Scanning electron microscopy of the papillary layer of the rat-incisor enamel organ.

The enamel organ isolated from the maturation zone was treated with hydrochloric acid to remove such extracellular materials as collagen fibrils and basal lamina covering the papillary layer. Two types of the ridges were classified on the basis of their morphology and distribution: (1) globular and irregularly arranged ridges at the periphery of the early maturation zone, (2) elliptical and parallel ridges in the central part of this zone and over the whole of the late maturation zone. The fine surface structures of the ridges were characterized by numerous interdigitating cell processes.