. Initially, the number of damaged cells per unit dose was approximately the same after neutron and gamma-ray irradiation. One hour after treatment, however, the number of normal cells per unit dose was much lower for neutrons than for gamma rays, suggesting a more efficient initial repair for gamma rays. Twenty-four hours after treatment, the numbers of damaged cells per unit dose of neutrons or gamma rays were again at comparable level. Cell cycle kinetic studies showed a strong G2/M arrest at equivalent unit dose (neutrons up to 8 Gy; gamma rays up to 5.6 Gy), suggesting a period in cell cycle for DNA repair. However, only cells treated with low doses (up to 2 Gy) seemed to be capable of returning into normal cell cycle within 4 days. For the highest dose of neutrons, decline in the number of normal cells seen at already 3 days after treatment was deeper compared with equivalent unit doses of gamma rays. Our present results support different mechanisms of action by these two irradiations and suggest the generation of locally multiply damaged sites (LMDS) for high linear energy transfer (LET) radiation which are known to be repaired at lower efficiency.     

Nonrandom distribution of mouse spermatogonial stem cells surviving fission neutron irradiation

Colony formation by surviving spermatogonial stem cells was investigated by mapping pieces of whole mounted tubuli at intervals of 6 and 10 days after doses of 0.75 and 1.50 Gy of fission neutron irradiation. Colony sizes, expressed in numbers of spermatogonia per colony, varied greatly. However, the mean colony size found in different animals was relatively constant. The mitotic indices in large and small colonies and in colonies in different epithelial stages did not differ significantly. This finding suggests that size differences in these spermatogenic colonies are not caused by differences in growth rate. Apparently, surviving stem cells start to form colonies at variable times after irradiation. The number of colonies per unit area varied with the epithelial stages. Many more colonies were found in areas that during irradiation were in stages IX-III (IX-IIIirr) than in those that were in stages IV-VII (IV-VIIirr). After a dose of 1.50 Gy, 90% of all colonies were found in areas IX-IIIirr. It is concluded that the previously found difference in repopulation after irradiation between areas VIII-IIIirr and III-VIIIirr can be explained not by differences in colony sizes and/or growth rates of the colonies in these areas but by a difference in the number of surviving stem cells in both areas. In area XII-IIIirr three times more colonies were found after a dose of 0.75 Gy than after a dose of 1.50 Gy. In area IV-VIIirr the numbers of colonies differed by a factor of six after both doses. This finding indicates that spermatogonial stem cells are more sensitive to irradiation in epithelial stages IV-VII than in stages XII-III. In control material, spermatogonia with a nuclear area of 70-110 micron2 are rare. However, especially 6 days after irradiation, single cells of these dimensions are rather common. These cells were found to lie at random over the tubular basement membrane with no preference for areas with colonies. It is concluded that the great majority of these cells were not or do not derive from surviving stem cells. These enlarged cells most likely represent lethally injured cells that will die or become giant cells (nuclear area greater than 110 micron2).     

Transformed growth phenotype of mouse mammary epithelium in primary culture induced by specific fetal mesenchymes.

When mesenchyme from fetal mammary or salivary gland is implanted into adult mouse mammary gland, adjacent epithelium responds with intense hyperplasia. The hyperplastic cells are more vulnerable than are non-stimulated cells to transformation in vivo by a chemical carcinogen or by mammary tumor virus. This system provides a potentially useful model for determining how stroma contributes to mammary tumorigenesis. We have developed co-culture systems and used them to investigate in more detail the nature of the signal produced by the mesenchyme cells. Monolayers of mesenchyme cells were prepared on tissue-culture wells. The mesenchyme cells were trapped on the surface by a thin overlay of agarose. Primary mammary epithelial cells were cultured atop this barrier layer, either as organoids in collagen gels for assessment of anchorage-dependent growth, or as single-cell dispersions in soft agarose for assessment of anchorage-independent growth. Our procedures for assay of anchorage-independent growth allow us for the first time to detect and measure this transformation-defining characteristic in non-immortalized mammary epithelial cells in primary culture. Fetal mammary fat pad precursor tissue and fetal salivary mesenchyme both stimulated anchorage-dependent growth of mammary epithelium, with cell number increasing as much as fifteenfold during a 6-day culture period. These same fetal tissues also stimulated anchorage-independent growth of the mammary epithelial cells, with colony-forming efficiencies of up to 40% in co-cultures with salivary mesenchyme. No colonies formed in the absence of mesenchyme. Cells of colonies contained keratin, which indicates that the colonies grew from epithelial cells and not from a contaminant of another cell type. When co-cultured epithelial cells were subsequently re-cultured in the absence of mesenchyme, they lost their ability to grow independent of anchorage. No colonies grew in co-cultures with fetal cells from heart, kidney, or lung, which is consistent with the lack of stimulation by these tissues in the mammary gland in vivo. A tumor promoter, 12-O-tetradecanoylphorbol acetate (TPA), also caused anchorage-independent growth of the dispersed mammary epithelial cells. Culture medium conditioned by primary or early-passage salivary mesenchyme cells was capable of stimulating growth under both anchorage-dependent and anchorage-independent conditions, confirming that these effects are mediated by a paracrine factor. The results indicate that stimulatory fetal mesenchymes produce soluble molecules that act analogously to transforming growth factors.     

