A freeze-fracture study of ruffle-ended post-secretory ameloblasts.

The post-secretory portion of the rat incisor enamel organ was prepared for routine transmission electron microscopy and freeze-fracture replication in order to define further the structural surface features of the ruffle-ended ameloblasts. Surface views of the distal plasma membrane of the ruffle-ended ameloblasts revealed a well-developed zonula occludens junction with from six to ten rows of tight junctional strands. Gap junctions were also observed just proximal to the tight junctional strands. The membranes of the ruffled border contained a rich supply of intramembrane particles (IMP). The IMPs were approximately 7 to 8 nm in diameter and preferentially located on the P-face profiles of the membrane. The density of IMPs on the membranes of the ruffled border was higher than that on the lateral borders of the cell. It is suggested that the IMPs of the ruffled border may represent enzymatic proteins in the basal cell membrane of absorptive ameloblasts. In addition, the large, highly-developed zonula occludens appeared structurally capable of sealing the intercellular spaces between the ruffle-ended ameloblasts.