Early in development, all animal embryos pass through an almost identical stage where they form a blastoderm, which is a hollow ball of a few thousand cells. From that hollow ball, layers of cells start to fold into shapes that will become different parts of the body. Although the behavior of cells and the patterning of the embryo after the blastoderm stage is well-understood, how the blastoderm itself is made has remained a longstanding mystery.
Now, a team of biologists and applied mathematicians at Harvard University in Massachusetts have developed a framework for understanding the general principles by which cell nuclei move and arrange themselves during the earliest stages of embryonic development to form the blastoderm. In their research, which was published July 6 in Nature Communications, the researchers made specific predictions about how the blastoderm would form in a variety of insect eggs and validated them using mathematical models and images of developing cricket (Gryllus bimaculatus) embryos.
Because so little was known about blastoderm formation in any animal, “I thought the best thing to do is to take the most unbiased view you can, and just observe and record the actual behaviors, and ask what patterns emerge,” says evolutionary developmental biologist Cassandra Extavour, who led the study with applied mathematician Chris Rycroft.
The team used multiview light-sheet and confocal microscopy to capture 3D timelapse images of the developing embryos and found that the multiplying cell nuclei in the cricket embryos rapidly move to fill the empty space in the egg. They don't divide and move randomly through the egg; rather, it’s as if each nucleus can sense the presence of other nuclei in their neighborhood and react accordingly to avoid crowding one another.
Despite their focus on crickets, “we wanted to really understand the general principles of how embryos form across a whole class of different insect species,” says Rycroft.
I thought the best thing to do is to take the most unbiased view you can, and just observe and record the actual behaviors, and ask what patterns emerge.
—Cassandra Extavour, Harvard University
The researchers approached the study with two hypotheses: that the nuclei might push away nearby neighbors, and that each nucleus experiences a force that pulls it away from other nuclei and into empty space. They formalized these two hypotheses with mathematical models and made computer simulations of the developing embryos. Ultimately, the latter hypothesis—that nuclei are somehow pulled into the egg’s remaining empty space—best matched their observations.
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To test the model further, the researchers tied human hairs around cricket embryos, creating a constriction in the middle. They found that the nuclei were slow to get through the constriction, but then accelerated into the open space on the other side—a finding that was in total agreement with their model’s predictions.
“For me, that's a really exciting and important aspect of this work,” says Extavour. “Generating a model that recapitulates biology isn’t in and of itself evidence that the model is capturing the parameters that are important in biology. And so, it was really great to find at least one way to try to falsify the model and test its realism.”
So how do the nuclei sense the space around them? The researchers hypothesize that microtubules attach to each nucleus self-assemble and grow outward, preferentially pulling the nuclei toward empty space because they can grow longer in that direction.
Ensuring that nuclei are where they need to be in the blastoderm sets the foundation for all subsequent development to happen correctly, explains Stefano Di Talia, a developmental biologist at Duke University in North Carolina who didn’t work on the study. “Even more importantly, a lot of the mechanisms that have to do with the formation of the blastoderm are really fundamental cell biological and biophysical questions,” such as how the cell cycle is coordinated with biochemical signals and mechanical forces generated by the cytoskeleton to position things correctly, he says.
Di Talia says the ability of microtubules to pull nuclei into position is a fascinating hypothesis and could be a more general mechanism to explain how cells move during development and morphogenesis. “I think there’s going to be lot of situations where microtubule-driven processes are going to control the positioning of nuclei or organelles,” he says, “and I think the systems where you can really learn the biophysics of how that works are going to be very helpful.”
