(a) Scope. This section is intended to meet the testing requirements under section 4 of TSCA.

(1) The mammalian erythrocyte micronucleus test is used for the detection of damage induced by the test substance to the chromosomes or the mitotic apparatus of erythroblasts by analysis of erythrocytes as sampled in bone marrow and/or peripheral blood cells of animals, usually rodents.

(2) The purpose of the micronucleus test is to identify substances that cause cytogenetic damage which results in the formation of micronuclei containing lagging chromosome fragments or whole chromosomes.

(3) When a bone marrow erythroblast develops into a polychromatic erythrocyte, the main nucleus is extruded; any micronucleus that has been formed may remain behind in the otherwise anucleated cytoplasm. Visualization of micronuclei is facilitated in these cells because they lack a main nucleus. An increase in the frequency of micronucleated polychromatic erythrocytes in treated animals is an indication of induced chromosome damage.

(b) Source. The source material used in developing this TSCA test guideline is the OECD guideline 474 (February 1997). This source is available at the address in paragraph (g) of this section.

(c) Definitions. The following definitions apply to this section:

Centromere (kinetochore) is a region of a chromosome with which spindle fibers are associated during cell division, allowing orderly movement of daughter chromosomes to the poles of the daughter cells.

Micronuclei are small nuclei, separate from and additional to the main nuclei of cells, produced during telophase of mitosis (meiosis) by lagging chromosome fragments or whole chromosomes.

Normochromatic erythrocyte is a mature erythrocyte that lacks ribosomes and can be distinguished from immature, polychromatic erythrocytes by stains selective for ribosomes.

Polychromatic erythrocyte is an immature erythrocyte, in an intermediate stage of development, that still contains ribosomes and therefore can be distinguished from mature, normochromatic erythrocytes by stains selective for ribosomes.

(d) Initial considerations.

(1) The bone marrow of rodents is routinely used in this test since polychromatic erythrocytes are produced in that tissue. The measurement of micronucleated immature (polychromatic) erythrocytes in peripheral blood is equally acceptable in any species in which the inability of the spleen to remove micronucleated erythrocytes has been demonstrated, or which has shown an adequate sensitivity to detect agents that cause structural or numerical chromosome aberrations. Micronuclei can be distinguished by a number of criteria. These include identification of the presence or absence of a kinetochore or centromeric DNA in the micronuclei. The frequency of micronucleated immature (polychromatic) erythrocytes is the principal endpoint. The number of mature (normochromatic) erythrocytes in the peripheral blood that contain micronuclei among a given number of mature erythrocytes can also be used as the endpoint of the assay when animals are treated continuously for 4 weeks or more. This mammalian in vivo micronucleus test is especially relevant to assessing mutagenic hazard in that it allows consideration of factors of in vivo metabolism, pharmacokinetics and DNA-repair processes although these may vary among species, among tissues and among genetic endpoints. An in vivo assay is also useful for further investigation of a mutagenic effect detected by an in vitro system.

(2) If there is evidence that the test substance, or a reactive metabolite, will not reach the target tissue, it is not appropriate to use this test.

(e) Test method—(1) Principle. Animals are exposed to the test substance by an appropriate route. If bone marrow is used, the animals are sacrificed at appropriate times after treatment, the bone marrow extracted, and preparations made and stained (test techniques described in the references under paragraphs (g)(1), (g)(2), and (g)(3) of this section may be used). When peripheral blood is used, the blood is collected at appropriate times after treatment and smear preparations are made and stained (the test techniques described in the references under paragraphs (g)(3), (g)(4), (g)(5), and (g)(6) of this section may be used). For studies with peripheral blood, as little time as possible should elapse between the last exposure and cell harvest. Preparations are analyzed for the presence of micronuclei.

(2) Description—(i) Preparations—(A) Selection of animal species. Mice or rats are recommended if bone marrow is used, although any appropriate mammalian species may be used. When peripheral blood is used, mice are recommended. However, any appropriate mammalian species may be used provided it is a species in which the spleen does not remove micronucleated erythrocytes or a species which has shown an adequate sensitivity to detect agents that cause structural or numerical chromosome aberrations. Commonly used laboratory strains of young healthy animals should be employed. At the commencement of the study, the weight variation of animals should be minimal and not exceed ±20% of the mean weight of each sex.

