This study investigated the expression and localization of APin (which was previously identified and cloned from a rat odontoblast cDNA library), during ameloblast differentiation in rat incisors, by using in situ hybridization and immunohistochemistry. The subcellular localization of APin varied during ameloblast differentiation, but was stage-specific. APin mRNA was not expressed in pre-ameloblasts, was weakly expressed in secretory ameloblasts, and was strongly expressed in maturation-stage ameloblasts as well as in the junctional epithelium attached to the enamel of erupted molars. In the maturation-stage ameloblasts, APin protein was conspicuous in the supranuclear area (Golgi complex) of smooth-ended ameloblasts as well as in both the supranuclear area and the ruffle end of ruffle-ended ameloblasts. During ameloblast-lineage cell culture, APin was expressed at a low level in the early stages of culture, but at a high level in the late stage of culture, which was equivalent to the maturation stage. APin protein was efficiently secreted from transfected cells in culture. Furthermore, its overexpression and inactivation caused an increase and decrease in matrix metalloproteinase-20 (MMP-20) and tuftelin expression, respectively. These findings indicate a functional role for APin in the mineralization and maturation of enamel that is mediated by the expression of MMP-20 and tuftelin.

Root formation in 14‐day‐old Sprague‐Dawley rats was studied by light and electron microscopy. Special attention was focused on initial cementoblast differentiation. Disruption of the epithelial root sheath appears to be a consequence of directed cell migration by cells of the dental follicle proper which undergo differentiation into precementoblasts. Precementoblasts rapidly develop polarity towards the dentin, exhibiting major cytoplasmic processes rich in cytoplasmic filaments. These processes grow toward and eventually contact the dentin matrix. It is suggested that the cells of the dental follicle proper are cementoblast precursors which respond to chemoattractant substances released from newly deposited dentin matrix‐ and/or basal lamina‐associated material of root sheath origin.

Fibroblast‐to‐matrix attachment sites were studied by routine electron microscopy and immunocytochemistry. Rabbit antibodies to beagle dog plasma fibronectin and sheep antirabbit antibodies conjugated with horseradish peroxidase or ferritin were used to localize fibronectin at fibroblast‐to‐matrix attachment sites. In fibroblasts of healthy periodontal ligament, the attachment sites consisted of rectangular patches of amorphous material juxtaposed to the external surface of the plasma membrane. At these sites, the cell membrane was more densely stained and the adjacent cytoplasm was characterized by increased density and a high concentration of cytoplasmic filaments. The extracellular plaques contained fibronectin. Morphometric analysis indicated that the attachment plaques were approximately 90 nm thick, 250 nm wide, and 550 nm long, and distributed uniformly over both the cell body and peripheral cytoplasmic processes. In inflamed gingiva, the attachment sites were larger, irregular in shape, and with greater amounts of extracellular amorphous material and fibronectin associated to the cell surface. Cytoplasmic filaments were more often bundled as stress fibers which terminated in fibronexus‐type junctions with extracellular fibronectin‐coated filaments.