Thanks to new time-lapse videos of equine embryo development, scientists are discovering the milestones associated with future implantation and pregnancy success. They’re also discovering just how active these microscopic equines are.
“We were all surprised by the amount of movement of the embryos and even of the cytoplasm (cell material) within the cells of the embryos,” said Niamh Lewis, BVM&S, PhD, Dipl. ACT, ECAR, equine reproduction specialist based in Dublin, Ireland. “They are by no means static during development. Cellular content and the attached cumulus cells (which surround the fertilized egg) are moving all the time.”
The movement she and her team saw was so significant that the embryos sometimes came in and out of focus or even shifted from one droplet of culture medium (the material in laboratory dishes that help the embryos grow) to another, Lewis said.
Seeing that movement—for the first time in equine science history—was “fascinating,” she said. But that wasn’t the primary goal of the time-lapse experiment. The scientists initially set out to look for specific milestones that could help predict which embryos were more likely to result in successful pregnancies.
Through studying the first several days of development in time-lapse videos, Lewis and her fellow researchers determined that early cleavage (the separation of a cell into two or three cells) is key, she said. Specifically, when the zygote (the single-cell product of egg fertilization) pushes out some of its cytoplasm and then divides within the first 24 hours, it’s much more likely to later become an embryo with a heartbeat.
Previous research had already pointed to this 24-hour window for cleavage, said Lewis. But their study visually confirmed the importance of the timing of this event.
In their study, Lewis and her fellow researchers created 144 equine zygotes through intracytoplasmic sperm injection (ICSI). They placed each injected oocyte (egg) in a droplet of culture medium and set it under a camera that would take images automatically every five to 10 minutes, for about the first 10 days of development. Afterward, they combined the images into a playback mode that generates a video of each embryo’s development.
The procedure allowed them to study, in precise detail, the timing of different developmental events in each embryo and then compare them, she said. For example, the timing of the first cleavage varied dramatically—ranging from 2.4 hours to more than four days after injection.
The 144 injected oocytes resulted in 19 blastocytes (an embryo at a stage that’s appropriate for implantation), Lewis said. They implanted 11 of those 19. Six resulted in pregnancies, and of those six, four reached heartbeat stage.
By following these successes and failures throughout the development process from the very first hour, the scientists were able to identify trends that could lead to better selection of “more promising embryos,” said Lewis.
“Time-lapse imaging can provide great insight into early equine embryo development and aid in predicting developmental potential,” she said. “As we gather more information on the topic, it will be great to be able to choose the embryos with the greatest chance of establishing an ongoing pregnancy.”
Beyond its practical advantages, the scientists are celebrating this important and historic step in equine reproduction science. While time-lapse imaging already exists in human medicine, where it has “taken the human IVF (in-vitro fertilization) field by storm,” it’s new in the horse world, Lewis explained.
“We were among the first researchers to ever view equine embryos under time-lapse,” she said. “It was fascinating to watch them develop in real time and to be able to see blastocyst formation so early on when it was not obvious on still image. That fascination never subsided in looking at each embryo develop, watching a miracle in real time! It certainly was extremely exciting.”