Effect of fungicide Euparen Multi (Tolylfluanid) on the induction of chromosomal aberations in cultivated bovine lymphocytes

The effect of the fungicide Euparen Multi (containing 50% tolylfluanid) was investigated on the induction of chromosomal aberrations (CA) in cultured bovine peripheral lymphocytes. Cultures from two healthy donors were treated with tolylfluanid-based fungicide at concentrations ranging from 1.7 to 17.5 μg/ml for the last 24 and 48 hours of cultivation. Conventional cytogenetic method (CA assay) with Giemsa staining as well as fluorescence in situ hybridization (FISH) with whole bovine chromosomes 1 and 5 painting probes were used in the experiment. In the CA assay, no clastogenic effect of the fungicide was found after Euparen Multi treatment for 24 hours. On the contrary, significant elevation in polyploidy induction was observed with dose-dependence in one of the donors. Using prolonged time of exposure to the fungicide (the last 48 h of the cultivation), a slight clastogenic effect was detected at the doses of 8.75 and 17.5 μg/ml (P < 0.05, P < 0.01, respectively) in donor 1 and at the dose of 8.75 μg/ml (P < 0.05) in donor 2. The highest doses tested caused reduction of the mitotic indices (MI) (P < 0.05, P < 0.01) in both donors as well as both treatment times. The evaluation of stable structural aberrations in lymphocytes by two-colour FISH (48 h exposure) using bovine chromosome painting probes revealed the presence of nonreciprocal translocations at two examined concentrations (3.5 μg/ml and 8.75 μg/ml).     

DNA and immunoglobulin synthesis by rabbit peripheral blood lymphocytes in vitro: complete and incomplete stimulation

Activation of resting (G0) rabbit peripheral blood lymphocytes (PBLs) into DNA synthesis and IgG synthesis was studied using sheep anti-rabbit IgG (SARIgG), protein A, pokeweed mitogen (PWM), and lipopolysaccharide (LPS). DNA synthesis was assayed by [125I]iododeoxyuridine incorporation. IgG synthesis was measured by determination of Ig in culture supernatants by an ELISA assay. Rabbit PBLs cultured with SARIgG or protein A for 48 hr and then without these reagents for 72 hr showed both DNA synthesis and Ig synthesis, whereas PWM and LPS had very little, if any, effect. PBLs stimulated with SARIgG for 6 hr and then without SARIgG for subsequent 114 hr did not become activated into DNA synthesis or IgG synthesis. However, PBLs prestimulated with SARIgG for 6 hr and then with PWM for 114 hr showed prominent DNA and IgG synthesis. LPS also maintained activation of PBLs after prestimulation of these cells with SARIgG, but the effect was much smaller than that of PWM. No evidence was found for production of factors by SARIgG-stimulated PBLs that could, by themselves, either stimulate resting cells or maintain activation of SARIgG-prestimulated cells. These results suggest that anti-IgG and protein A are complete activating mitogens for resting rabbit B cells to proliferate and differentiate into IgG-producing cells, whereas PWM and LPS are not able to activate G0 cells directly, but have a sustaining effect after activation of resting B cells with anti-IgG, either directly or via production of factors by accessory cells.     

Respiratory Syncytial Virus Fusion Protein Mediates Inhibition of Mitogen-Induced T-Cell Proliferation by Contact

Human respiratory syncytial virus (HRSV) and bovine respiratory syncytial virus (BRSV) are major pathogens in infants and calves, respectively. Experimental BRSV infection of calves and lambs is associated with lymphopenia and a reduction in responsiveness of peripheral blood lymphocytes (PBLs) to mitogens ex vivo. In this report, we show that in vitro mitogen-induced proliferation of PBLs is inhibited after contact with RSV-infected and UV-inactivated cells or with cells expressing RSV envelope proteins on the cell surface. The protein responsible was identified as the RSV fusion protein (F), as cells infected with a recombinant RSV expressing F as the single envelope protein or cells transfected with a plasmid encoding F were able to induce this effect. Thus, direct contact with RSV F is necessary and sufficient to inhibit proliferation of PBLs. Interestingly, F derived from HRSV was more efficient in inhibiting human PBL proliferation, while F from BRSV was more efficient in inhibiting bovine PBLs. Since various T-cell activation markers were upregulated after presenter cell contact, T lymphocytes are viable and may still be activated by mitogen. However, a significant fraction of PBLs were delayed or defective in G0/G1 to S-phase transit.     

