A micronucleus test is a test used in toxicological screening for potential

genotoxic Genotoxicity is the property of chemical agents that damage the genetic information within a cell causing mutations, which may lead to cancer. While genotoxicity is often confused with mutagenicity, all mutagens are genotoxic, but some genotoxic s ...

compounds. The assay is now recognized as one of the most successful and reliable assays for genotoxic

carcinogen A carcinogen () is any agent that promotes the development of cancer. Carcinogens can include synthetic chemicals, naturally occurring substances, physical agents such as ionizing and non-ionizing radiation, and biologic agents such as viruse ...

s, i.e., carcinogens that act by causing genetic damage and is recommended by the

OECD The Organisation for Economic Co-operation and Development (OECD; , OCDE) is an international organization, intergovernmental organization with 38 member countries, founded in 1961 to stimulate economic progress and international trade, wor ...

guideline for the testing of chemicals. There are two major versions of this test, one ''

in vivo Studies that are ''in vivo'' (Latin for "within the living"; often not italicized in English) are those in which the effects of various biological entities are tested on whole, living organisms or cells, usually animals, including humans, an ...

'' and the other ''

in vitro ''In vitro'' (meaning ''in glass'', or ''in the glass'') Research, studies are performed with Cell (biology), cells or biological molecules outside their normal biological context. Colloquially called "test-tube experiments", these studies in ...

''. The ''in vivo'' test normally uses mouse

bone marrow Bone marrow is a semi-solid biological tissue, tissue found within the Spongy bone, spongy (also known as cancellous) portions of bones. In birds and mammals, bone marrow is the primary site of new blood cell production (or haematopoiesis). It i ...

or mouse peripheral blood. When a bone marrow

erythroblast A nucleated red blood cell (NRBC), also known by #Nomenclature, several other names, is a red blood cell that contains a cell nucleus. Almost all vertebrate organisms have hemoglobin-containing cells in their blood, and with the exception of mamm ...

develops into a polychromatic

erythrocyte Red blood cells (RBCs), referred to as erythrocytes (, with -''cyte'' translated as 'cell' in modern usage) in academia and medical publishing, also known as red cells, erythroid cells, and rarely haematids, are the most common type of blood ce ...

, the main nucleus is extruded; any micronucleus that has been formed may remain behind in the otherwise anucleated

cytoplasm The cytoplasm describes all the material within a eukaryotic or prokaryotic cell, enclosed by the cell membrane, including the organelles and excluding the nucleus in eukaryotic cells. The material inside the nucleus of a eukaryotic cell a ...

. Visualisation of

micronuclei A micronucleus is a small cell nucleus, nucleus that forms whenever a chromosome or a fragment of a chromosome is not incorporated into one of the daughter nuclei during cell division. It usually is a sign of genotoxic events and chromosomal inst ...

is facilitated in these cells because they lack a main

nucleus Nucleus (: nuclei) is a Latin word for the seed inside a fruit. It most often refers to: *Atomic nucleus, the very dense central region of an atom *Cell nucleus, a central organelle of a eukaryotic cell, containing most of the cell's DNA Nucleu ...

. An increase in the frequency of micronucleated

polychromatic Polychrome is the "practice of decorating architectural elements, sculpture, etc., in a variety of colors." The term is used to refer to certain styles of architecture, pottery, or sculpture in multiple colors. When looking at artworks and a ...

erythrocytes Red blood cells (RBCs), referred to as erythrocytes (, with -''cyte'' translated as 'cell' in modern usage) in academia and medical publishing, also known as red cells, erythroid cells, and rarely haematids, are the most common type of blood cel ...

in treated animals is an indication of induced

chromosome A chromosome is a package of DNA containing part or all of the genetic material of an organism. In most chromosomes, the very long thin DNA fibers are coated with nucleosome-forming packaging proteins; in eukaryotic cells, the most import ...

damage.

