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The contribution of chromosomal abnormalities to recurrent pregnancy loss (RLP) is reviewed in the paper. Data from conventional cytogenetic analysis of the karyotype of parents and spontaneous abortions, as well as the results of molecular cytogenetic investigations and preimplantation genetic diagnostics, are discussed. Information about the significance of epigenetic impairments (abnormalities of imprinting and X-chromosome inactivation) for recurrent pregnancy loss is also considered. Cytogenetic analysis of products of conception enables ascertainment of the causes of embryonic death in a large proportion of families, more accurate estimation of the therapeutic efficiency of treatment and drugs (when women with abnormal embryos were excluded), and a statistically valid prognosis about the next pregnancy outcome.

The current knowledge on multiple epimutations in imprinted genes and their impact on the disturbances of human prenatal development and the emergence of hereditary diseases are reviewed and analyzed. Genetic mechanisms regulating the epigenetic state of imprinted loci in the human genome are discussed.

The methylation profiles of the placental tissues of human embryos with normal karyotype and trisomy 16 were compared using Infinium HumanMethylation27 BeadChip array (Illumina, United States). Numerous differences between the extraembryonic tissues with diploid and aneuploid karyotypes were observed. The extraembryonic mesoderm of embryos with trisomy 16 appeared to be less methylated compared to the diploid tissue, whereas the cytotrophoblast of aneuploid embryos was hypermethylated. The presence of the supernumerary chromosome was shown to influence the epigenetic profile of the genome changing the level of methylation of CpG sites of all chromosomes. However, the biggest number of differentially methylated loci was found on the chromosome 16. Besides, more often the epimutations were tissue-specific. The hypomethylated genes in both tissues belong to the groups of genes responsible for different metabolic processes, whereas the hypermethylated genes control the processes of development, cell adhesion, immune response, and response to stimulus.

To date the question of epigenetic mechanisms of gene regulation in the context of cardiovascular diseases is of a considerable interest. Here, for the first time DNA methylation profiles of vascular tissues of atherosclerotic patients have been analyzed with using the microarray Infinium HumanMethylation27 BeadChip ("Illumina", USA). As the result, within 286 genes 314 CpG-sites that varied significantly in the DNA methylation level between the tissue samples of carotid (in the area of atherosclerotic plaques and nearby macroscopically intact tissues of the vascular wall) and mammary arteries as well saphenous veins have been identified. The most pronounced differences in the methylation level were registered for CpG-sites of homeobox genes HOXA2 and HOXD4 as well as imprinted gene MEST. In particular, these genes were found to be hypomethylated in the carotid atherosclerotic plaques compared to their methylation patterns in intact tissues of internal mammary arteries and saphenous veins.

For the first time in a comparative perspective the epigenetic status of the benign proliferative processes, breast cancer, and metastases to regional lymph nodes was studied using DNA methylation microarray "GoldenGate Cancer Panel I" ("Illumina", USA). The functional groups of differentially methylated genes were identified in each set of samples. The genes that regulate cell proliferation and mobility were methylated in samples with benign proliferative processes. An aberrant methylation of the genes responsible for cell differentiation and proliferation, as well as protein phosphorylation and cell mobility was observed in the samples with malignant phenotype. Differential methylation of the genes that regulate cell adhesion, the formation of anatomical structures, angiogenesis, immune response, signal transduction, and protein phosphorylation was found in the samples with metastases to regional lymph nodes in comparison with the morphologically unaltered breast epithelium. The tissues from the benign proliferative processes and metastases to regional lymph nodes were generally characterized by a relatively lower level of epigenetic variability in comparison with the tissues of the primary tumor.