Clastogenic action of ellipticine over the cell cycle of human lymphocytes and influence of posttreatments with caffeine and ara-C at G2

The clastogenic potential of the intercalating compound ellipticine, an antitumor alkaloid, has been demonstrated in mammalian cells. To characterize the mechanism of action of this drug over the cell cycle, human lymphocyte cultures from 2 healthy donors were treated with 3 micrograms/ml ellipticine in 30-min pulses during different phases of the cell cycle and analyzed for chromosomal aberrations and sister-chromatid exchanges. The G2 phase was most sensitive in terms of induction of aberrations, followed by S and G1. Chromatid-type aberrations were the most common type of chromosomal damage. Induction of SCEs was significantly high only after treatment at G1, when the frequencies of SCEs doubled. The post-treatment effect of lymphocytes with inhibitors of DNA repair, 10(-3) M caffeine and 5 x 10(-6) M 1-beta-D-arabinofuranosylcytosine, was also tested by adding 3 micrograms/ml ellipticine at G2 in 30-min pulses and immediately followed by caffeine and/or ara-C during the last 3 h before harvesting. Three experiments performed on blood from 3 donors showed a moderate potentiation effect on the frequency of chromatid-type aberrations (about 2-3 times) by both inhibitors. Likewise, a 3-fold increase was observed in the frequencies of chromosomal aberrations when caffeine and ara-C were combined. The present data demonstrate that posttreatment with caffeine and ara-C at G2 can modify the response of human lymphocytes treated with ellipticine by increasing the clastogenic action of this compound or by changing the cell-cycle progression.     

Possible biomarkers for ionizing radiation exposure in human peripheral blood lymphocytes

Biomarkers to indicate past exposure to radiation have not been entirely satisfactory. Using cDNA microarray hybridization to find new potential biomarkers, we identified highly expressed genes in human peripheral blood lymphocytes (PBLs) after irradiation 1 Gy ex vivo. The present set of radiation markers in PBLs was identified 12 h after radiation. A total of 44 genes were identified. However, when RT-PCR was performed with mRNA from the PBLs of five individuals, only four genes, including TRAIL receptor 2, DRAL (now known as FHL2), cyclin G, and cyclin protein gene, showed greater than 50% agreement between gene induction as detected by microarray analysis and by RT-PCR. When more than 32 donors were tested for the above four genes, greater than 85% agreement was obtained between gene induction measured by microarray analysis and by RT-PCR. There was a linear dose-response relationship between 0.5 and 4 Gy 12 h after irradiation; however, there was less linearity at later times. These results suggested that the relative expression levels of genes such as TRAIL receptor 2, FHL2, cyclin G, and cyclin protein gene in PBLs may provide estimates of radiation exposures.     

Protective effects of topical alpha-tocopherol acetate on UVB irradiation in guinea pigs: importance of free radicals

Reactive oxygen species can be generated by daily exposure of the skin to ultraviolet light and may cause some subchronic and chronic skin disorders. The aim of this study was to investigate a possible preventive role of alpha-tocopherol acetate (ATA) on ultraviolet B (UVB) induced peroxidation by assessing lipid peroxide (LPO) levels and activity of reactive oxygen scavenging enzymes including glutathione peroxidase and superoxide dismutase (SOD) in guinea pigs. ATA was topically applied to the skin for three weeks before a single dose of 0.9 J/cm2 UVB irradiation on the skin and lipid peroxide levels and antioxidants in plasma, skin and liver and erythrocytes were determined after decapitation. Topical application of ATA prevented the UVB irradiation-induced reduction of scavenging enzyme activities in skin and erythrocytes. In conclusion, we suggest that topical applications of ATA before UVB irradiation is effective in protecting the skin from unwanted effects of UVB irradiation.     

