LEFT AND RIGHT: A wild type st 45 Xenopus embryo with rightward heart looping and counter-clockwise gut coiling is shown on the left. In contrast, a st 45 embryo that was injected with Pitx2 at the 4 cell stage exhibits leftward heart looping and clockwise gut coiling.
When it comes to organ orientation, one gene can make the difference between right and left. That gene, Pitx2 , has been identified as the key to asymmetry, or "handedness," the property describing on which side of the body the heart, liver, and gut form, and in what direction they grow (A.K. Ryan et al., Nature, 394:545, Aug. 6, 1998).

Earlier research had identified growth factors contributing to handedness, including sonic hedgehog ( Shh ) in chicks (M. Levin et al., Cell, 82:803-14, 1995) and nodal in mouse (J. Heymer et al., Mechanical Development, 66:5-11, 1997). But...

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