Finally, in the planarian species most commonly used in stem cell research, fission occurs on average approximately once per month per worm ( 7) and only lasts from a few minutes to tens of minutes (this study). Furthermore, planarians are photophobic ( 5), fission occurs primarily in the dark ( 4, 6), and even slight disturbances cause it to stop, complicating real-time imaging of the process. No induction mechanism has been identified, although decapitation has been shown to increase fission probability ( 2– 4). Planarian fission is fast, violent, and irregular. How was it possible for these animals to generate the forces necessary to rip themselves using only their own musculature and substrate traction? The question has remained unanswered to this day, because it is experimentally difficult to study the fission process in sufficient detail to figure out how it works. Michael Faraday and his contemporaries were intrigued by the observation that asexual freshwater planarians, squishy worms a few millimeters in length, reproduced by tearing themselves into a head and tail offspring, in a process called binary fission ( 1). Together, our results demonstrate that where and how a planarian rips itself apart during asexual reproduction can be fully explained through biomechanics.
Furthermore, we show that the location of waist formation, and thus fission, is determined by physical constraints. It asserts that fission execution is a mechanical process. The model fully captures the pulsation dynamics leading to rupture and reproduces empirical time scales and stresses. We developed a linear mechanical model with a planarian represented by a thin shell. We focus on Dugesia japonica fission and show that it proceeds in three stages: a local constriction (“waist formation”), pulsation-which increases waist longitudinal stresses-and transverse rupture. Because planarians stop “doing it” at the slightest disturbance, this remained a centuries-old puzzle. Understanding this process of ripping oneself into two parts poses a challenging biomechanical problem. The resulting head and tail pieces regenerate within about a week, forming two new worms. Asexual freshwater planarians reproduce by tearing themselves into two pieces by a process called binary fission.
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