Genetic variation of the cutaneous HPA axis: An analysis of UVB-induced differential responses

Mammalian skin incorporates a local equivalent of the hypothalamic–pituitary–adrenal (HPA) axis that is critical in coordinating homeostatic responses against external noxious stimuli. Ultraviolet radiation B (UVB) is a skin-specific stressor that can activate this cutaneous HPA axis. Since C57BL/6 (B6) and DBA/2J (D2) strains of mice have different predispositions to sensorineural pathway activation, we quantified expression of HPA axis components at the gene and protein levels in skin incubated ex vivo after UVB or sham irradiation. Urocortin mRNA was up-regulated after all doses of UVB with a maximum level at 50 mJ/cm² after 12 h for D2 and at 200 mJ/cm² after 24 h for B6. Proopiomelanocortin mRNA was enhanced after 6 h with the peak after 12 h and at 200 mJ/cm² for both genotypes of mice. ACTH levels in tissue and media increased after 24 h in B6 but not in D2. UVB stimulated β-endorphin expression was higher in D2 than in B6. Melanocortin receptor 2 mRNA was stimulated by UVB in a dose-dependent manner, with a peak at 200 mJ/cm² after 12 h for both strains. The expression of Cyp11a1 mRNA — a key mitochondrial P450 enzyme in steroidogenesis, was stimulated at all doses of UVB irradiation, with the most pronounced effect after 12–24 h. UVB radiation caused, independently of genotype, a dose-dependent increase in corticosterone production in the skin, mainly after 24 h of histoculture. Thus, basal and UVB stimulated expression of the cutaneous HPA axis differs as a function of genotype: D2 responds to UVB earlier and with higher amplitude than B6, while B6 shows prolonged (up to 48 h) stress response to a noxious stimulus such as UVB.     

NUTRIENT LIMITATION AND HIGH IRRADIANCE ACCLIMATION REDUCE PAR AND UV‐INDUCED VIABILITY LOSS IN THE ANTARCTIC DIATOM CHAETOCEROS BREVIS (BACILLARIOPHYCEAE)1

The effects of high PAR (400–700 nm), UVA (315–400 nm), and UVB (280–315 nm) radiation on viability and photosynthesis were investigated for Chaetoceros brevis Schütt. This Antarctic marine diatom was cultivated under low, medium, and high irradiance and nitrate, phosphate, silicate, and iron limitation before exposure to a simulated surface irradiance (SSI) treatment, with and without UVB radiation. Light‐harvesting and protective pigment composition and PSII parameters were determined before SSI exposure, whereas viability was measured by flow cytometry in combination with a viability stain after the treatment. Recovery of PSII efficiency was measured after 20 h in dim light in a separate experiment. In addition, low and high irradiance acclimated cells were exposed outdoors for 4 h to assess the effects of natural PAR, UVA, and UVB on viability. Low irradiance acclimated cells were particularly sensitive to photo induced viability loss, whereas no viability loss was found after acclimation to high irradiance. Furthermore, nutrient limitation reduced sensitivity to photo induced viability loss, relative to nutrient replete conditions. No additional viability loss was found after UVB exposure. Sunlight exposed cells showed no additional UVB effect on viability, whereas UVA and PAR significantly reduced the viability of low irradiance acclimated cells. Recovery of PSII function was nearly complete in cultures that survived the light treatments. Increased resistance to high irradiance coincided with an increased ratio between protective‐ and light‐harvesting pigments before the SSI treatment, demonstrating the importance of nonphotochemical quenching by diatoxanthin for survival of near‐surface irradiance. We conclude that a sudden transfer to high irradiance can be fatal for low irradiance acclimated C. brevis.     

The immunobiological development of human bone marrow mesenchymal stem cells in the course of neuronal differentiation

The central nervous system (CNS) has been referred to as the "immunological privileged site". However, it is now clear that the privileged status of the CNS is a result of a balance between immune privilege and effective response. In vitro, human bone marrow mesenchymal stem cells (MSCs) have the ability to differentiate into neurons. Based on this biological attribute we gain the possibility by means of using MSCs as the donors to develop a future cell therapy in clinical application. But using MSCs as donor cells inevitably raises the question as to whether these donor cells would be immunogenic, and if so, would they be rejected after transplantation. To investigate this, human MSCs were cultured in vitro and induced to differentiate along neuronal lineage. The expression of human leukocyte antigen (HLA) class I and class II molecules and the co-stimulatory protein CD80 were increased on the surface of MSCs in the course of neuronal differentiation. But neither of the co-stimulatory proteins, CD40 or CD86, was expressed. After IFN-gamma exposure, the expression of the HLA molecules was further enhanced, but the co-stimulatory proteins were unaffected. MSCs that had been differentiated along neuronal lineage were not capable of inducing the proliferation of peripheral blood lymphocytes (PBLs). Even after IFN-gamma exposure, PBLs remained unresponsive. Furthermore, MSCs differentiated along neuronal lineage suppressed the proliferation of PBLs induced by allogeneic PBLs and mitogens. The mechanisms involved in the immunosuppression may be related to the effect of soluble factors and cell-cell interactions of neuronal differentiated MSCs and PBLs. From the above data we suggested that the low immunogenicity and immunomodulatory function of MSCs in the course of neuronal differentiation in vitro, which will be helpful to further investigation in order to establish the new way for future medical application.     

