Chromosome segregation errors during early embryonic development

Abstract

Background

Mitosis maintains a genome's genetic information in daughter cells by accurately segregating chromosomes. However, chromosome aberrations are common during early mammalian embryogenesis. Chromosomal abnormalities during the early stages of embryogenesis result in the formation of mosaic embryos, wherein cells with normal genomes coexist with cells exhibiting abnormal genomes. The precise frequency and etiology of such abnormalities remain unclear. It is postulated that these aberrations contribute to the etiology of a number of conditions, including infertility and congenital diseases such as Down's syndrome.

Methods

This review synthesizes current literature and data to elucidate the causes and implications of chromosome aberrations in early mammalian embryos. It places particular emphasis on identifying patterns of mosaicism and investigating the underlying mechanisms responsible for these abnormalities.

Main Findings

The underlying causes of chromosome abnormalities in early embryos were examined in the context of DNA replication and embryonic development.

Conclusion

A deeper understanding of chromosome abnormalities in early embryos could help develop new infertility treatments and advance research on cancers caused by these abnormalities. This article reviews current knowledge and gaps in understanding chromosome segregation abnormalities during embryogenesis and future directions in this field.

1. INTRODUCTION

The maintenance of genomic integrity is essential for cells, as it ensures the accurate distribution of chromosomes to daughter cells during mitosis, which is a crucial process for the perpetuation of genetic information. In general, abnormalities in chromosome segregation are rare in somatic cells, occurring in less than 1% of cases and rarely resulting in chromosome aberrations. However, chromosome aberrations have been demonstrated to occur with high frequency during meiosis and early embryogenesis in mammalian oocytes, including those of humans ¹ , ² , ³ , ⁴ (Figure 1). These aberrations are postulated to be the underlying cause of congenital genetic disorders such as Down syndrome and infertility. In recent years, there has been a notable increase in interest within the field of assisted reproductive technologies (ART) with regard to chromosomal aberrations in early embryos. A substantial body of research has been conducted on chromosome aberrations during meiosis of oocytes, with a high frequency of aberrations, particularly in the initial meiotic phase, being observed. The underlying causes of these aberrations have also been elucidated in some cases. ⁵ , ⁶ , ⁷ Nevertheless, the precise causes and prevalence of chromosome aberrations in early embryos remain uncertain. In this article, we present our recent findings on the causes of chromosome aberrations in early embryos and discuss the current status and future prospects of chromosome aberrations in early embryos. ⁸

FIGURE 1.

Chromosome aberrations and mosaic embryo formation in early embryonic development. The conceptual diagram illustrates the developmental process of an early embryo and the formation of a mosaic embryo, which occurs when a dividing blastomere experiences an aberrant chromosome segregation during the 4‐cell stage.

2. CHROMOSOME ABERRATIONS AND GENETIC DISEASES

Chromosome distribution abnormalities represent a significant etiological factor in the pathogenesis of genetic disorders. Chromosome aberrations are classified into two main categories: numerical and structural aberrations (Figure 2). Each category is associated with a distinct set of genetic diseases.

FIGURE 2.

Types of chromosome segregation errors. A schematic diagram illustrates the process by which numerical and structural aberrations occur during chromosome segregation.

2.1. Numerical aberrations

Numerical aberrations are defined as structural alterations to the number of chromosomes within a cell. Abnormalities in the number of chromosomes are defined as aneuploidy.

2.2. Trisomy

Trisomy is defined as the presence of three copies of a chromosome in a cell, which is a numerical aberration. A condition in which an additional chromosome is present beyond the typical number. Notable examples include Down syndrome, which is caused by trisomy of chromosome 21; Patau syndrome, which is caused by trisomy of chromosome 13; Edwards syndrome, which is caused by trisomy of chromosome 18; and Klinefelter syndrome, which is caused by trisomy of the X chromosome.

2.3. Monosomy

Monosomy is defined as the absence of a single chromosome. A condition in which a single chromosome is absent. An illustrative example is Turner syndrome, which is caused by monosomy of the X chromosome. Autosomal monosomy typically results in embryonic lethality.

2.4. Polyploidy

Polyploidy is defined as the condition in which a chromosome has more than one multiple of the normal number of chromosomes. A condition in which a chromosome has a number of chromosomes that is a multiple of the normal number. For example, a triploid has three times the normal number of chromosomes and is often incompatible with life.

Structural aberration: A condition in which a portion of a chromosome is altered, whether by breakage, amplification, or reduction.