(B) Housing and feeding conditions. The temperature in the experimental animal room should be 22 °C ±3 °C). Although the relative humidity should be at least 30% and preferably not exceed 70% other than during room cleaning, the aim should be 50-60%. Lighting should be artificial, the sequence being 12 hrs light, 12 hrs dark. For feeding, conventional laboratory diets may be used with an unlimited supply of drinking water. The choice of diet may be influenced by the need to ensure a suitable admixture of a test substance when administered by this route. Animals may be housed individually, or caged in small groups of the same sex.

(C) Preparation of the animals. Healthy young adult animals shall be randomly assigned to the control and treatment groups. The animals are identified uniquely. The animals are acclimated to the laboratory conditions for at least 5 days. Cages should be arranged in such a way that possible effects due to cage placement are minimized.

(D) Preparation of doses. Solid test substances shall be dissolved or suspended in appropriate solvents or vehicles and diluted, if appropriate, prior to dosing of the animals. Liquid test substances may be dosed directly or diluted prior to dosing. Fresh preparations of the test substance should be employed unless stability data demonstrate the acceptability of storage.

(ii) Test conditions—(A) Solvent/vehicle. The solvent/vehicle shall not produce toxic effects at the dose levels used, and shall not be suspected of chemical reaction with the test substance. If other than well-known solvents/vehicles are used, their inclusion should be supported with reference data indicating their compatibility. It is recommended that wherever possible, the use of an aqueous solvent/vehicle should be considered first.

(B) Controls.

(1) Concurrent positive and negative (solvent/vehicle) controls shall be included for each sex in each test. Except for treatment with the test substance, animals in the control groups should be handled in an identical manner to animals of the treatment groups.

(2) Positive controls shall produce micronuclei in vivo at exposure levels expected to give a detectable increase over background. Positive control doses should be chosen so that the effects are clear but do not immediately reveal the identity of the coded slides to the reader. It is acceptable that the positive control be administered by a route different from the test substance and sampled at only a single time. In addition, the use of chemical class-related positive control chemicals may be considered, when available. Examples of positive control substances include:

(3) Negative controls, treated with solvent or vehicle alone, and otherwise treated in the same way as the treatment groups shall be included for every sampling time, unless acceptable inter-animal variability and frequencies of cells with micronuclei are demonstrated by historical control data. If single sampling is applied for negative controls, the most appropriate time is the first sampling time. In addition, untreated controls should also be used unless there are historical or published control data demonstrating that no deleterious or mutagenic effects are induced by the chosen solvent/vehicle.

(4) If peripheral blood is used, a pre-treatment sample may also be acceptable as a concurrent negative control, but only in the short peripheral blood studies (e.g., one to three treatment(s)) when the resulting data are in the expected range for the historical control.

(3) Procedure—(i) Number and sex of animals. Each treated and control group shall include at least 5 analyzable animals per sex (techniques described in the reference under paragraph (g)(7) of this section may be used). If at the time of the study there are data available from studies in the same species and using the same route of exposure that demonstrate that there are no substantial differences between sexes in toxicity, then testing in a single sex will be sufficient. Where human exposure to chemicals may be sex-specific, as for example with some pharmaceutical agents, the test should be performed with animals of the appropriate sex.

(ii) Treatment schedule.

(A) No standard treatment schedule (i.e. one, two, or more treatments at 24 h intervals) can be recommended. The samples from extended dose regimens are acceptable as long as a positive effect has been demonstrated for this study or, for a negative study, as long as toxicity has been demonstrated or the limit dose has been used, and dosing continued until the time of sampling. Test substances may also be administered as a split dose, i.e., two treatments on the same day separated by no more than a few hrs, to facilitate administering a large volume of material.

(B) The test may be performed in two ways:

(1) Animals shall be treated with the test substance once. Samples of bone marrow shall be taken at least twice, starting not earlier than 24 hrs after treatment, but not extending beyond 48 hrs after treatment with appropriate interval(s) between samples. The use of sampling times earlier than 24 hrs after treatment should be justified. Samples of peripheral blood shall be taken at least twice, starting not earlier than 36 hrs after treatment, with appropriate intervals following the first sample, but not extending beyond 72 hrs. When a positive response is recognized at one sampling time, additional sampling is not required.

(2) If two or more daily treatments are used (e.g. two or more treatments at 24 hr intervals), samples shall be collected once between 18 and 24 hrs following the final treatment for the bone marrow and once between 36 and 48 hrs following the final treatment for the peripheral blood (techniques described in the reference under paragraph (g)(8) of this section may be used).

(C) Other sampling times may be used in addition, when relevant.