Lentiviral transduction of mammary stem cells for analysis of gene function during development and cancer

The mouse mammary gland is the only epithelial organ capable of complete regeneration upon orthotopic transplantation, making it ideally suited for in vivo gene function studies through viral-mediated gene delivery. A hurdle that has challenged the widespread adoption of this technique has been the inability to transduce mammary stem cells effectively. We have overcome this limitation by infecting total primary mammary epithelial cells in suspension with high-titer lentiviruses. Transduced cells gave rise to all major cell types of the mammary gland and were capable of clonal outgrowth and functional differentiation in serial transplants. To demonstrate that this method is a valuable alternative to developing transgenic animals, we used lentiviral-mediated Wnt-1 overexpression to replicate MMTV-Wnt-1 mammary phenotypes and used a dominant-negative Xenopus Suppressor of Hairless to reveal a requirement for Notch signaling during ductal morphogenesis. Importantly, this method is also applicable to transduction of cells from other tissues.     

Myeloid leukemia in male RFM mice following irradiation with fission spectrum neutrons or gamma rays

The induction of myeloid leukemia following fission neutron irradiation was examined over the 0-80 rad dose range. Over this dose range the dose response could be described by the linear regression equation: y = 0.94 + 0.18X. A comparison of these data with data obtained following gamma irradiation from this study and a previous study indicated that the relative biological effectiveness for myeloid leukemia induction was 2.8. These results appear to be compatible with those reported by other investigators.     

Reduction in sperm levels after testicular irradiation of the mouse: a comparison with man

The potential and limitations of applying extrapolation factors (EFs) to the results of animal studies to predict effects of toxic agents on human male fertility were evaluated using radiation data. The EF is the ratio of the dose to produce a given effect in the mouse to that necessary to produce the same effect in man. Sperm counts in mouse testes were compared to those in the ejaculates of human males (D. K. Clifton and W. J. Bremner, J. Androl. 4, 387-392 (1981)) at several different times after irradiation. EFs of between 2.6 and 7 were obtained at the time when minimum counts occur. However, it must be noted that the sperm being measured arose from different cell types: differentiating spermatogonia in the mouse vs stem spermatogonia in man. Sperm counts performed at times at which the sperm develop from irradiated stem cells in both species yielded EFs between 11 and 44. However, if sufficient time was allowed for maximum recovery in both species, the EF was less than 1.7. These results indicate that man appears to be much more sensitive than the mouse to the testicular effects of irradiation at 2 to 9 months postexposure, but both species are comparable in their sensitivity to irreversible damage. The use of EFs may be appropriate, but since the EF is very dependent on the time at which the comparison was made, different values must be used for prompt and permanent testicular injury.     

Effects of 50 Hz electromagnetic fields on voltage-gated Ca2+ channels and their role in modulation of neuroendocrine cell proliferation and death

Possible correlation between the effects of electromagnetic fields (EFs) on voltage-gated Ca(2+) channels, cell proliferation and apoptosis was investigated in neural and neuroendocrine cells. Exposure to 50 Hz EFs significantly enhanced proliferation in human neuroblastoma IMR32 (+40%) and rat pituitary GH3 cells (+38%). In IMR32 cells EF stimulation also inhibited puromycin- and H(2)O(2)-induced apoptosis (-22 and -33%, respectively). EF effects on proliferation and apoptosis were counteracted by Ca(2+) channel blockade. In whole-cell patch-clamp experiments 24-72 h exposure to EFs increased macroscopic Ba(2+)-current density in both GH3 (+67%) and IMR32 cells (+40%). Single-channel recordings showed that gating of L and N channels was instead unaffected, thus suggesting that the observed enhancement of current density was due to increased number of voltage-gated Ca(2+) channels. Western blot analysis of plasma membrane-enriched microsomal fractions of GH3 and IMR32 cells confirmed enhanced expression of Ca(2+) channel subunit alpha(1) following exposure to EFs. These data provide the first direct evidence that EFs enhance the expression of voltage-gated Ca(2+) channels on plasma membrane of the exposed cells. The consequent increase in Ca(2+) influx is likely responsible for the EF-induced modulation of neuronal cell proliferation and apoptosis.     

