Sister chromatid exchange / editor, Avery A. Sandberg.

Date:
[1982], ©1982
Catalogue details

Licence: Attribution-NonCommercial-NoDerivatives 4.0 International (CC BY-NC-ND 4.0)

Credit: Sister chromatid exchange / editor, Avery A. Sandberg. Source: Wellcome Collection.

    70/736 (page 48)
    48 / Jenssen THE FORMATION OF MICRONUCLEI Indirect evidence suggests that micronuclei can be formed as a consequence of both chromosome breakage and spindle dysfunction. With the method of Schmid et al. [3,4], the induction of micronuclei is studied during erythropoie- sis. They took advantage of the fact that mammals expel their nuclei from pre¬ mature red blood cells, and for unknow^n reasons the micronuclei formed dur¬ ing the last cell cycle of the erythroblasts remain in the young erythrocytes. Of course, micronuclei can occur in any cell type, but in nucleated cells some of the micronuclei are not observable behind the main nuclei. Using micronuclei as an indicator of chromosomal damage, studies have been performed in plants [7,8] and with different animal systems both in vivo [3-5,9-11] and in vitro [12-14]. Micronuclei have also been studied in bone marrow cells and blood lymphocytes of exposed humans [15-18]. Micronuclei are mostly induced before and detected after mitosis. They are usually found as one per cell — very seldom more — and characterized as sharp¬ ly demarcated, rounded bodies with the staining properties of nuclear DNA and are usually 1/20-1/5 of the size of the main nucleus. In special cases mi¬ cronuclei can be induced during mitosis, indicating a destructive effect on spindle fibers and chromosome segregation. If micronuclei are induced in nucleated cells, they can be slowed down in their cell cycle in relation to the associated main nucleus of the cell [12]. The consequence is that the chromatin of the main nuclei may be forced to under¬ go premature chromosome condensation during the next mitosis [12]. It has been suggested [10] that premature chromosome condensation is an expres¬ sion of long-lived lesions, which prevent the chromosomes from replicating and/or condensing normally. METHODOLOGICAL ASPECTS Technical Procedure According to the standard protocol described by Schmid [19], the micronu- cleus tests are performed as follows: The mice are usually injected twice, 24 hours apart. Six hours after the second injection, the mice are killed and the bone marrow of the femora is dispersed into fetal calf serum. After centrifuga- tion, the pellet is resuspended in a small part of serum, and a small drop is transferred to the end of a slide and spread out by pulling it behind a cover glass. The air-dried preparations are stained with May-Gruenwald-Giemsa, rinsed with tap water, dried, cleared in xylene, and mounted. In the description by Schmid it is said that the bone marrow cells can only be suspended in fetal calf serum. We have found [20] that the fetal serum can be readily diluted at least 1:1 with phosphate buffer (Sörensen, 0.1 M, pH 7.0)