Access of horseradish peroxidase (HRP) to the extracellular spaces of the maturation zone of the rat incisor enamel organ.

Adult rats received a single dose of HRP intravenously and were killed from 10 min to 6 hr after injection. Following fixation with glutaraldehyde, the enamel organs were treated with a Graham-Karnovsky-type procedure for peroxidase activity, post-osmicated, and embedded in plastic. Sections were studied with light and electron microscopes. Ten minutes after injection, reaction product was found in all extracellular spaces of the enamel organ, at the enamel-ameloblast interface over smooth-ended and intermediate ameloblasts, and in apical surface invaginations and vesicles of the latter cell types. The enamel-ameloblast interface over the ruffle-ended aemloblasts and the extracellular spaces within the ruffled border were free of reaction product and remained so for up to 6 hr. The apical terminal bars of the ruffle-ended ameloblasts functioned as a barrier to HRP. The basal terminal bars of the smooth-ended ameloblasts likewise seemed to prevent the passage of the HRP. Possibly, HRP flows in a lateral direction from groups of ruffle-ended into groups of smooth-ended ameloblasts. Between 10 min and 6 hr, HRP was cleared more rapidly from the extracellular spaces of the papillary layer than from those of the ameloblast layer, and there was little backflow of tracer from the ameloblast into the papillary layer. Eventually, tracer was cleared also from the extracellular spaces of the ameloblast layer, probably mainly through micropinocytosis by the ameloblasts. A working model is proposed regarding the handling of large molecules by the enamel organ in the maturation zone.