Effects of UVB Radiation on Stomatal Density and Opening in Rice (Oryza sativa L.)

Ultraviolet-B (UVB, wavelength 280-320 nm) radiation has beendemonstrated to affect growth and development of many plants.This study was conducted to determine the effect of UVB radiationon stomatal density and opening of Oryza sativa and to testif the stomatal response to UVB was associated with differentsensitivity of growth to UVB in four cultivars. Ten-day-oldseedlings of IR45 and IR74 (UVB sensitive), and IR64 and IR30(UVB less sensitive), were subjected to UVB radiation in a glasshousefor 6 h d^-1 for 4 weeks. The unweighted UVB radiation was 1·94W m^-2 for UVB treatment and 0·15 W m^-2 for control. Leafarea and plant dry mass were determined every 2 weeks whilestomatal density and opening were recorded weekly. Results showedthat a 2-week UVB treatment had no effect on the leaf area orplant dry mass of any test cultivar, but significantly reducedstomatal density and opening in IR45 and IR74. Under 4-weekUVB exposure, leaf area and plant dry mass of IR45 and IR74were significantly reduced. Stomatal density decreased in allcultivars, except in IR64. Greater reduction of stomata on theadaxial surface than on the abaxial surface under 3 and 4 weeksof UVB exposure suggests a direct effect of UVB radiation onstomata. IR45 and IR74 showed significant reductions in stomatalopening after 2 weeks of exposure to UVB, while stomatal openingin IR30 and IR64 decreased significantly after only 4 weeksof UVB treatment. Difference in plant dry mass between UVB treatedand control plants was significantly correlated with the reductionsin stomatal opening and density on adaxial surface under UVBtreatment. Thus, reduction in dry mass of rice plants underUVB in the glasshouse could be attributed to decrease in stomataldensity and opening.Copyright 1995, 1999 Academic Press Oryza sativa, UVB radiation, stomatal density, stomatal opening     

Analysis of spontaneous and bleomycin-induced chromosome damage in peripheral lymphocytes of long-haul aircrew members from Argentina

Spontaneous and bleomycin (BLM)-induced chromosomal aberrations in G0 and G2 stages of the cell cycle have been analyzed in peripheral lymphocytes of 21 long-haul aircrew members from Argentina in order to assess BLM-induced clastogenesis as a first approach to determine the DNA repair capacity and thereby the susceptibility to environmental cancers in aircrew. The possibility that occupational exposure of flight personnel to cosmic radiation can induce an adaptive response in their peripheral lymphocytes that can be detected by a subsequent in vitro treatment with BLM was also investigated. For comparison, aberrations were also scored in the lymphocytes of 15 healthy volunteers matched by age, health, sex, drinking and smoking habits to the flight personnel group. Aircrew exhibited a higher frequency of spontaneous dicentrics and ring chromosomes than the control population (p<0.05). BLM sensitivity test showed that aircrew and controls are equally sensitive to BLM G2 clastogenic effects, since both groups exhibited a similar frequency of chromatid breaks per cell (p>0.05). However, the aircrew sampled population was almost two times more sensitive to BLM G0 clastogenic effects than controls (p<0.05). Therefore, our data suggest that chronic exposure of aircrew to cosmic radiation increases the in vitro chromosomal sensitivity of their peripheral lymphocytes to BLM (at least in the G0 stage of the cell cycle), and that occupational exposure of flight personnel to cosmic radiation does not induce an adaptive response to this radiomimetic compound. Our results justify further studies aimed at determine if those aircrew members hypersensitive to BLM are more prone to develop environmental cancer than BLM-insensitive individuals.     

