In addition to pruning cells out of the way during embryonic development, the much-studied process of programmed cell death, or 1) ___, has been newly found to exert significant mechanical force on surrounding cells. This mechanical force may be harnessed throughout biology by tissues to aid wound formation, organ development and other processes that require cell movement, according to a Duke University team that melds biology with physics to investigate force at the 2) ___ level. Cells are known to move in coordinated fashion during the closure of an eye-shaped opening on the back of a developing fruit fly embryo, a model system Duke biophysicists have been working on for nearly a decade. The newly discovered 3) ___ created by apoptotic cells imploding and withdrawing is making a force sort of like a friend helping you tuck the edge of the sleeping bag in. Dying cells appear to occur at random times across the plane of cells comprising the shrinking opening, in a pattern that totals about 10% of the population of cells. It was first hypothesized that the apoptotic 4) ___ might be particularly significant for force production. The next step was to carefully measure the motion of cells immediately surrounding a dying cell. What was observed by laser-induced fluorescence, was that as a dying cell collapsed and sunk beneath the surface, it contributed to the 5) ___ pulling the edges of the opening closer together. It was concluded that apoptosis is not a single cell event but is amplified by the five-to-seven surrounding cells. On balance, these dying cells exert perhaps a third to a half of the force that is moving the edges of the opening together, so it’s a very significant part of the 6) ___. The forces at work here are measured in perhaps billionths of a Newton, but that’s because you’re moving cells, [one pound = 4.45 newtons]. At the cellular scale, these forces are quite substantial. The group’s findings appear in the Sept. 19 edition of Science. Though this finding is so far limited to dorsal closure in the fruit fly embryo, next steps are to begin looking for the mechanical force of apoptosis elsewhere. Earlier findings on the fruit fly model so far have appeared applicable to wound closure and organ development in vertebrates like humans. It’s entirely possible, that evolution has harnessed the 7) ___ force created by dying cells in many other ways.

ANSWERS: 1) apoptosis; 2) cellular; 3) force; 4) cells; 5) forces; 6) process; 7) mechanical

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