2.5. Deletion

A deletion is defined as the loss of a portion of a chromosome. A reduction in the size of a chromosome. Examples of such conditions include cat's meow syndrome, which is caused by the deletion of 5p; retinoblastoma, which is caused by the deletion of 13q14; Wilms tumor, which is caused by the deletion of 11p13; and Li‐Fraumeni syndrome, which is caused by the deletion of 17p13.

2.6. Duplication

A condition in which a portion of a chromosome is duplicated, resulting in the production of excessive gene products and the subsequent manifestation of symptoms. The symptoms and severity of the disease vary according to the degree of duplication, and thus, no specific name has been assigned to the disease. However, diseases of chromosomes 1, 4, 8, 15, 17, and 22 have been reported.

2.7. Inversion

Inversion is defined as the process whereby a portion of a chromosome is fused in the opposite direction. A condition in which a portion of a chromosome undergoes a fusion in the opposite direction. Notable examples include genetic disorders associated with the Philadelphia chromosome. Additionally, inversions of chromosomes 9, 13, 15, 18, 21, and X have been documented in the medical literature.

2.8. Translocation

A translocation is defined as the movement of a portion of a chromosome to another chromosome. This phenomenon denotes the relocation of a portion of a chromosome to an alternative chromosome.

The majority of numerical and structural abnormalities are embryonic lethal; however, some have been observed to manifest as genetic disorders in the postnatal period.

3. EARLY EMBRYO DEVELOPMENT AND CHROMOSOME ABERRATION FREQUENCY

Immediately following fertilization, the fertilized zygote undergoes repeated cell division and rapidly develops into an embryo. This early cell division (cleavage) represents the initial process by which a single cell develops into an individual. It is imperative that the genetic information be accurately transmitted to the daughter cells through precise chromosome duplication and segregation. However, it is well known that abnormalities in chromosome segregation occur during this process, resulting in frequent chromosome aberrations, including aneuploidy (abnormality in the number of chromosomes). Chromosome aberrations have been identified in over 70% of fertilized eggs from infertile patients and are believed to be the primary cause of embryonic lethality and miscarriages.

The causes of chromosome aberrations can be classified into two principal categories. The initial cause is the presence of a chromosomal abnormality in the oocyte prior to the process of fertilization. Mammalian oocytes are susceptible to chromosome aberrations during meiosis prior to fertilization. ⁹ The prevalence of such aberrations is known to increase with maternal age. In other words, some fertilized zygotes commence development with inherited chromosomal abnormalities derived from the oocyte.

The second cause is the occurrence of new chromosome aberrations during the early stages of embryogenesis. The frequency of chromosome aberrations during early embryogenesis is significantly higher than in other divisions. ⁴ Indeed, it has been reported that the frequency is particularly elevated during the first three cleavages ¹ , ² , ³ , ⁴ (Figure 1). This process results the formation of mosaic embryos, wherein cells with accurate genomes and cells with inaccurate genomes are intermingled (Figure 1). The frequency of chromosomal aberrations in early human embryos is estimated from the frequency of mosaic embryos. However, there is considerable variation in the reported data, with studies based on a limited number of human embryo samples derived from assisted reproductive technology (ART) patients showing a wide range of frequencies, from 15% to over 90%. ¹ , ² , ³ , ⁴ The observed heterogeneity may be attributed to discrepancies in the origin of the samples and the methodologies employed for measurement. There is a paucity of reports that provide precise data on the frequency of mosaic embryo formation. The reason for the high frequency of unequal chromosome distribution that leads to mosaic embryo formation in early embryos has remained unclear for an extended period.

Abnormal chromosome segregation has a significant impact on embryonic development. In many cases, embryos with abnormalities fail to develop normally and are selected spontaneously as miscarriages. The majority of miscarriages occurring during the early stages of pregnancy are attributed to chromosomal abnormalities, particularly in the pre‐ and post‐implantation periods. Nevertheless, it is not inevitable that all embryos with chromosomal abnormalities will result in miscarriage. Indeed, some embryos with specific abnormalities, such as 21‐trisomy (Down syndrome) and X‐monosomy (Turner syndrome), may ultimately result in birth.

The advent of assisted reproductive technology (ART) techniques, including in vitro fertilization (IVF) and intracytoplasmic sperm injection (ICSI), has facilitated the detection of chromosome number abnormalities through preimplantation genetic screening (PGT‐A) prior to the transfer of fertilized eggs into the maternal body. It is anticipated that PGT‐A will mitigate the risk of miscarriage resulting from chromosomal aberrations by facilitating the identification of such aberrations in a timely manner. However, it should be noted that PGT‐A is not a comprehensive method for detecting all chromosomal abnormalities. Furthermore, even in embryos that do not exhibit chromosomal abnormalities, implantation and pregnancy may still be unsuccessful. While technological advances have expanded the range of strategies for preventing chromosomal abnormalities, their limitations have also underscored the intricate nature of embryonic development.

