A research group led by Associate Professor Shunsuke Yaguchi from the Institute of Life and Environmental Sciences at the University of Tsukuba and Specially Appointed Lecturer Koki Tsuyuzaki from the Graduate School of Medicine at Chiba University used microscopy techniques and molecular biology methods to elucidate the mechanism by which sea urchins can develop into complete individuals even when a single organism is divided in half during its early developmental stages, with each fragment autonomously redrawing its body plan. They successfully captured the cellular movements and gene functions underlying the "power to reorganize one's own body (self-organization)" and visualized the process of embryonic axis reconstruction in individuals. This discovery provides new insights into a long-standing mystery in life sciences: "Why do two lives emerge from a single fertilized egg?" It will also serve as an important clue for future developmental research in understanding the mechanisms of identical twins in humans. The findings were published in Nature Communications.
When a 2-cell stage sea urchin embryo is separated into individual cells, each can develop into a twin. Although the detailed process had remained unclear, we show here that the separated cells first grow as a sheet, then round up to form a complete individual.
Provided by Shimoda Marine Research Center, University of Tsukuba
At the end of the 19th century, German embryologist Hans Driesch first demonstrated that when sea urchin fertilized eggs are separated at the two-cell stage, each cell independently grows into a complete individual, essentially creating twins. However, the detailed developmental processes and molecular mechanisms of how embryos rebuild their embryonic axes (body axes necessary for forming normal bodies) and achieve normal development after separation had remained unelucidated for more than 100 years.
The research group aimed to elucidate at the molecular level the mechanism by which two individuals can develop even when embryos are divided in half, using sea urchin embryos.
First, they artificially separated sea urchin embryos of Hemicentrotus pulcherrimus at the two-cell stage and observed in detail how each cell (half-embryo) proceeded with development. While normal embryos undergo cell division to become hollow spherical blastulae, the half-embryos initially spread into a flat (sheet-like) shape, then changed form to become cup-shaped and finally became spherical, ultimately becoming small blastulae. Although these blastulae were somewhat smaller than normal embryos, they grew into healthy larvae through essentially the same developmental process as usual.
When the group investigated the mechanisms supporting these unique morphological changes using live imaging and molecular biology techniques, they found that the constriction of the protein complex actomyosin and septate junctions connecting cells work in coordination to transform the entire cell sheet into a spherical shape.
During this process, they confirmed that although the anterior-posterior axis (the body axis connecting the posterior end ("head") and anterior end ("tail")) is temporarily disrupted in the sheet-like cell arrangement, it subsequently normalizes, and the dorsal-ventral axis (the body axis connecting the back and belly) is also formed normally. At this time, the Wnt/β-catenin signal, which determines positional information on the anterior side of the body, is reactivated, causing the posterior-side region to move and the axis to be repaired. This allows the appropriate arrangement of the distribution of the transcription factor FoxQ2, which normally suppresses the function of the signaling molecule Nodal that determines the dorsal side, ultimately enabling dorsal-ventral axis formation.
From these results, the mechanism was revealed: even if embryos are divided in half during their early stages, each fragment autonomously adjusts its shape, reconstructs its body axes, and grows into a complete individual. This discovery represents important findings that provide new clues from a developmental biology perspective to the long-standing question of why robust individuals are somehow born even when accidentally divided in half during the growth process.
Journal Information
Publication: Nature Communications
Title: Unraveling the regulative development and molecular mechanisms of identical sea urchin twins
DOI: 10.1038/s41467-025-63111-z
This article has been translated by JST with permission from The Science News Ltd. (https://sci-news.co.jp/). Unauthorized reproduction of the article and photographs is prohibited.