Embryonic Stem Cells Are Redirected to Non-Tumorigenic Epithelial Cell Fate by Interaction with the Mammary Microenvironment

Experiments were conducted to redirect mouse Embryonic Stem (ES) cells from a tumorigenic phenotype to a normal mammary epithelial phenotype in vivo. Mixing LacZ-labeled ES cells with normal mouse mammary epithelial cells at ratios of 1∶5 and 1∶50 in phosphate buffered saline and immediately inoculating them into epithelium-divested mammary fat pads of immune-compromised mice accomplished this. Our results indicate that tumorigenesis occurs only when normal mammary ductal growth is not achieved in the inoculated fat pads. When normal mammary gland growth occurs, we find ES cells (LacZ+) progeny interspersed with normal mammary cell progeny in the mammary epithelial structures. We demonstrate that these progeny, marked by LacZ expression, differentiate into multiple epithelial subtypes including steroid receptor positive luminal cells and myoepithelial cells indicating that the ES cells are capable of epithelial multipotency in this context but do not form teratomas. In addition, in secondary transplants, ES cell progeny proliferate, contribute apparently normal mammary progeny, maintain their multipotency and do not produce teratomas.     

Inverse correlation between growth and degrading enzyme activity of seedlings after gamma and neutron irradiation of pea seeds

The effect of gamma-, 14 MeV neutron- and fission neutron irradiation was investigated on the growth rate and degrading enzyme activities of pea seedlings. Both dormant pea seeds and 4-day-old growing seedlings were used for the experiments. Depending on the gamma dose between 15 and 300 Gy the height of pea seedlings was found shorter, and parallel with this the endogenous RNase and peroxidase activities were higher than in the unirradiated controls. Seedlings proved to be more sensitive by about one order of magnitude than seeds. Irradiation of seeds between 5 and 10 Gy slightly enhanced the growth rate of seedlings (10 per cent) and parallel with this, the RNase activity measured was lower than that in the controls. On irradiation of seedlings with 14 MeV neutrons the growth inhibition and RNase activity enhancement was only 1.3 times more effective than in the case of irradiation of seeds. The following RBE values were obtained after irradiation of seeds, related to the biological effect of gamma rays: in growth inhibition, 6 for 14 MeV neutrons and 12 for fission neutrons, and the enhancement of two enzyme activities was 15-30 for 14 MeV neutrons and 45-58 for fission neutrons. In the case of seedling irradiation with 14 MeV neutrons the RBE was 1.0 for growth inhibition and between 3 and 6 for enhancement of enzyme activity. The isoenzyme pattern of RNase also changed: two isoenzymes became predominant after the gamma irradiation of seeds, characterized by molecular weights of 21,000 and 30,000, respectively. As a result of enhanced RNase activity, the degradation of longer polysomes to monomeric ribosomes occurred. Thus after ionizing irradiation of pea seeds and seedlings an inverse correlation was found between the growth rate of pea seedlings and the activities of degrading enzymes.     

An analysis of the cytogenetic effects of gamma and neutron irradiation in a human lymphocyte culture at the G0 and G1 stages of the mitotic cycle (a structural-functional approach)

A study was made of the dose dependence of the chromosome aberration frequency in human lymphocytes exposed to 60Co-gamma radiation and neutrons (mean energy of 0.85 MeV) at the G0 stage and in different periods of the G1 and G1/S stages of the cycle. With gamma irradiation the dose dependence for cells at the G1 and G1/S stages was at a higher level than that for cells at the G0 stage, whereas the opposite picture was observed for cells exposed to neutron radiation. The difference was also noted in the time-response curves where gamma radiation increased and neutrons, on the contrary, decreased the aberration yield in the cells that passed from G0 to G1 stage. The experimental data obtained are attributed to activation of repair system at the G1 stage which is mainly conditioned by chromatin decondensation; the activating, that is, the functional factor influences the aberration induction with gamma irradiation, while the decondensation, that is, the structural factor, with neutron irradiation.