If modern science is right, the great mystery of embryonic development is less about how life unfolds, and more about how it folds.
Embryos of many organisms grow from two cells to four, then eight, and so on until there are thousands in a kind of ball. Then sheets of cells start to make folds or furrows as the basic shape of the creature — fly or fish or human — begins to emerge.
One of the most striking examples is a moment in the development of Volvox, a kind of algae that forms one of the simplest multicellular organisms. When it is a sphere of a few thousand cells, it reaches adult size, but not adult shape. So it turns itself inside out.
Scientists at the University of Cambridge in England have made a time-lapse recording of the process that shows it in three dimensions for the first time and has enough detail that researchers can check their mathematical descriptions of the transformation.
The shape changing is no mere curiosity. Volvox embryos do it in two ways, and the one the Cambridge researchers studied is remarkably similar to something that occurs in the embryos of humans and other animals, when a ball of cells turns into a doughnut shape, with an inside and an outside. Although it’s not immediately obvious, grown up humans are still kind of messy doughnuts, with arms, head, legs and most of our internal organs all part of the doughnut itself. The hole, in terms of geometry, at least, is the digestive system, from beginning to end.
That process is called gastrulation, and it is enormously significant in the embryo’s growth, the beginning of a crucial distinction between what’s inside and what’s outside. The paper in Physical Review Letters describing the new research begins with a quote from the embryologist Lewis Wolpert: “It is not birth, marriage or death, but gastrulation which is truly the most important time in your life.”
Humans and other large, complicated animals are, however, very difficult to study. Even when they are embryos, their cells not only change shape during gastrulation but move around and turn into different kinds of tissues.
For Stephanie Höhn, Raymond E. Goldstein and their Cambridge colleagues who did the research, Volvox presented a process that is a little bit simpler. The spherical skin of a Volvox embryo is one cell deep. Each cell has a tail, and just before inversion they are all pointing inward. They need to point outward, so they can flutter and move the Volvox along, so the embryo turns inside out.
All the cells stay in place, maintaining their connections with other cells. But they change shape. Dr. Goldstein said, “If you were to take a basketball and you tried to turn the thing inside out,” that would be similar to what Volvox has to do.
But imagine that the basketball has a hole in the top and that the cells at the bottom change shape so that the bottom shrinks and curves inward. Then it would be easier to push it through the top.
The process was known but hadn’t been characterized mathematically, or seen in three dimensions. That’s what Dr. Goldstein and his colleagues did, using an advanced microscopic technique called selective plane illumination, which allowed them to make a time-lapse movie that shows the whole inversion in a few seconds. In real time, it takes several hours.
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