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Say hello to the scutoid, a brand new shape that has been discovered in聽our cells. This addition to geometry finally explains how nature packs cells efficiently into three-dimensional structures.

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聽the lining of blood vessels and organs.

It had been assumed that these cells adopted prism- or pyramid-like shapes to form these structures, but no one was sure as only thin cross-sections had been examined. 鈥淚t is difficult to image these tiny structures in 3D,鈥� says Luis Escudero of Seville University, Spain.

Now, Escudero and a team of researchers have taken a detailed look at this cell-packing puzzle.

They modelled curved tissues where the cells have to 鈥榩ave鈥� surfaces that have very different areas at their top and bottom. In particular, the team wanted to explain a strange finding from previous research showing that epithelial cells can have different types of neighbours at their top and bottom surfaces.

They found that the only way to achieve this pattern was for the cells to adopt a particular prism-like shape with five edges at the top, six at the bottom, and with one of the side edges divided into a Y shape (see diagram).

Along the curve

The researchers assumed that this shape would have already been described by mathematicians, but a detailed search revealed that it had not. 鈥淲e then had to come up with a name,鈥� says Escudero. They chose 鈥榮cutoid鈥� because the shape resembled the 鈥榮cutum鈥� of some beetles 鈥� the part on its back that includes its wings.

The team then looked to see if this shape existed in real tissues. They found that 75 per cent of epithelial cells in the salivary glands of the fruit fly Drosophila were scutoids, as were 50 per cent of these cells from the folds of its developing embryo. There was a higher ratio of scutoids in curvier tissues.

Scutoids were also identified in zebrafish tissues, and a preliminary check found them in mammalian cells too, indicating that they are widespread tool used by nature to create curved structures.

鈥淲hen we saw the shape in computational models, that was surprising. We were even more surprised when we saw it in real tissues,鈥� says Escudero.

The discovery of scutoid-shaped cells has important implications for tissue engineering and the creation of artificial organs, which rely on the ability to understand and control the 3D organisation of cells. It is important to know that the cells that make up artificial organs are packed in the same way as real tissue, says Escudo.

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