To assess the frequency and spectrum of chromosome aberrations and micronuclei in peripheral blood lymphocytes of patients with parotid salivary gland tumors and relapse of breast cancer during the course of neutron therapy.Samples of peripheral blood were obtained from 9 patients with parotid salivary gland tumors (T3N0-3M0) and 8 patients with relapse of breast cancer before, after first fraction and at the end of neutron therapy. The treatment course specified 5.5-8.4 Gy (equivalent to 23-44 Gy of photon irradiation) with 1.3-2.2 Gy per fraction for patients with parotid salivary gland tumors and 4,8-8.0 Gy (equivalent to 30-40 Gy of photon irradiation) with 1.6 Gy per fraction for patients with relapse of breast cancer. Control group established for conventional cytogenetic analysis consisted of 15 healthy persons. Assessment of chromosome aberrations frequency was performed on routinely stained metaphase plates. Lymphocytes from the same patients were analyzed by micronucleus test in combination with fluorescent in situ hybridization (FISH) using pancentromeric DNA probe.Level of chromosome aberrations and micronuclei significantly increased in lymphocytes of patients from both groups during neutron therapy (P0.05). This increase was mainly due to chromosome-type aberrations and centromere-negative micronuclei. The prevalent types of aberrations are in agreement with theoretical mechanisms of neutron effects on cells.Cytogenetic effects of fast neutron therapy in lymphocytes of patients with parotid salivary gland tumors and relapse of breast cancer were observed. A positive dynamics of radiation-induced chromosomal damages formation during the course was denoted in lymphocytes of cancer patients in both groups.

Genomic imprinting is one of the most significant epigenetic phenomena, which is involved in the support of eutherians and human embryo development. Molecular mechanisms of imprinting disturbance in the pathology of pre- and postnatal ontogeny are related to a considerable degree to aberrant DNA methylation of imprinted genes. At present time data about multiple abnormalities of DNA methylation arising simultaneously in several imprinted loci are accumulated. This fact brings up the problem of interpretation of imprintome structural and functional organization, as well as interaction of imprinted genes. At present study DNA methylation analysis of 51 imprinted genes in placental tissues of human spontaneous abortions was performed. The presence of several epimutations affected from four to 12 imprinted genes was observed in each embryo. Majority of epimutations (78%) had a postzygotic origin. It was shown for the first time that the total incidence of abnormal DNA methylation of maternal and paternal alleles of imprinted genes, which lead to suppression of embryo development, is significantly higher than the incidence of epimutations, which can lead to stimulation of ontogenesis processes. This fact supports at the epigenetic level the "sex conflict" hypothesis, which explains the appearance of monoallelic imprinted genes expression in the evolution of mammals.

Effects of ionizing radiation registered in cells after low dose irradiation are still poorly understood. A pulsed mode of irradiation is even more problematic in terms of predicting the radiation-induced response in cells. Thus, the aim of this paper was to study and analyze the effects of dose and frequency of pulsed X-rays on the frequency of radiation-induced DNA double-strand breaks and their repair kinetics in human peripheral blood lymphocytes in vitro. Analysis of radiation-induced gammaH2AX and 53BP1 repair foci was used to assess the DNA damage in these cells. The dose-response curve of radiation-induced foci of both proteins has shown deviations from linearity to a higher effect in the 12-32 mGy dose range and a lower effect at 72 mGy. The dose-response curve was linear at doses higher than 100 mGy. The number of radiation-induced gammaH2AX and 53BP1 foci depended on the frequency of X-ray pulses: the highest effect was registered at 13 pulses per second. Moreover, slower repair kinetics was observed for those foci induced by very low doses with a nonlinear dose-response relationship.

Metastasizing is one of the key stages in tumor development. Understanding this process is necessary for effective diagnosis, therapy and prediction of clinical outcome. Some recent data suggest the possibility of metastatic phenotype cells appearance at the early stages of tumor evolution in contravention with generally accepted hypothesis of linear metastatic process development. In this study we have performed a comparative analysis of the array-based DNA methylation profile in biopsy samples of patients with benign breast disorders, breast cancer and lymphogenous metastases. In some cases the biopsy samples dated back to different stages of the same tumor. For the analysis the GoldenGate Methylation Cancer Panel I was used. The DNA methylation level in 1,505 CpG-sites was similar in samples from patients with benign breast disorders and lymphogenous metastases. Our data support the hypothesis of the early appearance of cell clones responsible for the tumor limphogenous dissemination. Epigenetic component apparently plays an important role in this process.