In order to gain insight into the relationship between the complexity of embryogenesis and chromosome aberration frequencies, we have recently conducted a comprehensive analysis of chromosome aberration frequencies in early mouse embryos. The lack of reliable single‐cell analysis methods has previously hindered the accurate analysis of chromosome aberration frequencies. However, the use of microscopic manipulation techniques and the single‐cell DNA replication sequencing method (scRepli‐seq) has made it possible to analyze numerical and structural aberrations at the sequencing level. ¹⁰ , ¹¹ scRepli‐seq is a one‐cell whole‐genome sequencing method that can be used to detect DNA replication aberrations and chromosomal aberrations. In the course of our investigation, we discovered that the incidence of chromosomal aberrations in early mouse embryos was markedly elevated in the third oocyte division occurring between the 4‐ and 8‐cell stages. ⁸ While the frequency of chromosome aberrations in somatic cells is less than 1%, the frequency observed in the early embryos in this study was markedly higher than that observed in somatic cells.

4. MECHANISM OF CHROMOSOME ABERRATION DEVELOPMENT IN EARLY EMBRYOS

The analysis of chromosome aberrations using scRepli‐seq revealed that approximately 80% of the aberrations were structural, involving the fragmentation of a chromosome. Furthermore, live cell imaging revealed that chromosomes were broken during chromosome segregation, suggesting that the chromosomes were not initially broken but were unable to withstand the tensile forces exerted during this process. This suggests the possibility of the existence of fragile regions within chromosomes.

One potential cause of chromosome breaks is the issue of DNA replication. It has been documented that incomplete DNA replication prior to mitosis results in abnormal chromosome segregation in unreplicated regions. ¹² , ¹³ The visualization of DNA unreplicated regions revealed the presence of numerous unreplicated regions at the four‐cell stage, which is a period during which chromosome aberrations are most likely to occur. Moreover, the level of replication stress markers was found to be elevated at the four‐cell stage, indicating that this stage presents a significant challenge for DNA replication.

It is therefore pertinent to inquire as to why mouse 4‐cell stage embryos are susceptible to stress in DNA replication. A detailed analysis was conducted to examine the status of DNA replication. Typically, DNA replication is executed in a programmed manner, whereby replication occurs initially and is subsequently followed by a subsequent phase (Figure 3). However, in mouse embryos at the 1‐ and 2‐cell stages, replication does not follow this pattern and is observed to occur randomly (Figure 3). Conversely, from the 4‐cell stage onward, the replication pattern is observed to be programmed, similar to that observed in somatic cells.

FIGURE 3.

DNA replication patterns of somatic and embryonic S‐phase. The following diagram offers a visual representation of the temporal dynamics of replication in mouse chromosome 16. The figure displays the binarized scRepli‐seq profiles of 1‐, 2‐, and 4‐cell embryos. The top of the image features red bands that demarcate the DNA that has undergone replication during the indicated phase of the cell cycle.

Normally, random replication increases the number of replicated regions and decreases the time required for DNA replication. However, the DNA replication time of 1‐ and 2‐cell stage mouse embryos was approximately 5 hours, which was not significantly different from that of somatic cells with an established replication pattern. This suggests that although replication occurs in many regions, the rate of DNA replication forks is relatively slow. The replication fork speed is observed to increase gradually after the 8‐cell stage, suggesting that 4‐cell stage embryos are in an imbalance of having a programmed replication pattern but a slow replication fork speed. This results in a slow and fragile DNA replication process.

Some reports have shown that an imbalance in replication status can readily lead to chromosomal abnormalities in somatic cells. ¹⁴ , ¹⁵ , ¹⁶ In embryos, genome instability has been shown to be dependent on DNA replication in experiments using nuclear transfer embryos. ¹⁷ The present study shows that the frequency of chromosomal aberrations changes depending on the cell cycle phase into which the donor cells are transferred. If the donor nucleus begins DNA replication before it has passed through the M and G1 phases of the first mitotic cell cycle, the genome becomes more unstable and more prone to chromosome segregation abnormalities. In light of these findings, it can be postulated that the failure of the somatic nucleus to undergo M and G1 phases results in an inability to eliminate the replication timing program, leading to the establishment of a state where replication timing persists but the replication fork rate is abnormally slow. This would have resulted in prolonged replication and an increased incidence of chromosome segregation abnormalities, which is consistent with the results of our recent study.

