The extracellular nature of enamel in the rat.

Developing incisal enamel of the rat has been examined in sections with the electron microscope. Staining the sections with heavy metal and sandwiching them has revealed details hitherto unvisualized because of low contrast and destruction by the electron beam. In particular, it is seen that the cell membrane always lies between the ameloblast and the enamel and therefore that enamel is extracellular and not intracellular. Implications of this with regard to the possible keratinous nature of enamel matrix are discussed.