Micronucleus scoring: An available approach in the evaluation of genomic damage in exfoliative cervicovaginal cells

Micronucleus is small chromatin extranuclear bodies when chromosomes or chromosomal fragments are not included in the nucleus during cell division. Micronucleus formation usually serves as a sensible indicator of genotoxic damage and also a morphological marker of chromosomal instability. Genomic damage is crucial for the development of degenerative diseases, including cancer. MN assay is a reliable and applicable method on many cell types such as erythrocytes, leucocytes and epithelial cells and it represents an important tool for evaluating DNA damage and defects in mitosis. The purpose of this article is to provide an overview of the current status of the micronuclei scoring in exfoliated epithelial cells and to highlight the importance of this procedure. Review Article Micronucleus scoring: An available approach in the evaluation of genomic damage in exfoliative cervicovaginal cells Zehra SAFI OZ* Department of Medical Biology, Zonguldak Bülent Ecevit University School of Medicine, Zonguldak, Turkey Received: 15 June, 2020 Accepted: 20 June, 2020 Published: 22 June, 2020 *Corresponding author: Zehra SAFI OZ, Department of Medical Biology, Zonguldak Bülent Ecevit University School of Medicine, Zonguldak, Turkey, Tel: +90 (372) 2613228; Fax: +90 (372) 2610264; E-mail:


Review of the literature
Micronucleus (the plural is micronuclei) was fi rst identifi ed by Howell and Jolly in red cell precursors and was also referred as Howell-Jolly bodies [1]. At that time, these bodies were described as nuclear remnants (clusters of DNA) in the circulating erythrocytes. After a while, MN was described in lymphocytes, exfoliated buccal and cervicovaginal epithelial cells [1][2][3]. Extranuclear cytoplasmic bodies of the damaged chromosome fragments and/or whole chromosomes form MN. It is not incorporated into the nucleus after cell division [2]. MN is also a sensible indicator of the genetic damage and linked to various chromosome aberrations such as defective mitotic fi gures, chromosome fragmentations, mitotic cell death and catastrophe, giant nuclear, and especially genome chaos [4][5][6][7]. Genomic damage may occur due to medical factors such as radiation and chemicals and also due to the defi ciency of some micronutrients. Lifestyle factors such as alcohol, smoking, stress and genetic factors are also important for the development of genomic damage [6][7][8][9][10][11][12][13]. MN assay can be applied to exfoliative epithelial cells to screen population groups at risk for cancer [14][15][16]. It can also be used as a biomarker to examine the effects of infection agents [17][18][19].

History of micronuclei
Micronucleus test was suggested for the fi rst time by Boller, Schmidt and Heddle in the early 1970s. A few years later Countryman and Heddle could be used for the micronucleus approach in the peripheral blood lymphocytes [3]. In 1982, MN technique was fi rst described by Stich and colleaques in exfoliative cells of the buccal mucosa for human biomonitoring studies [11]. Exfoliated cells can be used in several types of cell obtained from buccal and cervical mucosa, bronchi, urinary bladder etc. MN assay seems to represent a useful 'internal dosimeter' for estimating exposure to genotoxic agents [8,14,20].

Micronuclei formation
The basal layer of squamous epithelium contains the basal cells which are cuboidal-shaped stem cells. During nuclear division, genetic damage may express as MN in these cells [2,3].
According to the knowledge accumulated in literature, some genetic and epigenetic mechanisms effects the formation of MN [2]. There are concisely three mechanisms that may contribute to this process. These are chromosomal breakage, dysfunction of the mitotic apparatus and broken anaphase bridges [16,19].
Agents that stimulate aneuploidy cause centromere division errors and mitotic spindle failure. The classtogens also contribute to MN formation lead to chromosome breaks [2].

Criteria for the evaluation of micronuclei in exfoliated cells
One of the most important criteria for the evaluation of MN in exfoliated epithelial cells is the counting of nuclei and cells with intact boundaries [18,19]. Heddle (1973) initially described

Exfoliative cytology and Papanicolaou test (Pap test)
The cervicovaginal epithelium is composed of several distinct layers or strata. This rapid turnover of epithelial tissue brings the cells to the surface, where they exfoliate [3,22]. Pap test is an easily performed, quick, noninvasive, inexpensive and safe methods to screen for preinvasive, invasive cervical cancer and the effects of gynecological infection agents [24]. Cervical cancer is the fourth most common cancer in female. Pap test can help prevent cervical cancer [25][26][27].  This test can be performed via the conventional method and liquid based cytology. In the conventional method, exfoliated epithelial cells were layered on a glass slide and immediately fi xed with appropriate fi xative. In LBC, cells are stored in a vial containing a special liquid. LBC had better performance than conventional Pap test [26,28]. Light microscopic analysis of MN in cervical smears increases the sensitivity and specifi city of cytology in the evaluation of cellular pictures due to genomic instability [19].

Micronucleus scoring and the interaction between MN and gynecological infections
MN scoring can be performed immediately by using microscopy and it can be screened in various diseases, infection agents, cervical intraepithelial lesions and carcinoma [17][18][19]29]. MN scoring on the epithelial cells of cervix can be used as supplemental in cervical cancer screening. MN requency appears to increase in carcinogen-exposed tissues long before any clinical symptoms are evident [27]. Candidiasis and Trichomoniasis [18,19]. Cortés Gutiérrez, et al. demonstrated that MN frequencies were higher in HPV infected Mexican females [17]. Rosin and Anwar stated that MN frequencies were higher in the S. haematobium infected group [34]. In some studies, the limitations of the micronuclei scoring has been reported [3]. A few studies reported at possibility of false-positive results as the bacteria or bacterial colonies, nuclear debris, small stain deposits and keratohyalin granules may resemble to MN [3,29,30]. Safi , et al. indicated that bacteria and bacterial colonies can be differentiated from MN by their characteristic shape, color, staining intensity and smaller size, which is also apparent in the background. Also stain deposits are polymorphic granules in the smear, generally over the cells. Pap staining ensures that the nucleus and cytoplasm of the epithelial cells are clearly stained, ensuring visible and identifi able MN [30]. MN monitoring could be incorporated into routine screening procedures as an additional criterion for the early detection of cytogenetic damage as used in evaluation of cancer and infectious agents [35].