From the Technology Review: "Growing living tissue and organs in the lab would be a life-saving trick. But replicating the complexity of an organ, by growing different types of cells in precisely the right arrangement - muscle held together with connective tissue and threaded with blood vessels, for example - is currently impossible. Researchers at MIT have taken a step toward this goal by coming up with a way to make 'building blocks' containing different kinds of tissue that can be put together. ... The MIT group [put] embryonic stem cells into 'building blocks' containing gel that encouraged the cells to turn into certain types of cell. These building blocks can then be put together [to] make more complex structures. The gel degrades and disappears as the tissue grows. Eventually, the group hopes to make cardiac tissue by stacking blocks containing cells that have turned into muscle next to blocks containing blood vessels, and so forth. ... The researchers expose clusters of stem cells called embryoid bodies to a physical environment that mimics some of the cues the cells experience during embryonic development. ... The result is a hydrogel block, half gelatin, half polyethylene glycol, with a sphere of embryonic stem cells inside. ... within an individual embryoid body, cells on the squishier, gelatin side took a different path from cells on the polyethylene glycol side. The gelatin is easier for the cells to push into, and this affects how they grow, directing them to become blood vessels." This sort of technology is a potential path to replicating the complexity of the extracellular matrix from the bottom up, necessary to the goal of producing highly structured tissue from scratch while ensuring it is laced with the required tiny blood vessels to support the cells.