The researchers used SPY650-DNA, a fluorescent dye that labels genomic DNA, and SPY555-actin, which labels the protein F-actin that forms the skeleton of cells. They then visualized dozens of live embryos during the first 40 hours of development using powerful laser scanning microscopes.

“We could see those cells dividing and chromosomes segregating, and we could even capture in real time chromosome segregation defects,” says Plachta.

For example, the researchers observed that cells in the outer layer of the embryo, known as the trophectoderm, lose some of their DNA during a stage of cell replication called interphase — in which cells are replicating their DNA.

Such errors could be linked to chromosomal abnormalities such as aneuploidy, a condition that is marked by extra or missing chromosomes in the early embryo and is associated with pregnancy loss and failure of implantation.

“Knowing when aneuploidies occur allows us to get opportunities to intervene and try to correct the issue,” says Zev Williams, an obstetrician at Columbia University in New York City. The latest images reveal the first days of embryo development “with a clarity never seen before”, he adds.

Not like mice

The researchers were also able to compare key events in human embryos and mouse ones — which are often used as models to study embryonic development. They observed some important differences. For example, a process called compaction, which involves alterations in cell shape, starts at the 12-cell stage in human embryos compared with the 8-cell stage in mice; the process is also more asynchronous in human embryos, leading to variations in inner- and outer-cell formation.

“Detecting these small changes is what makes this paper so novel,” says Sade Clayton, a cell biologist at Washington University in St. Louis, Missouri. “These small differences could actually [translate to] quite large differences in terms of uterine development.”

The authors hope to build on this research by imaging human embryos for longer periods, using lower-intensity laser microscopes, and incorporating other dyes that can label different structures, such as cell membranes.

The technique might even have clinical applications one day, says Plachta. “In the future, we could use this type of live-imaging approach to follow embryos non-invasively in the clinic,” he says. This could form part of tests to determine “which embryo is likely to have the best potential” before implantation, he adds.