During a long journey from egg to adult, series of elaborate tissue movements, such as contractions, foldings, and rearrangements are involved. For instance, gastrulation in Drosophila embryo is one of the dramatic developmental stages (see movie). To make this kind of movement happen, every single cell coordinates their role in time and space perfectly and makes a huge movement as a result. It’s simply AMAZING.

                                              Drosophila gastrulation

From biophysics/bioengineering point of view, this is predominantly a biomechanical process. One key question is which cells generate force and which cells are passively effected by forces generated elsewhere.

We propose that cells that commit suicide (program cell death, apoptosis) actively produce forces and promote tissue dynamics.

As a cell in an epithelia tissue undergoes apoptosis, the dying cell is squeezed out from the tissue. In a gap-closing system in Drosophila embryo, dorsal closure, a transient tissue called the amnioserosa occupies the gap in between sheets of epithelial cells (see movie). A small fraction (10%) of amnioserosa cells undergo apoptosis prior to the complete closure. Each apoptotic cell (white cells in figure) is surrounded by 5 to 7 neighboring cells that are also deformed (red cells in figure). This observation can interpolate as an increasing of mechanical stress in the surrounding tissue(s).

Top: time lapsed images showing apoptotic cells in the amnioserosa. 

Bottom: color forced images. apoptotic cells (white) and surrounding non-apoptotic cells (red).

To control the dynamics of closure, the occurrence of apoptosis is altered genetically. The closure slows down by suppressing apoptosis. In contrast, when the apoptosis is enhanced only in the amnioserosa cells, the speed of closure is increased.

Dorsal closure in control and apoptotically altered Drosophila

embryos.

Top: apoptosis “enhanced” embryo.

Middle: control embryo.

Bottom: apoptosis “surpressed” embryo.

We conclude that the suicide of a cell can be a source of mechanical force.

More details and stories behind this research can be found in

- Y. Toyama et al., Science 321, 1683 (2008)  [PubMed]

and

- K.L. Bates, Programmed Cell Death Contributes Force to the Movement of Cells. (Sep. 2008) [ScienceDaily].

Background

Apoptotic force