Although the human body is externally symmetric across the left-right axis, there are remarkable left-right asymmetries in the shape and positioning of most internal organs including the heart, lungs, liver, stomach, and brain. Left-right asymmetry is known to be established during early embryogenesis by a small cluster of cells termed the left-right organizer. Within this organizer, motile cilia, hair-like structures on the cell surfaces, beat rapidly to create a leftward directional flow of extracellular fluid, which is the first outward sign of a left-right difference. This early flow has been shown to be critical to the distinction of right from left; however, how this flow is sensed and translated into left-right asymmetry has been unknown. Results of a new study led by Massachusetts General Hospital (MGH) researchers now reveal that cilia in the organizer function as the creators of the flow--they also act as sensors for the biomechanical forces exerted by the flow to shape the left-right body plan of the developing embryo. The findings were published on January 5, 2023 in Science. The article is titled “Cilia Function As Calcium-Mediated Mechanosensors That Instruct Left-Right Asymmetry.”