A good demonstration of dental tissue engineering: researchers "used stem cells obtained from the periodontal ligament of molars extracted from mice, expanded them in an incubator, and then seeded them on barren rat molars. The stem cell-treated molars were reinserted into the tooth sockets of rats. After two and four months, the stem cells aligned and formed new fibrous attachments between the tooth and bone, firmly attaching the replanted tooth into the animal's mouth ... Tissue sections showed that the replanted tooth was surrounded by newly formed, functional periodontal ligament fibers and new cementum, the essential ingredients of a healthy tooth attachment. ... To verify that the ligament was formed by the transplanted stem cells and not by the animal's own cells, stem cells were labeled with green fluorescent protein prior to seeding them on the molars and re-inserting the teeth into the animal's mouth. ... Our research uncovered the code required to reattach teeth - a combination of natural tooth root surface structure together with periodontal progenitor cells. ... Our strategy could be used for replanting teeth that were lost due to trauma or as a novel approach for tooth replacement using tooth-shaped replicas."