For the first time using genome-wide Infinium HumanMethylation27 BeadChip Array (Illumina, USA) DNA methylation pattern was determined in the human cytotrophoblast and extraembryonic mesoderm. These tissues are the derivatives of trophectoderm and inner cell mass of blastocyst and have substantial differences in dynamics of epigenetic genome reprogramming during early stages of differentiation. The genome of the extraembryonic mesoderm cells has been shown to be more methylated compared to the cytotrophoblast similarly to other mammalian species. Differences in methylation pattern of single CpG-dinucleotides and dinucleotides localized within promoter CpG-islands have been found. It has been shown that the majority of single CpG-dinucleotides in both tissues were methylated whereas promoter CpG-island sites were not. Comparative analysis revealed 202 differentially methylated genes in extraembryonic mesoderm and 40 genes in cytotrophoblast. These genes are responsible for diverse biological processes. However, in the extraembryonic mesoderm the main functional groups included genes responsible for DNA binding and transcriptional factors' activity whereas in the cytotrophoblast--for transport and protein and cytokine secretion.

The analysis of plutonium production factors has been carried out by using two methodical approaches: assessment of chromosomal aberrations level in routine and G-banded metaphases and molecular-cytogenetic investigation of aneugenic/clastogenic damages in cytokinesis-block binuclear lymphocytes by FISH with centromere specific DNA probes. The obtaining data point out for the first time about both aneugenic and clastogenic influences of incorporated 239Pu with activity range from 0.37 to 6.95 kBq. Correlation analysis of chromosome aberrations with cytome abnormalities allowed finding significant connection between number parameters of metaphase and interphase approaches. The results of this study support the suggestion that aberrant chromosomes are involved preferable in aneugenic events. The FISH technique in binucleated cytokinesis-blocked lymphocytes allows extending of detecting spectrum of chromosome damages and glance of aneugenic mechanisms. Correlations between metaphase and interphase-FISH results point out a high sensitivity of FISH cytome assay, which could be used as an independent test for detection both clastogenic and aneugenic environment influences.

Risks related to occupational exposure of humans to ionizing radiation remains a focus of attention. Of special importance is the poorly understood effect of ionizing radiation on the rate of numerical chromosome aberrations in human somatic cells. The purpose of this study was characterization of a radiation-induced increase in the rate of aneuploidy induced by incorporated plutonium-239 (239Pu) in somatic cells of chemonuclear industry workers. Significant differences in the chromosome 2, 7, 8, 12, and Y aneuploidy rate have been found between subjects with incorporated 239Pu and control subjects have been demonstrated. Chromosome nondisjunction has been found to be the main mechanism of aneuploidy induced by incorporated 239Pu.

Aneuploidy is among the most serious impairments of hereditary material in somatic and germline cells of living organisms. Chromosome loss or the appearance of an extra homolog in the chromosome set can result in either cell death or the development of various neoplasms with high probability of malignancy. It was traditionally believed that ionizing radiation produces primarily a clastogenic effect. However, there is apparently an aneugenic component of radiation, with mechanism different from that of structural chromosome damage. The present review focuses on the evidence for the existence of the aneugenic effect of ionizing radiation in mammalian and human somatic cells.

In this study we present the results of molecular-cytogenetic investigation of blood cells of fertile women of reproductive age. We used cultivated and uncultivated cells and two sets of FISH probes with direct and indirect labeling. The mean aneuploidy levels of 4 chromosomes were determined and statistical limits were scored. Aneuploidy levels determined in cultivated lymphocytes by multicolor FISH varied from 0.1 to 1.3%. The mean aneuploidy level for all chromosomes (13, 18, 21 and X) was 1.39%. The limit mutation detection was 3.4%. We found differences in results obtained with the use of direct and indirect labeling. It was shown that cultivating process influenced the aneuploidy level. Absolute error of FISH technique was 0.13%, relative error - 4.08%.