Micronuclei and nuclear abnormalities in peripheral blood erythrocytes of penguins Pygoscelis papua 1

Micronuclei A micronucleus is a small cell nucleus, nucleus that forms whenever a chromosome or a fragment of a chromosome is not incorporated into one of the daughter nuclei during cell division. It usually is a sign of genotoxic events and chromosomal inst ...

were first used to quantify chromosomal damage by H.J. Evans et al., in root tips of the Broad Bean, ''Vicia faba''. Subsequently, the ''in vivo'' assay was developed independently by W. Schmid and by J.A. Heddle and their colleagues. The mouse peripheral blood assay was developed by J.T. MacGregor and has now been adapted for measurement by

flow cytometry Flow cytometry (FC) is a technique used to detect and measure the physical and chemical characteristics of a population of cells or particles. In this process, a sample containing cells or particles is suspended in a fluid and injected into the ...

by A. Tometsko and colleagues. The first use of

in cultured cells was by J.A. Heddle and colleagues in human

lymphocytes A lymphocyte is a type of white blood cell (leukocyte) in the immune system of most vertebrates. Lymphocytes include T cells (for cell-mediated and cytotoxic adaptive immunity), B cells (for humoral, antibody-driven adaptive immunity), and ...

. The assay has been improved by M. Fenech and colleagues for use in

and other cells in culture cells. Simple

Giemsa stain Giemsa stain (), named after German chemist and bacteriologist Gustav Giemsa, is a nucleic acid stain used in cytogenetics and for the histopathological diagnosis of malaria and other parasites. Uses It is specific for the phosphate groups o ...

ing was originally used for MN scoring. Later, the cytokinesis-block micronucleus (CBMN) method was established, where Cyt-B, an inhibitor of the spindle assembly, was used to prevent

cytokinesis Cytokinesis () is the part of the cell division process and part of mitosis during which the cytoplasm of a single eukaryotic cell divides into two daughter cells. Cytoplasmic division begins during or after the late stages of nuclear division ...

occurring after nuclear division. The CBMN method is used for the assessment of chromosomal loss, breakage, and associated

apoptosis Apoptosis (from ) is a form of programmed cell death that occurs in multicellular organisms and in some eukaryotic, single-celled microorganisms such as yeast. Biochemistry, Biochemical events lead to characteristic cell changes (Morphology (biol ...

and

necrosis Necrosis () is a form of cell injury which results in the premature death of cells in living tissue by autolysis. The term "necrosis" came about in the mid-19th century and is commonly attributed to German pathologist Rudolf Virchow, who i ...

induced by different

mutagen In genetics, a mutagen is a physical or chemical agent that permanently changes genetic material, usually DNA, in an organism and thus increases the frequency of mutations above the natural background level. As many mutations can cause cancer in ...

s. A micronucleus is the erratic (third) nucleus that is formed during the

anaphase Anaphase () is the stage of mitosis after the process of metaphase, when replicated chromosomes are split and the newly-copied chromosomes (daughter chromatids) are moved to opposite poles of the cell. Chromosomes also reach their overall maxim ...

mitosis Mitosis () is a part of the cell cycle in eukaryote, eukaryotic cells in which replicated chromosomes are separated into two new Cell nucleus, nuclei. Cell division by mitosis is an equational division which gives rise to genetically identic ...

meiosis Meiosis () is a special type of cell division of germ cells in sexually-reproducing organisms that produces the gametes, the sperm or egg cells. It involves two rounds of division that ultimately result in four cells, each with only one c ...

. Micronuclei (the name means 'small nucleus') are cytoplasmic bodies having a portion of acentric chromosome or whole chromosome which was not carried to the opposite poles during the anaphase. Their formation results in the daughter cell lacking a part or all of a chromosome. These chromosome fragments or whole chromosomes normally develop nuclear membranes and form as micronuclei as a third nucleus. After cytokinesis, one daughter cell ends up with one nucleus and the other ends up with one large and one small nucleus, i.e., micronuclei. There is a chance of more than one micronucleus forming when more genetic damage has happened. The micronucleus test is used as a tool for genotoxicity assessment of various chemicals. It is easier to conduct than the chromosomal aberration test in terms of procedures and evaluation. Using

fluorescent in situ hybridization Fluorescence ''in situ'' hybridization (FISH) is a cytogenetics, molecular cytogenetic technique that uses hybridization probe, fluorescent probes that bind to only particular parts of a nucleic acid sequence with a high degree of sequence Com ...

(FISH) with probes targeted to the

centromere The centromere links a pair of sister chromatids together during cell division. This constricted region of chromosome connects the sister chromatids, creating a short arm (p) and a long arm (q) on the chromatids. During mitosis, spindle fiber ...

region, it can be determined if a whole chromosome, or only a fragment is lost.

References and notes

{{reflist Mutation

See also

References and notes