Scutoid: A New Shape in Science

All animals are formed from tissues that bend into complex shapes. The building blocks of these structures are epithelial cells, which pack tightly together to form the lining of blood vessels and organs. For a long time, scientists have assumed that these cells are shaped like prisms or pyramids for efficient packing, but no one was sure since only thin cross-sections had been examined. But recent mathematical models confirm that cells adopt a shape that is new to science. It is, to say the least, a highly unusual shape and it goes by the name of scutoid.

With the help of mathematical modelling, scientists have recently discovered that epithelial cells – those covering the surface of animal organs – use a new geometrical shape, the scutoid, that enables them to pack together in the most efficient way to form tissues and organs. Image: University of Seville.

Discovered only in 2018, the scutoid is hard to describe. It looks like a twisted prism, although that’s not so helpful. “It’s a prism with a zipper,” says Javier Buceta, a biophysicist at Lehigh University and one of the scutoid’s discoverers. This, too, was not so helpful. What matters is that scutoids turn out to be everywhere, especially in living things.

One of the many mysteries of living cells is how they manage to blossom into coherent many-celled units. All of us begin as a single cell, which divides into more cells, which again divide and subdivide. Some of these, the epithelial cells, are destined to become tissues and organs. The cells collect into layers, which then bend and fold into three-dimensional organs like ovaries, kidneys, a heart and so on. The challenge for nature is how to pack the cells as efficiently as possible. In other words, it is a problem in geometry, for which mathematical models are helpful in uncovering insights into how nature “thinks.”

To gain insights into nature’s mode of thought, researchers led by Buceta developed a computer model to simulate what happens to individual cells in curved tissue. The cells pack together along faces and edges, but building and maintaining these borders takes energy, and each cell wants to use as little as possible. The team found that the total length of those contacting borders is smallest, and the packing most efficient, when at least some of the cells take the shape of scutoids. In other words, where you have curvature, you have scutoids.

A mathematical model for curved epithelia uncovers a novel geometrical solid, the scutoid. Scheme (a) represents planar columnar/cubic monolayer epithelia. Here, cells are simplified as prisms. Scheme (b) illustrates an fold in a columnar/cubic monolayer epithelium. Cells adopt the called “bottle shape” known as a frusta. Image: Nature Communications.

Having predicted the existence of the scutoid through computer simulations, the researchers went looking for it in nature and soon found it in abundance. For example, examined the developing embryos of fruit flies and watched as individual epithelial cells divided, aggregated, curved, and folded to form salivary glands and egg chambers. Visible within the tissue matrices were scutoids: more where the tissue curved the most, none where the tissue was flat.

As with fruit flies, so with humans. Are we made of scutoids? The answer is a definite “Yes”. The discovery not only shows nature’s mathematical cunning, it might also help scientists in efforts to create artificial organs and tissues in the lab. Such efforts often begin with some sort of scaffolding on which the epithelial cells grow and self-organize; adding scutoids to the mix could help the cells pack together more efficiently.

This post is adapted from “We Are All Scutoids: A Brand-New Shape, Explained” by Alan Burdick, The New Yorker, July 30, 2018.

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