Intramembrane particle distribution on the plasma membrane of ruffle-ended and smooth ended maturation ameloblasts of the rat incisors.

Plasma membrane structure and intramembrane particle (IMP) distribution in ruffle-ended (RA) and smooth-ended (SA) maturation ameloblasts were investigated by means of freeze-fracture replication of the rat incisor enamel organ. On the plasma membranes of both the lateral and distal cell surfaces, mean values of IMPs per 1 micron2 obtained from the SAs were lower than that from the RAs. This does not exclude the ameloblast modulation phenomenon but rather supports a radical shift of functional activity between RA and SA and argues in favour of the possible existence of two definite groups of maturation ameloblasts. In the distal plasma membranes of these ameloblasts, there were also some morphologic evidence of endocytic activity (resorptive functions).