Distressed amoebas can call for help.

Some single-celled amoebas that become stalled while dividing into two daughter cells summon help from fellow amoebas, according to a report in March 22 NATURE. When such cellular reproduction hits a snag, a member of Entamoeba invadens produces a chemical signal that brings other amoebas rushing in. An arriving cell acts as a "midwife," severing the connection between the nascent daughter cells, say the researchers who documented the unusual behavior.

Single-celled organisms typically reproduce by simply pinching in two. Known as fission, this type of reproduction normally requires no partner. But fission often fails with Entamoeba, a gut parasite of reptiles akin to the organism that causes amoebic dysentery in people.

Graduate students working with Elisha Moses at the Weizmann Institute of Science in Rehovot, Israel, were attempting to record the mechanics of normal cell division of amoebas cultured in laboratory flasks. The scientists noted that up to a third of the cells trying to divide failed to complete the process by themselves, but then were able to fully divide with the help of a nearby midwife cell. One out of 10 dividing cells never completed the process and ended up as a single cell with two nuclei.

In sparse populations of amoebas--in which potential midwife cells were few and far between--this type of reproductive failure reached one in four, Moses' team found.

Scientists may never before have noted cellular midwifery because cell division happens so quickly, says Moses. Cells spend only about 7 minutes dividing at the end of their 12-hour life cycle.

Watching many divisions, the Weizmann Institute researchers found that amoebas would make a beeline for other cells stalled in the middle of dividing. The helper cells traveled up to 40 times their own length. Investigating further, the team used a fine pipette to siphon up fluid near cells in the process of fission and then released the fluid near other amoebas. More than a third of these amoebas moved toward the pipette tip and even chased it as the researchers moved it.

David Mirelman, a biologist on the team, likens this amoebic midwifery to the signaling and collective action of other microorganisms that researchers have studied. Although the identity of the amoeba attractant remains unknown, the researchers speculate that it's a molecule, perhaps shed from a dividing amoeba's surface, that's perceived as a "delicacy" by the midwife cell, says Moses.

Another cell biologist cautions that the observed failure rate of Entamoeba to fully divide could be an artifact of the experiment. The reptile gut may be more conducive to division than the labware in the Israeli experiments, suggests Paul A. Janmey, who studies cells mechanics at the University of Pennsylvania in Philadelphia. By sticking themselves to gut tissue, a dividing amoeba might have a better anchor for pulling itself apart than did the amoebas in the new experiments, he speculates.

The attraction of amoebas toward dividing cells is interesting, notes Richard Firtel, who studies a different kind of amoeba--one that has complex signaling and aggregation behaviors. "But the question is whether they are doing it for the purpose of cleaving other cells," he says.