Michael Sheetz

William R. Kenan Jr. Professor of Biological Sciences, Columbia University
 
On Cellular Motility

"The membrane is not like that of a balloon in the sense that it can't expand, but instead it has to move from other portions of the cell out there. So there's this process of continual disassembly at the back and reassembly at the front and that enables the cell to worm its way between the cells in the tissue to ultimately reach its goal of finding those bacteria and eating them. It would be like moving a house by taking the back of the house down and moving those components to the front and reassembling them."

Interview transcript: Michael Sheetz

Adults have relatively few cells that migrate and those are located mostly in the immune system to fight infections or to kill cells that are unwanted because of the signals that those cells give off. So therefore, the cells are, by necessity, chemo-tactic, which means that they respond to a cue from the environment.

In those types of cases, the cell is often in a passive situation. One of the best examples is that of immune cells, which are rolling macrophages that move down the blood stream and make periodic contact with the endo-philial cells that line the blood vessel. And by that periodic contact, they in essence sample that surface. Because there's flow, they can't rely on chemo-tactic cues; they have to rely on tactile cues, and those tactile cues have to be elicited and responded to in a very short period of time. In an area where there's inflammation or bacterial infection, the blood vessels in that region (because of the death of some of the neighboring cells) become activated and once activated, they will bind or cause the macrophages to bind. The macrophages will then extravasate, i.e., go between the endo-philial cells and move into the tissue and then migrate to the site of infection and then start killing bacteria in that situation.

With the actomyosin contraction the cell squeezes much like a muscle in three dimensions and that pushes that membrane out front. The membrane is not like that of a balloon in the sense that it can't expand, but instead it has to move from other portions of the cell out there. So there's this process of continual disassembly at the back and reassembly at the front and that enables the cell to worm its way between the cells in the tissue to ultimately reach its goal of finding those bacteria and eating them. It would be like moving a house by taking the back of the house down and moving those components to the front and reassembling them. Because the machinery of the cell (the cytoskeleton) is made up of filaments and cross-linking proteins and motors that generate the force, they are continually dynamic on the time scale of minutes.

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