GROWING human cells in a laboratory is easy. Making those cells arrange themselves into something that resembles human flesh is, however, anything but. So-called tissue engineers have mastered the arts of artificial skin and bladders, and recently they have managed to rig up a windpipe for a patient whose existing one was blocked. But more complicated organs elude them. And simpler ones, too. No one, for instance, has managed to grow bone marrow successfully.

At first sight, that is surprising. The soft and squishy marrow inside bones does not look like a highly structured tissue, but apparently it is. That does not matter for transplants. If marrow cells are moved from one bone to another they quickly make themselves at home. But it matters for research. Bone marrow plays an important role in the immune system, and also in bodily rejuvenation. Stem cells that originate within the marrow generate various sorts of infection-fighting blood cells and also help to repair damaged organs. However, many anti-cancer and anti-viral drugs are toxic to marrow. That leaves patients taking them susceptible to disease and premature ageing. Experiments intended to investigate this toxicity using mice have proved unsatisfactory. Nicholas Kotov of the University of Michigan in Ann Arbor and his colleagues have therefore been trying to grow human marrow artificially.

When they started their research, Dr Kotov and his team knew that the stem cells from which marrow is derived grow naturally in specialised pores within bone. These pores are lined by a mixture of connective-tissue cells, bone cells and fat cells, which collaborate to nurture the stem cells. The researchers also knew that the cells in this lining send chemical signals to one another and to those stem cells they touch. That suggests a stem cell's fate may depend on its surroundings in three dimensions, rather than the two dimensions of the bottom of a Petri dish—the type of vessel traditionally used to grow cell cultures. If correct, this would explain why attempts to make marrow in Petri dishes have failed.

To test their idea, Dr Kotov and his colleagues tried to replicate the interior of a bone using a material, known as a hydromel, that is similar in composition to a soft contact lens. To make the ersatz bone, liquid hydromel is densely seeded with tiny polystyrene spheres (they have diameters of between 50 and 300 microns). When the hydromel has solidified, the spheres are dissolved using a solvent called tetrahydrofurane, leaving a porous matrix. The diameters of the pores in this matrix match those of natural bone.

For added verisimilitude, the team then coated the internal surfaces of the pores with a material similar to mother-of-pearl. Marrow cells do not like to attach themselves to squishy materials such as hydromels, but the team's new material has enough stiffness to pass for bone. It thus fooled the marrow cells into setting up home.

Once the matrix was completed in this way, the researchers took the final step and seeded the pores with marrow harvested from donors. They report in Biomaterials that the transplanted cells behaved as if they were in real bone-marrow tissue, growing and dividing as they would normally. Then, to test their artificial marrow further, they added influenza viruses. They found that it released antibodies to fight those viruses in exactly the way that natural marrow would.

The anti-cancer and anti-viral drugs that damage natural marrow have not been studied in the artificial version yet, but that should happen soon. Dr Kotov's new tool should also allow researchers to study the marrow's response to pathogens such as influenza in more detail than is now possible. An unusual application of tissue engineering, then. But a valuable one.