Disturbance of the epigenetic status of the H19 and KCNQ1OT1 imprinting centers of chromosome 11 and the CDKN1C imprinted gene in early human embryolethality have been studied. This is the first study to detect hypomethylation of KCNQ1OT1 in the maternal homolog in placental tissues from some (9.5%) spontaneous abortions with impaired cell proliferation during the first trimester of pregnancy. Tissue specificity of the aberrant methylation status of the imprinting center has been found. A hypothesis on the postimplantation origin of epimutations in somatic cells of the developing embryo is put forward. The selective role of epimutations of imprinted genes in early human ontogeny as compared to uniparental chromosome inheritance is considered; estimation of the epigenetic risk entailed in using assisted reproductive technologies is discussed.

The methylation status of the cell cycle control gene RB1 has been studied in placental tissues of spontaneous abortions of the first trimester of pregnancy with mosaic variants of numerical chromosomal abnormalities verified by a molecular genetic examination. Aberrant methylation of the gene promoter region has been revealed for the first time in 20% of embryos with chromosomal mosaicism that died in utero. A maximum frequency of epimutations was recorded in a group of embryos with a low level of abnormal cells for which mitotic errors are most likely to determine the formation of mosaic aneuploidy in primary euploid zygotes. It has been suggested that aberrant epigenetic genomic modifications at early stages of human embryonic development can be one of the mechanisms promoting genomic instability realized in the form of mosaic abnormalities of the karyotype that are incompatible with the normal course of embryogenesis.

Genomic imprinting is an epigenetic phenomenon characterized by monoallelic expression of the genes depending on their parental origin. The molecular basis of this expression is covalent modifications of DNA and histones that are formed during maturation of germline cells. Abnormalities of the establishment of genome imprinting during gametogenesis or its maintenance at various stages of development, caused by aberrant epigenetic modifications of the chromatin, predominantly disturbance of DNA methylation state, are a form of mutational variability of imprinted genomic loci. In this review, we consider the spectrum of epimutations of imprinted genes, present their classification, and discuss possible causes of their appearance and their role in etiology of hereditary human diseases.

To date, a wide range of assisted reproductive technologies is available for patients with impaired fertility. In general, the current methods of reproductive medicine are considered safe and do not significantly increase the frequency of birth of children with diseases or congenital malformations. However, the evidence has been accumulating in literature on higher risk of genomic imprinting diseases (Beckwith-Wiedemann and Angelman syndromes) as a result of using assisted reproductive technologies. In most cases examined, the appearance of these syndromes was explained by defective methylation status of imprinted genes. It has been suggested that manipulations with gametes and embryos during the period of total epigenetic modification of their genomes may act as potential risk factors of assisted reproductive technologies. Moreover, overcoming many natural reproductive barriers may contribute to the development of some pathological phenotypes. The review summarizes current views on epigenetic risk factors associated with assisted reproductive technologies.

The ploidy level of noncultivated extraembryonic tissues was studied by fluorescence in situ hybridization in 30 human I trimester spontaneous abortions with tetraploid or diploid-tetraploid karyotype after conventional cytogenetic analysis. Only thirteen embryos (43 %) were verified to be tetraploid that provides evidence for the hypothesis of placental cell polyploidization during long-term in vitro cultivation. It is shown that preferred compartmentalization of tetraploid cells in the inner cell mass derivatives is associated with blighted ovum - the most severe type of human embryo dysmorphogenesis.

