Cyclic localization of actin and its relationship to junctional complexes in maturation ameloblasts of the rat incisor.

The patterns of fluorescence associated with maturation ameloblasts of mandibular incisors labeled with 7-nitrobenz-2-oxa-1,3-diazole-phallacidin (NBD-phallacidin) for the detection of F-actin were investigated in normal and fluoride-treated rats. In normal rats, bands of smooth-ended ameloblasts (SA) exhibited intense fluorescence at their proximal ends only. Bands of ruffle-ended ameloblasts (RA) exhibited strong fluorescence at their distal ends as well as at their proximal ends. Regional differences in degree of intensity within the bands and between bands were displayed. In the apical part of the RA bands the proximal fluorescence was intense; it then decreased in an incisal direction; and it finally was absent close to the adjacent SA band. The incisal extension of strong proximal fluorescence in RA bands was short in early maturation and long in late maturation. The fluorescence pattern at both ends of the ameloblasts was cyclically repeated throughout the region of ameloblast modulation corresponding to the numbers of SA bands. In rats receiving 113 ppm fluoride in their drinking water for 2 months the number of fluorescence and ameloblast modulation cycles was reduced equally indicating that the cyclic F-actin localization is a phenomenon related to ameloblast modulation. Electron microscopy revealed that areas of strong fluorescence contained filament bundles, presumably actin filaments, in relation to continuous junctions occluding the interameloblast spaces. Areas of weak or no fluorescence were related to discontinuous macular junctions. The results suggest that the changes in F-actin distribution correlate well with junctional complex development, and therefore, possible functions related to the intermeloblast spaces within the RA bands may be redistributed as the ameloblasts are carried incisally by the erupting incisor.