Scientists captured a fascinating 3D video of an algal embryo as it turned itself inside out and back again.
These findings could provide insight into the mechanical processes involved in the "most important time in your life," the University of Cambridge reported.
A research team used fluorescence microscopy to observe the behavior of Volvox embryos, and also tested a mathematical model of morphogenesis to show how the shape of cells drives this inversion phenomenon. In the observations, the algal embryos changed from a sphere to a mushroom shape and back again.
The observed processes are similar to those that are seen in animal embryo gastrulation. In this process, the embryo folds inwards to form a "cup-like" shape, creating a primary germ layer that is the origin of the body's organs. Volvox embryos also demonstrate this process, but also turn themselves right-side out.
The algal embryos complete their shape change by switching the cell shape and location of connections between cells, allowing them to be used as a model for the understanding of cell sheet folding. The process of inversion starts when the embryo folds inward, forming two hemispheres. One of these hemispheres moves inside the other, an opening at the top widens, and the outer hemisphere glides over the inner, until the embryo has returned to a spherical shape. The entire process takes place over the course of only about an hour. Past research has suggested changes in cell shape drive the peculiar process.
"Until now there was no quantitative mechanical understanding of whether those changes were sufficient to account for the observed embryo shapes, and existing studies by conventional microscopy were limited to two-dimensional sections and analyses of chemically fixed embryos, rendering comparisons with theory on the dynamics difficult," said Professor Raymond E. Goldstein of the Department of Applied Mathematics and Theoretical Physics, who led the research.
The new time-lapse recordings taken by the scientists revealed that one hemisphere shrinks while the other expands. Mathematical modeling showed the "mushroom shape" could only be achieved through "active contraction of one hemisphere and active expansion of the other."
"It's exciting to be able to finally visualise this intriguing process in 3D," said Stephanie Höhn, the paper's lead author. "This simple organism may provide ground-breaking information to help us understand similar processes in many different types of animals."
The findings suggest cell shape changes that occur away from the invagination region are a result of active forces within the cell, and not passive deformations.
"The power of this mathematical model is that we can identify which cell deformations are needed to cause the embryo movements that we observe in nature," said Aurelia Honerkamp-Smith, one of the study's co-authors.
The findings were published in a recent edition of the journal Physical Review Letters.