In most cases, the cause of embryo and fetus death remains unclear although the multifactorial causes are suspected. The polymorphic C677T and A1298C variants of the MTHFR gene are associated with an increase in the level of homocysteine, which is risk factor of pregnancy loss. The subject of this study is analysis of genotypes and haplotypes of C677T and A1298C polymorphic variants of MTHFR genes in the groups of spontaneous abortions with the normal karyotype and newborns in the Tomsk population. Between these groups, no statistically significant differences were determined in the allele, genotype, and haplotype distributions of C677T and A1298C polymorphisms of the MTHFR gene. The haplotype frequencies of C677T and A1298C polymorphic variants of MTHFR gene in the Russian populations, which proved to be similar to those in most European populations, are presented.

We have previously reported a high rate of tetranucleotide DNA repeat mutations, including mutations of both germline and somatic origin, in spontaneous human abortuses. To analyze in more detail mutational microsatellite (MS) variability in meiosis and its possible association with disturbed embryonic development, we have conducted a comparative study of mutation rates of a complex of 15 autosomal tetranucleotide MSs in 55 families with healthy children and in 103 families that have had spontaneous abortuses with normal karyotypes. In the families with miscarriage, the gametic MS mutation rate was higher than in the families with normal reproductive function (4.36 x 10(-3) versus 2.32 x 10(-3) per locus per gamete per generation), but this difference was statistically nonsignificant (P = 0.25). No association of MS mutations with familiar miscarriage was found. Mutations at the MS loci studied were recorded almost 3 times as often in spermatogenesis as in oogenesis, which is likely to result from a greater number of DNA replication cycles in male germline cell precursors than in female ones. Mutations increasing and reducing the MS sequence length appeared at virtually the same rate. Changes in MS DNA sequence length per one repeated element, i.e., single-step mutations (93% of cases) exceeded all other events of allele length change. The highest number of mutations (81.2%) was found in longer alleles. This distribution of mutations by size, direction, and parental origin corresponds to the multistep mutation model of their emergence via mechanism of DNA strand slippage during replication.

We carried out systematic studies of the contribution of uniparental disomy for eight human chromosomes, 2, 9, 11, 15, 16, 19, 20, and 21, to the etiology of spontaneous mortality of human embryos. Most of these chromosomes have regions with orthologous imprinted genes syntenic with those on mouse chromosomes, the disturbed expression of which is related to embryolethality in mice. Screening of uniparental disomy in spontaneous 5- to 16-week abortuses was performed by evaluation of the pattern of inheritance of alleles of polymorphic microsatellite loci located in the studied chromosomes. A total of 100 human embryos with cytogenetically determined normal karyotype were studied, in which arrest at the early stages of intrauterine development was determined by ultrasound examination of pregnant women. During this study, 13 embryos were discarded due to karyotype anomalies or nonpaternity. No cases of uniparental disomy were found among the 87 studied abortuses for any of chromosomes studied. The analysis of the results of this study and four other studies concerning the search for uniparental disomy in dead embryos and fetuses did not reveal its elevated frequency in spontaneous abortuses as compared to the theoretically expected value based on evaluation of the probable combination of meiotic errors in human gametes. The data we obtained suggest that, first, uniparental disomies for human chromosomes that have regions with orthologous imprinted genes syntenic with mouse chromosomes do not contribute noticeably to the death of human embryos at the early developmental stages and, second, the mechanisms underlying embryolethality as a result of disturbed expression of imprinted loci differ markedly in mammals evolutionarily remote from one other.

