What appears chaotic is actually a well-orchestrated process for embryonic head-to-tail elongation.

Few sights are as captivating as the movements of a developing embryo. Shapeless tissue remodels itself as cells migrate en masse to form elongated structures that, depending on the species, will become a frog spinal cord, a human gut, or the main body of a fly.

Jennifer Zallen, an HHMI early career scientist at Memorial Sloan-Kettering Cancer Center, studies how embryonic tissues stretch along an anterior-posterior axis using the fruit fly Drosophila melanogaster as a model system. Shortly after formation of the embryo, the mass of fruit fly cells dramatically elongates over about a two-hour period and establishes a head at one end and a tail at the other, a process known as convergent extension.

Over the past seven years, Zallen’s lab has used high-resolution, time-lapse imaging to track embryonic cell migration during that two-hour window. She has found that what looks like chaotic pushing and shoving between neighboring cells in elongating tissue is actually a highly cooperative and orderly process.

Zallen’s group established the ground rules of the game in a 2006 Developmental Cell paper. Tracking single cells in a living embryo, they showed that cells consistently join and then exit pinwheel-like structures known as rosettes as a fly embryo becomes longer and thinner. Computational analysis indicated that most cells repeatedly move in and out of multiple 5-8 cell clusters as the embryo stretches outward. Each group of cells reorganizes to become longer and narrower, suggesting that rosette formation could drive morphological change.

Cells in the Drosophila embryo form multicellular rosettes that assemble and resolve in a directional fashion, promoting body axis elongation. A subset of rosettes is highlighted.
Image courtesy of Ori Weitz, Dene Farrell, and Jennifer Zallen.

Since then, her laboratory has focused on understanding the multiple signals that encourage cells to move in and out of transient groups. “It is forces generated at the contacts between cells that cause the cells to rearrange and the tissue to elongate,” says Zallen. “These forces place special demands on the cell junctions, which must be dynamic enough to dismantle individual contacts but strong enough to prevent the group from ripping apart under tension.”

Illustration: Brendan Monroe

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