(iii) Dose levels. If a range finding study is performed because there are no suitable data available, it shall be performed in the same laboratory, using the same species, strain, sex, and treatment regimen to be used in the main study (guidance on dose setting is provided in the reference in paragraph (g)(9) of this section). If there is toxicity, three dose levels shall be used for the first sampling time. These dose levels shall cover a range from the maximum to little or no toxicity. At the later sampling time only the highest dose needs to be used. The highest dose is defined as the dose producing signs of toxicity such that higher dose levels, based on the same dosing regimen, would be expected to produce lethality. Substances with specific biological activities at low non-toxic doses (such as hormones and mitogens) may be exceptions to the dose-setting criteria and should be evaluated on a case-by-case basis. The highest dose may also be defined as a dose that produces some indication of toxicity in the bone marrow (e.g. a reduction in the proportion of immature erythrocytes among total erythrocytes in the bone marrow or peripheral blood).

(iv) Limit test. If a test at one dose level of at least 2,000 mg/kg body weight using a single treatment, or as two treatments on the same day, produces no observable toxic effects, and if genotoxicity would not be expected based upon data from structurally related substances, then a full study using three dose levels may not be considered necessary. For studies of a longer duration, the limit dose is 2,000 mg/kg/body weight/day for treatment up to 14 days, and 1,000 mg/kg/body weight/day for treatment longer than 14 days. Expected human exposure may indicate the need for a higher dose level to be used in the limit test.

(v) Administration of doses. The test substance is usually administered by gavage using a stomach tube or a suitable intubation cannula, or by intraperitoneal injection. Other routes of exposure may be acceptable where they can be justified. The maximum volume of liquid that can be administered by gavage or injection at one time depends on the size of the test animal. The volume should not exceed 2 ml/100g body weight. The use of volumes higher than these must be justified. Except for irritating or corrosive substances which will normally reveal exacerbated effects with higher concentrations, variability in test volume should be minimized by adjusting the concentration to ensure a constant volume at all dose levels.

(vi) Bone marrow/blood preparation. Bone marrow cells shall be obtained from the femurs or tibias immediately following sacrifice. Cells shall be removed from femurs or tibias, prepared and stained using established methods. Peripheral blood is obtained from the tail vein or other appropriate blood vessel. Blood cells are immediately stained supravitally (the test techniques described in the references under paragraphs (g)(4), (g)(5), and (g)(6) of this section may be used) or smear preparations are made and then stained. The use of a DNA specific stain (e.g. acridine orange (techniques described in the reference under paragraph (g)(10) of this section may be used) or Hoechst 33258 plus pyronin-Y) can eliminate some of the artifacts associated with using a non-DNA specific stain. This advantage does not preclude the use of conventional stains (e.g., Giemsa). Additional systems (e.g. cellulose columns to remove nucleated cells (the test techniques described in the references under paragraph (g)(12) of this section may be used)) can also be used provided that these systems have been shown to adequately work for micronucleus preparation in the laboratory.

(vii) Analysis. The proportion of immature among total (immature = mature) erythrocytes is determined for each animal by counting a total of at least 200 erythrocytes for bone marrow and 1,000 erythrocytes for peripheral blood (techniques described in the reference under paragraph (g)(13) of this section maybe used). All slides, including those of positive and negative controls, shall be independently coded before microscopic analysis. At least 2,000 immature erythrocytes per animal shall be scored for the incidence of micronucleated immature erythrocytes. Additional information may be obtained by scoring mature erythrocytes for micronuclei. When analyzing slides, the proportion of immature erythrocytes among total erythrocytes should not be less than 20% of the control value. When animals are treated continuously for 4 weeks or more, at least 2,000 mature erythrocytes per animal can also be scored for the incidence of micronuclei. Systems for automated analysis (image analysis) and cell suspensions (flow cytometry) are acceptable alternatives to manual evaluation if appropriately justified and validated.

(f) Data and reporting—(1) Treatment of results. Individual animal data shall be presented in tabular form. The experimental unit is the animal. The number of immature erythrocytes scored, the number of micronucleated immature erythrocytes, and the number of immature among total erythrocytes shall be listed separately for each animal analyzed. When animals are treated continuously for 4 weeks or more, the data on mature erythrocytes should also be given if it is collected. The proportion of immature among total erythrocytes and, if considered applicable, the percentage of micronucleated erythrocytes shall be given for each animal. If there is no evidence for a difference in response between the sexes, the data from both sexes may be combined for statistical analysis.

(2) Evaluation and interpretation of results.