The results of standard cytogenetic analysis of the long-term cultures of embryonic fibroblasts of 478 first-trimester spontaneous abortions were retrospectively reviewed. In 16% of embryos with cytogenetically confirmed karyotype 46,XX, the Y chromosome was found by molecular genetic methods. Prior to obtaining the chromosome preparations, the cell cultures of Y chromosome-carrying embryos were maintained for a longer period than the cultures of embryos without the Y chromosome. Thus, a late entry of a culture into the logarithmic growth phase serves as marker of maternal cell contamination. We developed a mathematic model for assessment of karyotype incidence and the "sex ratio" of spontaneous abortions, taking into account risk of maternal cell contamination in extraembryonic tissue cultures. Thus estimated, the incidence of chromosomal abnormalities in the studied sample increased from 54.6 to 60.3% and the expected sex ratio increased from 0.66 to 1.02 in abortions with normal karyotype. Using molecular analysis of inheritance of polymorphic DNA markers of six autosomes (2, 11, 16, 19, 20, and 21), the proposed model was tested on 60 embryos with karyotype 46,XX and their parents. Numerical chromosome abnormalities were revealed in uncultured tissues of seven abortions (11.7%), including four without the Y chromosome, which is in a good agreement with the expected incidence of karyotype abnormalities (8.3%) predicted by our model. In view of this, estimating risk of maternal cell contamination in embryonic cell cultures seems necessary for correctly assessing the effect of natural selection in humans, for understanding the mechanisms that determine the sex ratio, and for evaluating the precision of prenatal cytogenetic diagnosis of chromosomal abnormalities.

Autosomal monosomies represent a severe form of genomic disbalance which determines elimination of human embryos already at the preimplantation stages. As a rule, they occur very rarely in the materials of spontaneously aborted embryos and fetuses. Molecular-cytogenetic studies were carried out on the karyotype of cells of 60 spontaneous abortuses of I trimester of pregnancy with cell degeneration or absence of cell proliferation in the cultures, as a result of which the cells could not be studied using the standard metaphase analysis. The embryos were characterized by an unexpectedly high frequency of mosaic variants of monosomies for chromosomes 7, 15, 21, and 22, which amounted to 19% of all chromosome aberrations. Lethal forms of monosomies for human chromosomes 7 and 15 were described for the first time, since they are not found in spontaneous abortuses by standard cytogenetic methods. A hypothesis was proposed which accounts for the possibility of early postimplantation lethality of the embryos with mosaic forms of autosomal monosomies. The differences were found between the cells with monosomies for different autosomes in the mechanisms of origin, intertissue localization, and phenotypic effects. It was shown that monosomies for chromosomes 7, 15, 21, and 22 in a mosaic state with the normal cell line can be compatible with the early stages of postimplantation differentiation of the cytotrophoblast. Predominant compartmentalization of the cells with monosomies for chromosomes 21 and 22 in the extraembryonic mesoderm, a derivative of epiblast, can be a critical factor, which makes it impossible the normal morphogenesis of embryonic structures.

Karyotyping of noncultivated cells of 60 first-trimester spontaneous abortions (blighted ova and missed abortions) was carried out using fluorescence in situ hybridization (FISH) with centromere-specific DNA probes for all chromosomes of the karyotype. Conventional cytogenetic study of these abortions was impossible because of cell culture failures. The algorithm is proposed for molecular cytogenetic FISH analysis of interphase karyotypes. Chromosome abnormalities were found in 32 fetuses (53.3%). In groups of missed abortions and blighted ova, the frequency of numerical chromosome abnormalities was 50 and 60%, respectively. Both the numerical chromosome abnormalities typical of spontaneous human abortions (autosomal trisomies, sex chromosome aneuploidy, and polyploidy) and a relatively rare type of genomic imbalance unidentifiable by standard cytogenetic analysis (autosomal monosomies 7, 15, 21, and 22 in mosaic state) were observed. The frequency of these type of chromosome abnormalities comprised 19% of all known karyotype abnormalities determined in spontaneously perished embryos. Note that the level of confined placental mosaicism in embryos with low cell proliferative activity was 25%, which is substantially higher than the corresponding parameter (1-2%) determined by prenatal diagnosis of chromosome abnormalities in developing embryos. The results of interphase FISH analysis of cells with low proliferative activity in vitro suggest that the pathology of early fetal development and missed abortion in humans are associated with a wider spectrum of chromosome abnormalities.