Activation of T-cell Protein-tyrosine Phosphatase Suppresses Keratinocyte Survival and Proliferation following UVB Irradiation

Chronic exposure to UV radiation can contribute to the development of skin cancer by promoting protein-tyrosine kinase (PTK) signaling. Studies show that exposure to UV radiation increases the ligand-independent activation of PTKs and induces protein-tyrosine phosphatase (PTP) inactivation. In the present work, we report that T-cell PTP (TC-PTP) activity is stimulated during the initial response to UVB irradiation, which leads to suppression of keratinocyte cell survival and proliferation via the down-regulation of STAT3 signaling. Our results show that TC-PTP-deficient keratinocyte cell lines expressed a significantly increased level of phosphorylated STAT3 after exposure to low dose UVB. This increase corresponded with increased cell proliferation in TC-PTP-deficient keratinocytes following UVB irradiation. Loss of TC-PTP also reduced UVB-induced apoptosis. Corroborating with these results, overexpression of TC-PTP in keratinocyte cell lines yielded a decrease in phosphorylated STAT3 levels, which corresponded with a significant decrease in cell proliferation in response to low dose UVB. We demonstrate that TC-PTP activity was increased upon UVB exposure, and overexpression of TC-PTP in keratinocyte cell lines further increased its activity in the presence of UVB. Treatment of TC-PTP-deficient keratinocytes with the STAT3 inhibitor STA21 significantly reduced cell viability following UVB exposure in comparison with untreated TC-PTP-deficient keratinocytes, confirming that the effect of TC-PTP on cell viability is mediated by STAT3 dephosphorylation. Combined, our results indicate that UVB-mediated activation of TC-PTP plays an important role in the STAT3-dependent regulation of keratinocyte cell proliferation and survival. Furthermore, these results suggest that TC-PTP may be a novel potential target for the prevention of UVB-induced skin cancer.     

氢化可的松诱导特发性血小板减少性紫癜淋巴细胞凋亡和增殖抑制

探讨氢化可的松对儿童急性特发性血小板减少性紫癜（AITP）外周血淋巴细胞增殖和凋亡的影响,对12例确诊的儿童AITP外周血淋巴细胞进行体外培养,用流式细胞术观察糖皮质激素处理后外周血淋巴细胞凋亡数量的变化,并用Wst-1（四氮唑盐）测定代表增殖的活细胞数。结果显示,激素处理组淋巴细胞凋亡率明显高于对照组,而活细胞数则明显低于对照组;激素处理后的外周血淋巴细胞凋亡率明显高于对照组,而活细胞数则明显低于对照组;激素处理后的外周血淋巴细胞凋亡率也明显高于处理前和正常对照组,均具有显性差异。结果表明,氢化可的松具有明显的诱导淋巴细胞凋亡和增殖抑制作用,说明糖皮质激素可通过诱导AITP外血淋巴细胞凋亡和增殖抑制发探治疗作用。     

Elevated sister chromatid exchange frequencies in dividing human peripheral blood lymphocytes exposed to 50 Hz magnetic fields

The in vitro cytomolecular technique, sister chromatid exchange (SCE), was applied to test the clastogenic potentiality of extremely low frequency (ELF) electromagnetic fields (EMFs) on human peripheral blood lymphocytes (HPBLs). SCE frequencies were scored in dividing peripheral blood lymphocytes (PBLs) from six healthy male blood donors in two rounds of experiments, R1 and R2, to determine reproducibility. Lymphocyte cultures in the eight experiments conducted in each round were exposed to 50 Hz sinusoidal (continuous or pulsed) or square (continuous or pulsed) MFs at field strengths of 1 microT or 1 mT for 72 h. A significant increase in the number of SCEs/cell in the grouped experimental conditions compared to the controls was observed in both rounds. The highest SCE frequency in R1 was 10.03 for a square continuous field, and 10.39 for a square continuous field was the second highest frequency in R2. DNA crosslinking at the replication fork is proposed as a model which could explain the mechanistic link between ELF EMF exposure and increased SCE frequency.     

Different individual immune responses elicited by in vitro immunization

We have previously demonstrated that the addition of muramyl dipeptide (MDP), interleukin-2 (IL-2) and IL-4 effectively raises antibody production from L-Leucyl-L-leucine methyl ester (LLME)-treated human peripheral blood lymphocytes (PBLs) against specific soluble antigen when immunized in vitro. However, PBLs from individual donors were separate optimal conditions regarding concentrations for IL-2 and IL-4, which in turn required us to optimize each individual PBLs to effectively produce antigen specific human antibody by in vitro immunization. These individual differences in the requirement for IL-2 and IL-4 reflects the differences in individual immune responses against a specific soluble antigen, which can be elicited by in vitro immunization. In the present study, we investigated these individual differences in the requirement for IL-2 and IL-4 to induce antibody productionin vitro in the PBLs of 12 volunteers (9 healthy donors and 3 allergenic patients). IL-2 requirements for antibody production varied dependent upon each donor, while higher amounts of IL-4 inhibited IgM and IgG production in all of the healthy donors. However, some of the characteristic features for PBLs donated from allergenic included lowered IgM production compared to PBLs derived from healthy donors, and very high IgE production in the absence of cytokines and allergen. These results demonstrate that the sensitivity of PBLs against antigen sensitization differs between healthy donors and atopic patients, which suggests that the frequency of antigen sensitization might be reflected in differing activation states and/or differing subpopulations of lymphocytes in vivo. This revised version was published online in August 2006 with corrections to the Cover Date.     

