Fig. 1.

Mmp20-null mouse ameloblasts can initiate, terminate, and reinitiate the secretory stage of enamel development. a–d Successive sections of an Mmp20-null mouse incisor. White arrows point to prominent Tomes’ processes that disappear in b and c. d The arrowhead indicates the return of the Tomes’ processes, with arrows indicating prominent processes. It appears that signals necessary for the ameloblasts to progress through the secretory stage are absent. Am = Ameloblast; d = dentin; e = enamel.