The frequencies of autosomal trisomies in extraembryonic human tissues were estimated in the cases of different abnormalities of prenatal development, from the confined placental mosaicism (CPM) with either relatively normal embryogenesis or restricted intrauterine growth to spontaneous abortion. A tissue-specific compartmentalization was found to be characteristic of cell lines with trisomies for individual autosomes. Analysis of various phenotypical effects of chromosomal aberrations associated with mosaicism is necessarily required to understand the mechanisms and factors responsible for tissue chromosomal mosaicism. Based on analysis of the cell karyotype during prenatal diagnosing of chromosome aberrations in tissues of both extraembryonic and embryonic origin, in 1996, Wolstenholme proposed a model of CPM for individual chromosomes. According to the model, the distribution of cell lines with autosomal trisomies between extraembryonic tissues depends on the ratio between meiotic and mitotic mutations early in embryonic development. However, the model cannot be used to study tissue chromosomal mosaicism in spontaneous abortions, because little information is available on cell karyotype in embryonic tissues themselves after intrauterine fetal death. In this work, a model of tissue-specific chromosomal mosaicism was suggested based on the data on cell karyotype determined in extraembryonic tissues alone, which can be helpful in evaluating the contribution of tissue chromosomal differences into the etiology of early intrauterine death. Along with the experimental evidence, comparative analysis of the two models indicated that the meiotic chromosome nondisjunction plays the major role in trisomy formation and the resultant spontaneous arrest of embryonic development. Other factors responsible for tissue-specific distribution of chromosomal aberrations are also discussed. These are differences in cell proliferative activity, as well as changes in compartmentalization and migration of cells with abnormal karyotypes.

The problem of the functioning specificity of sex chromosomes during the early stages of embryogenesis in man and the associated problem of the sex ratio in spontaneous and induced abortions, as well as in newborns, remains open. We have conducted a cytogenetic examination of 342 spontaneous abortions divided into three clinical groups on the basis of the severity of the developmental disturbances of the embryo: spontaneous abortions sensu stricto with a developed embryo without any significant intrauterine delay of development (n = 100), nondeveloping pregnancies (n = 176), and anembryonic fetuses (n = 66). The frequency of chromosomal mutations in these groups was 22.0, 48.3, and 48.5%, respectively. Statistical analysis has demonstrated significant differences between the studied groups in the frequencies of the normal and abnormal karyotypes: the major contributions to these differences were associated with autosomal trisomy, triploidy, and 46,XY karyotype. The presence of 46,XY may reflect specific genetic mechanisms of prenatal mortality of embryos with normal karyotype, associated with sex and/or with the imprinting of X-chromosomes. The sex ratio in spontaneous abortions with normal karyotype was as follows: 0.77 for spontaneous abortions with well-developed embryos without any significant intrauterine delay of development; 0.60 for non-developing pregnancies; and 0.31 for anembryonic fetuses. An analysis of DNA from the embryos and their parents has demonstrated a low probability of contamination of cell cultures with mother cells as a possible source of prevalence of embryos with 46,XX karyotype among spontaneous abortions. Nondeveloping pregnancies and anembryonic fetuses showed statistically significant differences in the sex ratio (1.11) from the control group consisting of medical abortions. Differences in the sex ratio were due to an increasingly lower proportion of embryos with karyotype 46,XY (relative to the expected one) among the fetuses with an increased severity of developmental disturbances. The statistical "chances ratio" index also provided evidence that embryos with 46,XY karyotype had a higher propensity to produce a well-formed fetus as compared with the female embryos. We propose that the expression of genes of the maternal X-chromosome in XY embryos supports a more stable development during early embryogenesis as compared with XX embryos. In the latter case, normal development is coupled with the operation of an additional mechanism for compensation of the dose of X-linked genes. Operation of this mechanism increases the probability of disturbances in female embryos. A higher viability of XY embryos during the early stages of ontogenesis in man appears to explain their underrepresentation in samples of spontaneously aborted embryos and appears to be the major factor responsible for the deviation of the sex ratio from the theoretically expected value.