Telling Left from Right in Organ Development

New study maps the work of tiny cellular sensors that determine the proper positioning of our organs

microscopy image of cilia helping shape organ positioning during development

Scientists have found how cilia — tiny whiplike projections on the surfaces of cells — help shape the left-right body plan of the developing organism. Image: Shiaulou Yuan Lab, Mass General

Although the human body is externally symmetric across the left-right axis, there are remarkable left-right asymmetries in the shape, size, and positioning of many internal organs, including the heart, lungs, liver, stomach, and brain. These asymmetries can range from benign to serious, causing a range of conditions that affect multiple organs.

Developmental biologists have long been fascinated by how this asymmetry arises in the first place.

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Scientists have long known that left-right asymmetry occurs during early embryonic development, driven by a structure called the left-right organizer, made up of a small cluster of cells. Within the “organizer,” motile cilia, hairlike structures on the surface of cells, beat rapidly to create a leftward directional flow of extracellular fluid as the first outward sign of a left-right difference.

And while research has shown that this early flow is critical in the distinction of right from left, just how this flow is sensed and translated into left-right asymmetry has remained unknown.