Demonstration of lateral asymmetry in G-banded mouse embryo chromosomes

High Resolution Mapping of Ribosomal DNA in Early Mouse Embryos by Fluorescence In Situ Hybridization1

The nucleolar precursor bodies (NPBs) are numerous discrete entities present in the nuclei of early mammalian embryos, which structurally support active rRNA genes. However, whether all rRNA genes, including those not transcribed, are spatially associated with NPBs, and moreover what is the general arrangement of ribosomal DNA (rDNA) in early mouse embryos, still remain unanswered questions. In our study, we examined the localization of rDNA in transcriptionally silent (one-cell and early two-cell) and transcriptionally active (late two-cell) mouse embryos by highly sensitive fluorescence in situ hybridization with probes complementary to mouse rDNA repeats. The results obtained showed that irrespective of the rDNA transcriptional status, one or more NPBs per nucleus were not structurally associated with rDNA. These observations support the idea that NPBs are heterogeneous in their ability to recruit rRNA genes and thus to participate in reassembly of the mature nucleolus. As in somatic cells, and despite the absence of the characteristic nucleoli, the general arrangement of rRNA genes in early mouse embryos reflected the intensity of rDNA transcription. Ribosomal RNA genes were unequally distributed with respect to repeat putative copy numbers between nucleolar organizing region (NOR)-bearing chromosomes at the first cleavage division, and more strikingly, between sister chromatid NORs of a single nucleolar organizing chromosome. The latter indicates that sister chromatids might harbor various numbers of rRNA gene copies, and that the genes might be unequally distributed between the two blastomeres during the first cleavage mitosis.

Sister chromatid differentiation and isolabeling of chromosomes

Isolabeling observed during sister chromatid differentiation (SCD) was studied from human skin fibroblasts by the fluorescence-plus-Giemsa (FPG) technique. Bromodeoxyuridine (BrdU) was fed to exponentially dividing cells for 52 h to enable completion of two consecutive cycles of DNA replication. During this period, the late-replicating regions of some chromosomes were able to go through three replication cycles. These chromosome regions had evidently incorporated BrdU bifiliarly in both chromatids and hence, on staining with FPG, appeared isostained (isolabeled). Thus, incubation of exponentially dividing cells with BrdU for a period longer than that required for two cell cycles appears to be a suitable method for revealing the late-replicating regions of the genome, such as the X chromosome in a human female, as isolabeled. In another experiment with Indian muntjac chromosomes, isolabeled segments were darkly stained, which suggested unifilar incorporation of BrdU. In this case, unequal crossing-over or an unequal distribution of thymine residues probably is responsible for the isolabel.

Detection of lateral asymmetry in the C-bands of mouse-embryo chromosomes exposed in utero to mitomycin C

Chromosomes from 10-day mouse embryos were C-banded and examined for aymmetric bands between sister chromatids. No occurrences of lateral asymmetry (LA) were observed in this series. However, when 10-day embryos were exposed in utero to a single maternal injection of 10 mg/kg mitomycin C 4--5 h prior to metaphase arrest with colcemid, a significant level of asymmetric C-banding was demonstrated in their chromosomes. As many as 4 LAs per cell were seen in 2 of the 50 metaphase spreads examined.