Morphological studies on the distribution of enamel matrix proteins using routine electron microscopy and freeze-fracture replicas in the rat incisor.

Enamel contains two categories of biochemically characterized proteins. Amelogenins are dissociated from enamel without physical disruption of the tissue whereas enamelins are obtained only when the crystallites are dissolved. Ultrastructural visualization of these proteins was attempted using routine electron microscopy and freeze-fracture replicas. Fresh, fixed, and 4.0 M guanidine-HCl-extracted samples of enamel from the secretory (young) and maturation (maturing) stages were compared. Decalcified and stained thin sections of fixed enamel revealed intercrystallite particulate material and "crystallite ghosts" which were identical to the crystallites themselves in young enamel and which corresponded to the periphery of the crystallites in maturing enamel. In contrast, 4.0 M guanidine-extracted enamel contained no intercrystallite particulate material but only "crystallite ghosts." Globular particles observed in freeze-fracture replicas of fresh and fixed enamel samples were also removed by 4.0 M guanidine extraction. Incubation of guanidine-extracted enamel with albumin and ovalbumin solutions restored the globular particles. It was concluded that amelogenins are the nonstructural, heterodispersed particulate material in the intercrystallite space. Enamelins constitute the integral template protein which initially provides for elongation of enamel crystallites. They then regulate the continuous growth in width and thickness during maturation and are progressively displaced to the periphery. The illusion that these "protein ghosts" are contained within the crystallite profile can be explained by the parallelepiped shape of the crystallite segment in thin sections.