Our results indicate that chromosomal aberrations occur more frequently in embryos at the four‐cell stage and that approximately 50% of embryos form mosaic embryos. Normally, when an abnormality occurs in DNA replication, the cell cycle repair system is activated. However, in early embryos, the cell cycle is synchronized, and the cell with the abnormality may not be sufficiently repaired because the cell cycle proceeds in synchrony with the surrounding cells. In early mouse embryos, cell cycle synchronization is typically broken after the 8‐cell stage. ¹⁸ This suggests that the repair system begins to function after the 8‐cell stage and that the frequency of chromosome aberrations subsequently decreases. This finding is consistent with the previously reported high frequency of chromosome aberrations observed in the initial three cleavages prior to the 8‐cell stage.

Finally, as the slow replication fork speed of 4‐cell stage embryos was identified as a contributing factor to chromosome aberrations, a rescue experiment was conducted to enhance the replication fork speed. The addition of nucleosides, which are known to increase replication fork speed, at the 4‐cell stage in a time‐specific manner resulted in a reduction in the frequency of chromosome aberrations.

5. QUESTIONS AND PROSPECTS FOR THE FUTURE

It has been demonstrated that micronuclei are formed in early mouse ¹⁹ and bovine ²⁰ embryos as a consequence of chromosomal aberrations. The percentage of micronuclei is observed to increase in accordance with the advancement of the developmental stage. Specifically, the absence of telomere formation at chromosome breaks has been demonstrated to result in the occurrence of further chromosomal structural abnormalities, a phenomenon that has been observed with particular frequency in human and bovine embryos. ²⁰ , ²¹ In the present study, approximately 50% of mouse 8‐cell stage embryos exhibited mosaicism and were found to contain micronuclei. ⁸ However, studies that have examined the relationship between chromosome aberrations and fertility by observing chromosome dynamics in live mouse and bovine fertilized eggs and then transplanting them have demonstrated that even embryos with abnormalities in early mitosis can be fertile if they reach the blastocyst stage. ²² , ²³ , ²⁴ In human embryos, it has been demonstrated that mosaic embryos with abnormalities can develop normally. ²⁵ This indicates that early embryos may be capable of recovering from the mosaic state through an as‐yet unidentified mechanism (Figure 4). In human embryos, chromosomal aneuploidy has recently been demonstrated to exert effects on alterations in cell number and inadequate differentiation and development into trophic ectoderm. ²⁶

FIGURE 4.

Mechanisms exist for early embryos to recover from mosaic embryos. Early embryos showed a very high rate of mosaic embryo formation, but when these embryos were transferred, the progeny rate was as high as 60%–80%. This indicated that the embryos must have some mechanism to recover from mosaicism.

Furthermore, it is essential to determine whether alterations in DNA replication patterns are a contributing factor to the high prevalence of mosaic embryos observed in early embryos of animals other than mice. In particular, the potential applications of nucleoside addition to suppress chromosome aberrations in the context of infertility treatment and the improvement of diagnostic criteria for preimplantation diagnosis (PGT‐A) based on the mosaic embryogenesis rate are particularly promising in the case of human embryos. Further research is required to address these issues, and it is anticipated that numerous researchers will contribute to this field of study.

6. CONCLUSION

It has been well‐documented that chromosomal abnormalities occur with high frequency during the early stages of embryonic development. Furthermore, embryos in the earliest stages of development exhibit a markedly elevated incidence of mosaic embryogenesis. However, the specific details regarding which cell cycle is most frequently aberrant, which chromosome number tends to cause the aberration, and the underlying reasons why early embryos are more prone to these aberrations have remained unclear. Our recent study of early mouse embryos revealed that chromosomal aberrations are particularly likely to occur in 4‐cell stage embryos, where changes in DNA replication patterns occur, with a mosaic embryo formation rate of up to 50%. The results also indicate the existence of a mechanism by which early embryos may recover from chromosome aberrations. Further research is anticipated to elucidate this recovery mechanism, thereby enabling the development of strategies to actively protect cells from chromosome aberrations and promote their recovery.

The establishment of such technology would contribute not only to the avoidance of the risk of genetic disorders in infertility treatment and the improvement of fertility rates, but it would also be applicable in the medical field, for example, to the inhibition of the cancerous transformation of cells. In particular, the nucleoside‐added therapy has the potential to become a new option for infertility treatment. Furthermore, it may contribute to improving the criteria for preimplantation diagnosis (PGT‐A) in the future.

CONFLICT OF INTEREST STATEMENT

The authors declare no conflict of interest.

Kyogoku H. Chromosome segregation errors during early embryonic development. Reprod Med Biol. 2025;24:e12631. 10.1002/rmb2.12631