There's nothing simple about the liver. The organ's many different cell types are arranged in precise three-dimensional patterns to filter toxins from the blood, convert nutrients into forms usable by body tissues, and perform a broad range of other functions. Now scientists are taking basic steps toward growing this complicated organ in the lab. Tissue engineering, as this field is known, has been around for a little more than a decade. Its researchers have already developed laboratory-grown versions of tissues such as skin and cartilage that are now being tested as replacements for damaged tissues. These are, however, comparatively simple body parts, consisting of one or two cell types layered on a synthetic polymer scaffold or a mesh made of the structural protein collagen (see box).A lab-grown liver, however, calls for a new level of sophistication and control: A variety of cells need to be arranged and grown in particular orientations. That means a scaffold with a high degree of selectivity, something polymer and collagen scaffolds lack. But by attaching specific cellular recognition molecules, such as protein fragments, to synthetic scaffolds, biologists and chemists have been able to demonstrate that such feats are indeed possible.
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