The proportion of babies born from in vitro fertilization (IVF) is rising at an exponential rate, highlighting the importance that we have a comprehensive understanding of human preimplantation development and determinants of embryo quality. The early divisions of the embryo after fertilization but before implantation and resulting mitotic errors have been studied primarily in the mouse embryo; however, thus far we have lacked the technology to characterize these critical steps in humans. Establishing an approach that can bypass genetic manipulation and microinjections of DNA or mRNA into human embryos would allow us to better uncover the processes patterning preimplantation human development.

This study aimed to evaluate human blastocyst preimplantation development with noninvasive imaging using membrane permeable fluorescent dyes. First, the dyes SPY650-DNA, which labels genomic DNA, and SPY555-actin, which labels F-actin, were validated in the mouse embryo where they produced high contrast labelling with similar results to microinjection of fluorescent mRNAs and produced offspring at a similar rate to nondyed control embryos. Live imaging data using these dyes enabled 3-dimensional scans of the embryo at 5- to 10-minute intervals of the following central events during preimplantation development: the main phases of mitosis, visualizing the major changes in cell shape that characterize embryo compaction at the 8-cell stage, detecting cell polarization at the 8-cell stage, establishing apical F-actin rings at the 16-cell stage, tracking the expansion and zippering of F-actin rings, detecting the first internalized cells within the 16-cell embryo, and visualizing blastocyst expansion and hatching from the zona pellucida.

Cleavage-stage human IVF embryos were then studied using the fluorescent dyes to track early cellular and morphogenetic processes. Early cell-cycle dynamics and mitotic stages revealed that the duration of human mitosis is similar to that of mice, but interphase is 27% ± 4% longer in humans (16.1 ± 0.9 vs 12.7 ± 0.4 hours; P < 0.01). Using live imaging, compaction dynamics such as increased cell-cell contact and angle between apical membranes with a decrease in cell sphericity were observed beginning at the 12-cell stage and differed significantly from development in mice. In comparison to mice, human compaction was found to be more asynchronous and did not have clear links to apical polarization or inner-outer segregation.

Next, chromosomal segregation was analyzed to determine if this approach allowed detection of segregation errors in the human embryo leading to aneuploidy. Lagging chromosomes detected in human embryos using SPY-DNA appeared morphologically similar to those found in mouse embryos and had similar segregation dynamics. During blastocyst expansion, a subset of trophectoderm cell nuclei forms protruding bud-like that is shed into cytoplasm producing cytoplasmic DNA structures (cytDNA), suggesting an additional process of DNA loss different from chromosome segregation errors during mitosis. A perinuclear keratin network developing at the early cavitation stage appears to protect from formation of cytDNA structures, and the mechanical stress of blastocyst expansion appeared to disrupt the keratin network in certain cells, increasing DNA shedding. Next, trophectoderm biopsy was conducted in mouse, and human embryos to determine if the mechanical stress would increase DNA shedding. Biopsy of human blastocysts resulted in a significant increase in nuclear budding (6.0% vs 0.70%; P = 0.0022), suggesting a link between mechanical stress and DNA shedding.

This study demonstrates the use of fluorescent dyes and live imaging in characterization of key cellular and morphogenetic processes patterning the preimplantation human embryo and describes a process of DNA shedding in response to mechanical stressors that warrants future research.