(i) There are several criteria for determining a positive result, such as a dose-related increase in the number of micronucleated cells or a clear increase in the number of micronucleated cells in a single dose group at a single sampling time. Biological relevance of the results should be considered first. Statistical methods may be used as an aid in evaluating the test results (the test techniques described in the references paragraphs (g)(14) and (g)(15) of this section may be used). Statistical significance should not be the only determining factor for a positive response. Equivocal results should be clarified by further testing preferably using a modification of experimental conditions.

(ii) A test substance for which the results do not meet the criteria in paragraph (f)(2)(i) of this section is considered non-mutagenic in this test.

(iii) Although most experiments will give clearly positive or negative results, in rare cases the data set will preclude making a definite judgement about the activity of the test substance. Results, may remain equivocal or questionable regardless of the number of times the experiment is repeated. Positive results in the micronucleus test indicate that a substance induces micronuclei which are the result of chromosomal damage or damage to the mitotic apparatus in the erythroblasts of the test species. Negative results indicate that, under the test conditions, the test substance does not produce micronuclei in the immature erythrocytes of the test species.

(iv) The likelihood that the test substance or its metabolites reach the general circulation or specifically the target tissue (e.g. systemic toxicity) should be discussed.

(3) Test report. The test report shall include the following information:

(i) Test substance:

(A) Identification data and CAS no., if known.

(B) Physical nature and purity.

(C) Physiochemical properties relevant to the conduct of the study.

(D) Stability of the test substance, if known.

(ii) Solvent/vehicle:

(A) Justification for choice of vehicle.

(B) Solubility and stability of the test substance in the solvent/vehicle, if known.

(iii) Test animals:

(A) Species/strain used.

(B) Number, age, and sex of animals.

(C) Source, housing conditions, diet, etc.

(D) Individual weight of the animals at the start of the test, including body weight range, mean and standard deviation for each group.

(iv) Test conditions:

(A) Positive and negative (vehicle/solvent) control data.

(B) Data from range-finding study, if conducted.

(C) Rationale for dose level selection.

(D) Details of test substance preparation.

(E) Details of the administration of the test substance.

(F) Rationale for route of administration.

(G) Methods for verifying that the test substance reached the general circulation or target tissue, if applicable.

(H) Conversion from diet/drinking water test substance concentration parts per million (ppm) to the actual dose (mg/kg body weight/day), if applicable.

(I) Details of food and water quality.

(J) Detailed description of treatment and sampling schedules.

(K) Methods of slide preparation.

(L) Methods for measurement of toxicity.

(M) Criteria for scoring micronucleated immature erythrocytes.

(N) Number of cells analyzed per animal.

(O) Criteria for considering studies as positive, negative or equivocal.

(v) Results:

(A) Signs of toxicity.

(B) Proportion of immature erythrocytes among total erythrocytes.

(C) Number of micronucleated immature erythrocytes, given separately for each animal.

(D) Mean = ±standard deviation of micronucleated immature erythrocytes per group.

(E) Dose-response relationship, where possible.

(F) Statistical analyses and method applied.

(G) Concurrent and historical negative control data.

(H) Concurrent positive control data.

(vi) Discussion of the results.

(vii) Conclusion.

(g) References. For additional background information on this test guideline, the following references should be consulted. These references are available at the addresses in §700.17(b)(1) and (2) of this chapter.

(1) Heddle, J.A. A Rapid In Vivo Test for Chromosomal Damage. Mutation Research. 18, 187-190 (1973).

(2) Schmid, W. The Micronucleus Test. Mutation Research. 31, 9-15 (1975).

(3) Mavournin, K.H., Blakey, D.H., Cimino, M.C., Salamone, M.F., and Heddle, J.A. The In Vivo Micronucleus Assay in Mammalian Bone Marrow and Peripheral Blood. A report of the U.S. Environmental Protection Agency Gene-Tox Program. Mutation Research. 239, 29-80 (1990).

(4) Hayashi, M., Morita, T., Kodama, Y., Sofuni, T., and Ishidate, Jr., M. The Micronucleus Assay with Mouse Peripheral Blood Reticulocytes Using Acridine Orange-Coated Slides. Mutation Research. 245, 245-249 (1990).

(5) The Collaborative Study Group for the Micronucleus Test (1992). Micronucleus Test with Mouse Peripheral Blood Erythrocytes by Acridine Orange Supravital Staining: The Summary Report of the 5th Collaborative Study by CSGMT/JEMS. MMS. Mutation Research. 278, 83-98.

(6) The Collaborative Study Group for the Micronucleus Test (CSGMT/JEMMS.MMS, The Mammalian Mutagenesis Study Group of the Environmental Mutagen Society of Japan) Protocol recommended for the short-term mouse peripheral blood micronucleus test. Mutagenesis. 10, 153-159 (1995).

