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Every human life begins with the fertilization of an egg (1). Once the egg and the sperm have fused, the parental chromosomes need to be united. To this end, the egg and sperm chromosomes are first packaged into two separate membrane-enclosed nuclei. These nuclei then slowly move toward each other and break down in the center of the fertilized egg, called the zygote. Only then are the maternal and paternal chromosomes united—but not quite. Surprisingly, the parental chromosomes do not mix immediately but instead occupy distinct territories in the zygote throughout the first cellular division (2, 3). How the autonomy of parental genomes is retained after fertilization has remained unclear. On page 189 of this issue, Reichmann et al. (4) used elegant microscopy methods to illuminate this special moment, when the parental chromosomes first meet in live mouse zygotes, and follow how the chromosomes become distributed as the zygote divides. Their findings reveal an unexpected mechanism that keeps the parental genomes apart during the first division of the embryo: The male and female chromosomes each assemble their own chromosome separation machineries. This increases the probability that chromosomes are separated into multiple, unequal groups, which may compromise embryo development and give rise to spontaneous miscarriage.
Reichmann et al. labeled the maternal and paternal chromosomes in different colors by taking advantage of distinct DNA sequences (5) in the parental chromosomes, which came from different mouse strains (6). To follow in detail how the chromosomes are united, zygotes have to be imaged at very high spatial and temporal resolution. However, embryos are light sensitive, which has hindered a detailed analysis in the past. The authors overcame this limitation by using an innovative light-sheet microscope, which illuminates the embryo selectively in the region of interest but not in adjacent regions, as is the case with standard microscopy approaches (7). This reduces the amount of light that the embryo is exposed to. Moreover, this method is fast and allowed the authors to reconstruct the entire volume that is occupied by the chromosomes with unprecedented spatial and temporal resolution.
The authors tracked the maternal and paternal chromosomes as they first met. In addition, they imaged microtubules, the proteinaceous fibers that form the spindle apparatus that captures, aligns, and distributes the chromosomes equally between the two daughter cells of the dividing zygote. Surprisingly, Reichmann et al. found that the maternal and paternal chromosome masses each assemble a separate spindle structure that autonomously initiates chromosome alignment (see the images). Later, the two spindles merge into a single spindle. However, the maternal and paternal chromosomes remain in separate regions of the merged spindle and do not mix.
Whether the spatial separation of parental chromosomes has any advantages for the developing mammalian embryo is unclear. However, the fact that zygotes have a dual spindle creates a previously unforeseen source of potential error. The final task the zygote has to accomplish before it divides is to align the two spindle axes in parallel to each other, so that the two spindles can merge into a compact dual structure. If the poles of the spindles fail to align and merge, the genetic material of the zygote could be pulled into three or four directions instead of two (see the figure). Reichmann et al. demonstrate that spindle misalignment leads to the formation of multinucleated embryos that have more than one nucleus per cell. It is easy to envision that such an undesired partitioning of the DNA would have a negative impact on the fidelity of subsequent cell divisions and hence might compromise embryo development. Indeed, experience from in vitro fertilization (IVF) clinics shows that early human embryos cultured in vitro before implantation frequently have multiple nuclei in their cells (8) and then fail to develop further (9).
Upon fertilization, parental chromosomes need to be united. Unexpectedly, in mice, male and female chromosomes are kept apart on separate mitotic spindles during the first division of the zygote. This increases the probability of forming multinucleated cells.
To evaluate the implications of these findings for human infertility, it is important to investigate whether cells with multiple nuclei in human embryos are also caused by defects during dual-spindle merging, as reported for mouse embryos in this study. In humans, the sperm delivers a centrosome into the zygote (10, 11). Centrosomes are microtubule-organizing centers that help to generate a bipolar spindle in somatic cells (12). However, in mice, the sperm does not carry a centrosome, and the zygotic spindle needs to assemble without the help of centrosomes (1). Reichmann et al. demonstrate that chromosomes play a key role in driving spindle assembly in the mouse zygote. The chromosome-based spindle assembly mechanism seems to create a permissive environment for the formation of two independent spindle structures.
The presence of centrosomes in human zygotes could facilitate the assembly of a single bipolar spindle. However, it is still unclear whether spindle assembly in human zygotes is primarily centrosome driven, or whether they also assemble two distinct spindles from chromosome surfaces. Interestingly, spindle assembly in unfertilized human eggs is largely chromosome driven (13), which raises the possibility that also in human zygotes, even in the presence of centrosomes, chromosome-driven spindle assembly may occur.
In many countries, the law considers that new human life begins when parental chromosomes are united upon fertilization (14). This work is a reminder that the definition of the unification of parental genomes as the beginning of life is not as clear cut as was previously assumed. How we define the “beginning of life” not only has ethical implications but also practical repercussions for fertility treatments. This is because in some countries, such as Germany (14), parental nuclei are allowed to merge and hence life is allowed to “begin” in IVF clinics only in a very limited number of in vitro–cultured zygotes, all of which have to be transferred to the mother. This policy increases the rate of multiparous pregnancies, which are associated with severe risks to the health of the mother and her children. Additionally, because identification of the most promising fertilized eggs has to legally occur before the parental genomes have merged (so that embryos with merged genomes, defined as being human life, are not discarded), embryologists have to select embryos for implantation at the zygotic stage. Because our understanding of early human embryo-genesis is still poor, it is difficult to select at this early stage the embryos that have the highest chance of dividing correctly, implanting, and giving rise to a healthy pregnancy. Considering that there is now comprehensive evidence that the parental DNA does not mix in live zygotes, this study highlights that a dialogue is needed to reshape the legal definition of when life begins, so that it is supported by up-to-date scientific knowledge. By revisiting this definition, we could increase the chances of a healthy pregnancy among couples who otherwise struggle to conceive.