Micronucleus

Micronuclei are commonly seen in cancerous cells and may indicate genomic damage events that can increase the risk of developmental or degenerative diseases.

This improper segregation of chromosomes may result from hypomethylation of repeat sequences present in pericentromeric DNA, irregularities in kinetochore proteins or their assembly, a dysfunctional spindle apparatus, or flawed anaphase checkpoint genes.

[3] Many micronucleus assays have been developed to test for the presence of these structures and determine their frequency in cells exposed to certain chemicals or subjected to stressful conditions.

[4] Micronuclei in newly formed red blood cells in humans are known as Howell-Jolly bodies because these structures were first identified and described in erythrocytes by hematologists William Howell and Justin Jolly.

The relationship between formation of micronuclei and exposure to environmental factors was first reported in root tip cells exposed to ionizing radiation.

These full chromosomes or chromatid fragments are eventually enclosed by a nuclear membranes and are structurally similar to conventional nuclei, albeit smaller in size.

If there is a marked increase in the number of cells with micronuclei, it can be concluded that the chemical induces structural and/or numerical chromosomal damage.

These cells experience constant, rapid turnover and the lack of a true nucleus in erythrocytes makes micronuclei easily visible under a microscope.

The cytokinesis-block micronucleus (CBMN) assay was first developed to score micronuclei in cells that completed nuclear division by blocking them at the binucleate stage before cytokinesis.

The major drawback of using micronucleus tests is that they cannot determine different types of chromosomal aberrations and can be influenced by the mitotic rate and proportion of cell death, skewing the results.

The deficiency of micronuclei in some of the oldest age groups may be explained by the fact that micro nucleated cells are preferentially eliminated by apoptosis.

However, higher micronuclei frequency corresponds to a decreased efficiency of DNA repair and increased genomic instability, which are typical in older subjects.

Alternatively, the leveling off of frequency of micronuclei in older subjects would suggest a threshold of genomic instability that cannot be crossed if the person is to survive.