Repeated doses of UVR cause minor alteration in cytokine serum levels in humans

The aim of our study was to compare serum concentration of IL-1beta, IL-6, IL-8, IL-10, and TNF-alpha in 105 healthy volunteers before and after exposure to UVR: 25 subjects (10 days of UVB), 55 (10 days of UVB or solar-simulated radiation, followed by acute UVB dose), and 25 (local high dose of UVB). In all the individuals blood samples were analyzed before and after final irradiation by chemiluminescence assay. After 10 days of UVB irradiation a statistically significant increase in serum concentration only in IL-8 (P<.05) and strong tendency in TNF-alpha (P=.05) were observed. The applied schedules of irradiation have minor impact on serum cytokine level and still a threshold dose of UVR causing systemic immune impairment is unknown.     

Exposure to 900 MHz radiofrequency radiation induces caspase 3 activation in proliferating human lymphocytes

Palumbo, R., Brescia, F., Capasso, D., Sannino, A., Sarti, M., Capri, M., Grassilli, E. and Scarfì, M. R. Exposure to 900 MHz Radiofrequency Radiation Induces Caspase 3 Activation in Proliferating Human Lymphocytes. Radiat. Res. 170, 327- 334 (2008).In this study, the induction of apoptosis after exposure to 900 MHz radiofrequency radiation (GSM signal) was investigated by assessing caspase 3 activation in exponentially growing Jurkat cells and in quiescent and proliferating human peripheral blood lymphocytes (PBLs). The exposure was carried out at an average specific absorption rate of 1.35 W/kg in a dual wire patch cell exposure system where the temperature of cell cultures was accurately controlled. After 1 h exposure to the radiofrequency field, a slight but statistically significant increase in caspase 3 activity, measured 6 h after exposure, was observed in Jurkat cells (32.4%) and in proliferating human PBLs (22%). In contrast, no effect was detected in quiescent human PBLs. In the same experimental conditions, apoptosis was also evaluated in Jurkat cells by Western blot analysis and in both cell types by flow cytometry. To evaluate late effects due to caspase 3 activity, flow cytometry was also employed to assess apoptosis and viability 24 h after radiofrequency-radiation exposure in both cell types. Neither the former nor the latter was affected. Since in recent years it has been reported that caspases are also involved in processes other than apoptosis, additional cell cycle studies were carried out on proliferating T cells exposed to radiofrequency radiation; however, we found no differences between sham-exposed and exposed cultures. Further studies are warranted to investigate the biological significance of our findings of a dose-response increase in caspase 3 activity after exposure to radiofrequency radiation.     

Ultraviolet light‐induced impairment of goldfish embryo development and evidence for photorepair mechanisms

Goldfish Carassius auratus embryos were subjected to artificial ultraviolet‐B (UVB) radiation (280–320 nm) at various times during development to evaluate the effects on production of anatomically normal larvae. The UVB radiation used in these experiments included a higher proportion of shorter wavelengths compared to the natural spectrum. The development of embryos exposed to UVB for 2 or 4 h at 26 h post‐fertilization was severely impaired whereas similar exposures at 50 or 74 h post‐fertilization had no effect. A 2 h exposure to UVB commencing at 2 h post‐fertilization did not adversely affect embryonic development whereas a 4 h exposure to a lower dose did. At 50 h post‐fertilization, when embryos were normally resistant to UVB, denial of access to visible light and UVA before, during and after exposure to UVB caused impairment of development. Analysis of DNA fragment length after incubation with an endonuclease suggested that UVB damage at 50 h was caused by formation of pyrimidine dimers. This study demonstrated that the sensitivity of goldfish embryos to UVB varied during development and that resistance to UVB in later developmental stages included a photorepair mechanism.