Breathe till you burst

To shed its shell for a new one, a crab needs a swollen head. And a swollon torso, legs and claws, for that matter. Expanding the body makes the old exoskeleton crack, so the crab can worm its way out of it. Aquatic crabs expand by taking in more water, creating a temporary hydrostatic 8220;skeleton8221; of pressurised water that supports the body while the new shell hardens. But what about crabs that spend their time on land?

Jennifer R A Taylor, a doctoral student at the University of North Carolina, and her adviser, William M Kier, have discovered that one land crab, at least, uses air. But it doesn8217;t just pump itself up like a ball; it combines air pressure with water pressure. 8220;It8217;s kind of like blowing up a balloon inside the body,8221; Taylor said. She and Kier studied red land crabs, Gecarcinus lateralis, which are found in the Caribbean and other tropical regions. They measured the pressure within the crab at various points in the molting process. Their findings were published in Nature. These crabs, whose bodies are about three inches wide, can take up only small amounts of water when they are on moist sediments. So to molt, the crab takes in air, trapping it in a cavity right behind the head. This inflated gut then puts pressure on the hemolymph, the bloodlike fluid within the crab. Because crabs have an open circulatory system, pressurising the hemolymph causes expansion throughout the body and provides the stiffness and support the crab needs while the shell hardens. How do the crabs take in the air? The researchers suspect they swallow it, which is what many molting insects do.

The Right culture mix

Culturing cells in the lab8212;tumour cells, perhaps, or stem cells8212;is normally done on a flat surface, like glass. But the real world exists in three dimensions, not two. Because cells respond to cues from surrounding cells, it makes sense that the response may be more natural, and the resulting tissue may better mimic the real world, if cells are surrounded in three dimensions rather than two.

But building a three-dimensional matrix for culturing tissue is not easy. A new technique has been developed by Sageeta N Bhatia and Dirk R Albrecht of the Massachusetts Institute of Technology and colleagues. As described in the journal Nature Methods, the technique uses a gel that sets by ultraviolet light. If you are making a chocolate chip cake, the chips will all settle to the bottom unless you distribute them by stirring so they can be locked in place when the cake bakes. Similarly, in their 3-D culturing technique, the researchers had to come up with a way to distribute the cells8212;cartilage cells from cows8212;in the liquid gel before it sets. The method they came up with was to put the cells in a layer of the gel solution sandwiched between two electrodes. The electrodes create nonuniform electric fields in various patterns that can propel the cells sideways and upward, even though they are not electrically charged. Once the cells are precisely distributed, they are locked in place by formation of the gel. The gel allows nutrient transport, so the technique may prove to be a good way to study the growth of certain tissues under more lifelike conditions.