(7) Hayashi, M., Tice, R.R., MacGregor, J.T., Anderson, D., Blakey, D.H., Kirsch-Volders, M., Oleson, Jr. F.B., Pacchierotti, F., Romagna, F., Shimada, H., Sutou, S., and Vannier, B. In Vivo Rodent Erythrocyte Micronucleus Assay. Mutation Research. 312, 293-304 (1994).

(8) Higashikuni, N. and Sutou, S. An optimal, generalized sampling time of 30 =/- 6 h after double dosing in the mouse peripheral blood micronucleus test. Mutagenesis. 10, 313-319 (1995).

(9) Fielder, R.J., Allen, J.A., Boobis, A.R., Botham, P.A., Doe, J., Esdaile, D.J., Gatehouse, D.G., Hodson-Walker, G., Morton, D.B., Kirkland, D. J., and Richold, M. Report of British Toxicology Society/UK Environmental Mutagen Society Working Group: Dose Setting in In Vivo Mutagenicity Assays. Mutagenesis. 7, 313-319 (1992).

(10) Hayashi, M., Sofuni, T., and Ishidate, Jr., M. An Application of Acridine Orange Fluorescent Staining to the Micronucleus Test. Mutation Research. 120, 241-247 (1983).

(11) MacGregor, J.T., Wehr, C.M., and Langlois, R.G. A Simple Fluorescent Staining Procedure for Micronuclei and RNA in Erythrocytes Using Hoechst 33258 and Pyronin Y. Mutation Research. 120, 269-275 (1983).

(12) Romagna, F. and Staniforth, C.D. The automated bone marrow micronucleus test. Mutation Research. 213, 91-104 (1989).

(13) Gollapudi, B. and McFadden, L.G. Sample size for the estimation of polychromatic to normochromatic eruthrocyte ratio in the bone marrow micronucleus test. Mutation Research. 347, 97-99 (1995).

(14) Richold, M., Ashby, J., Bootman, J., Chandley, A., Gatehouse, D.G., and Henderson, L. Ed. Kirkland, D.J. In Vivo Cytogenetics Assays. Basic Mutagenicity Tests, UKEMS Recommended Procedures. UKEMS Subcommittee on Guidelines for Mutagenicity Testing. Report. Part I revised (Cambridge University Press, Cambridge, New York, Port Chester, Melbourne, Sydney, 1990) pp. 115-141.

(15) Lovell, D.P., Anderson, D., Albanese, R., Amphlett, G.E., Clare, G., Ferguson, R., Richold, M., Papworth, D.G., and Savage, J.R.K. Ed. D.J. Kirkland. Statistical Analysis of In Vivo Cytogenetic Assays. Statistical Evaluation of Mutagenicity Test Data. UKEMS Sub-Committee on Guidelines for Mutagenicity Testing, Report, Part III. (Cambridge University Press, Cambridge, New York, Port Chester, Melbourne, Sydney, 1989) pp. 184-232.

(16) Heddle, J.A., Salamone, M.F., Hite, M., Kirkhart, B., Mavournin, K., MacGregor, J.G., and Newell, G.W. The Induction of Micronuclei as a Measure of Genotoxicity. Mutation Research. 123: 61-118 (1983).

(17) MacGregor, J.T., Heddle, J.A., Hite, M., Margolin, G.H., Ramel C., Salamone, M.F., Tice, R.R., and Wild, D. Guidelines for the Conduct of Micronucleus Assays in Mammalian Bone Marrow Erythrocytes. Mutation Research. 189: 103-112 (1987).

(18) MacGregor, J.T., Wehr, C.M., Henika, P.R., and Shelby, M.E. (1990). The In Vivo Erythrocyte Micronucleus Test: Measurement at Steady State Increases Assay Efficiency and Permits Integration with Toxicity Studies. Fundamental Applied Toxicology. 14: 513-522.

(19) MacGregor, J.T., Schlegel, R. Choy, W.N., and Wehr, C.M. Eds. Hayes, A.W., Schnell, R.C., and Miya, T.S. Micronuclei in Circulating Erythrocytes: A Rapid Screen for Chromosomal Damage During Routine Toxicity Testing in Mice. Developments in Science and Practice of Toxicology (Elsevier, Amsterdam, 1983) pp. 555-558.

[62 FR 43824, Aug. 15, 1997, as amended at 64 FR 35079, June 30, 1999; 77 FR 46294, Aug. 3, 2012]