Over the past few decades, elegant studies in model systems including yeast, flies, worms, fish and frogs have compared chromosome dynamics in wildtype and mutant cells that display diverse phenotypes32
. Particularly in Caenorhabditis elegans
, studies of mutations in genes that affect the first embryonic cell divisions are enlightening. For example, in 1994 Gonczy et al
reported a collection of 48 maternal-effect embryonic lethal mutations on chromosome III that they characterized phenotypically by time-lapse DIC video microscopy. The mutations mapped to 34 loci and were characterized by defects in pronuclear migration, rotation of centrosomes and associated pronuclei, spindle assembly, chromosome segregation, anaphase spindle positioning and cytokinesis. Subsequent studies have examined the molecular mechanisms that underlie these phenotypic classes and others32
In other species such as the zebrafish, Danio Rerio
, and the frog, Xenopus laevis,
karyomeres form to accommodate the large cells formed following fertilization38
. Karyomeres are thought to provide mitotic intermediates that are comprised of chromatin masses surrounded by nuclear envelope, which then fuse to form a single nucleus as recently described in more detail by Mullins and colleagues38
. When we compare these studies to our data, we observe that a subset of images from these organisms clearly resemble those we observed with formation of multiple micronuclei in human embryos. This suggests the possibility that embryonic micronuclei observed in human embryos may be formed via conserved pathways that allow encapsulation of chromosomes in nuclear envelope either under normal developmental conditions or in response to chromosome detachment from the spindle during cleavage divisions.
In this study, we observed that chromosomally normal embryos display strict and tightly clustered cell cycle parameters, whereas chromosomally abnormal embryos exhibit more diverse parameters that may or may not overlap those of euploid embryos. By the four-cell stage of development, we observed that dynamic assessment of cell cycle parameters in conjunction with fragmentation analysis and blastomere asymmetry assists in the differentiation between the type of error (meiotic versus mitotic), detects chromosomal duplications (trisomies) and deletions (monosomies) and provides a reliable readout of the degree of mitotic mosaicism (high versus low) in human embryos. Furthermore, we observed that the generation of partial chromosomal gains and losses was restricted primarily to embryos with mitotic errors, suggesting a relationship between subchromosomal instability and aneuploidy in the human embryo. We also demonstrate that human embryonic aneuploidy and mosaicism of chromosome content between blastomeres5
may have contributions from a phenomenon that encompasses the formation of embryonic micronuclei, cellular fragmentation and resorption. Thus, practices that were once promoted such as embryo surgery for the removal of fragments are likely to result in the removal of micronuclei and perhaps genetic information and may potentially be deleterious42
On the basis of the timing of fragmentation and our time-lapse image analysis, we suggest that the human embryo may initially undergo fragmentation, rather than cell death, in response to aneuploidy. We do not currently understand the mechanisms underlying the increased aneuploidy rates and frequent fragmentation observed in cleaving human embryos. However, we relate our data here to that in other organisms where genetic instability may be associated with unique epigenetic programs of preimplantation development44
. Alternatively, generation of human embryonic aneuploidy may be related to the paternal contribution of the centrosome (and other spindle components) by the sperm for the first mitotic division47
. Regardless of mechanism, it is likely that non-invasive assessment of development of human embryos as previously described1
and further confirmed in the retrospective studies of clinical embryos will assist in the prediction of embryo viability49
. Moreover, distinction between euploid and aneuploid embryos before transfer may contribute to improvements in IVF outcomes by potentially reducing the inadvertent transfer of embryos that would most likely result in embryonic lethality and spontaneous miscarriage.