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Researchers at University of Tsukuba examined how twin organisms can emerge from a single fertilized egg by studying sea urchin embryos. Their findings revealed that when an early-stage embryo is split in half, each portion is able to regenerate its own developmental plan. The team uncovered the dynamic cellular processes and genetic programs that allow each fragment to reorganize and grow into a complete individual.

In the late 19th century, German embryologist Hans Driesch showed that when fertilized sea urchin eggs were separated at the two-cell stage, each cell could still develop into a complete organism. Yet, for more than a century, the developmental process and molecular mechanisms that allow the embryo to re-establish its body axes (anterior-posterior, dorsal-ventral, and left-right) and resume normal development have remained largely unclear.

For a new published in Nature Communications, the researchers combined advanced microscopy with molecular biology to uncover how each embryo fragment is able to reset its developmental blueprint and grow into a full individual. They also succeeded in visualizing axis reconstruction by tracking the cellular movements and activation of the gene that drives this remarkable self-organizing ability.

This discovery offers fresh insight into a long-standing question in life sciences: How two complete individuals can emerge from one fertilized egg. The sea urchin model may also provide valuable insights for future developmental studies, including the origins of monozygotic twinning in humans.