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An apoptotic force enables dorsal closure during Drosophila melanogaster development, thus defining a new role for apoptotic forces in tissue sculpting and development.
PHOTODISC
Apoptosis provides a force that enables dorsal closure in Drosophila melanogaster development, report Yusuke Toyama and colleagues in Science. This finding not only demonstrates a new role for apoptosis in dorsal closure, it also suggests that apoptotic forces might be generally important in tissue sculpting and development.
Dorsal closure is a crucial step in D. melanogaster embryogenesis. During closure, two sheets of epithelial cells progressively join up to cover an eye-shaped opening that is otherwise covered by amnioserosa, an extra-embryonic tissue. Dorsal closure is highly synchronized and is dependent on, among other things, the motor protein myosin-II, which provides an active closure force for the joining epithelial cells, and apoptosis, which eliminates amnioserosa cells that are no longer necessary after closure. Using confocal microscopy, Toyama et al. observed that cells next to apoptotic amnioserosa cells (and even cells next to apoptotic-neighbouring cells) become distorted and pull on surrounding tissues as the apoptotic cells degrade. That is, apoptosis provides a mechanical force that promotes closure.
To test this theory, the authors either prevented or upregulated apoptosis of amnioserosa cells by overexpressing relevant proteins and then measured the rate of closure (vnative), the force at the leading edge of the closing epithelial cells (
AS) and other kinetic variables. Indeed, vnative (normally 6.3 
0.6 nm per second) was slower when apoptosis was inhibited (3.6 1.1 nm per second) and was faster when apoptosis was upregulated (10.2 0.8 nm per second). Similarly, AS was about 30% lower or higher when apoptosis was inhibited or upregulated, respectively.
Using their measurements, the authors estimate that apoptosis provides one-third to one-half of the net force that drives dorsal closure. The authors also propose that "tension and apoptosis may contribute to a positive feedback mechanism that serves as a force regulator". Given the importance of apoptosis in development and tissue sculpting, apoptotic forces may also drive such processes as adult abdomen closure in D. melanogaster, digit individualization, joint formation and wound healing.