Fig. 8. Hypothesis of HERS formation. Schematic illustration of our hypothesis of the process of HERS formation (A-D) and of mutant incisors grown under kidney capsules (E). The more active proliferation of OEE and stellate reticulum elongates beyond the IEE after cessation of crown formation, leading to the formation of HERS. OEE, stellate reticulum and HERS expressed both Notch2 and CK14 with strong intensity (A-C, dark-yellow cells). The epithelial sheaths proliferate, and fragmentation occurs at the surface of the dentin. Dental follicle cells migrate among the fragmented epithelium and make up the periodontal tissues (D). (E) In mutant incisors, the histological features at the labial side mimic the development of mouse molar germ. A broken red line indicates the border between the crown analog and the root analog at the labial side. d, dentin; e, enamel; iee, inner enamel epithelium; oee, outer enamel epithelium; sr, stellate reticulum.