Calmodulin immunocytochemistry in rat incisor enamel organ through its life cycle

The enamel organ of the growing rat incisor was perfusion-fixed with a mixture of formaldehyde and glutaraldehyde and processed for indirect immunogold labeling of calmodulin on post-embedded ultrathin sections. Throughout the zones of presecretion, secretion, and maturation of enamel, specific protein A-immunogold labeling was localized on polyribosomes and those attached to endoplasmic reticulum, mitochondria, nuclear chromatin, phagolysosomes, and cytoplasm adjacent to the plasma membrane, and tonofilaments associated with desmosomes of ameloblasts and cells of outer layer of enamel organ. Golgi membranes, condensing vacuoles, secretion granules, primary lysosomes, and micropinocytotic coated vesicles were hardly labeled. In the presecretion zone, the basal lamina of the preameloblasts and the matrix vesicles and collagen fibrils of the predentin matrix were not immunoreactive. Tomes' process of secretory ameloblast and adjacent enamel crystals were labeled. In addition to the above immunoreactive structures, some phagolysosomes, ferritin granules, and the cytoplasm of the ruffled border zone of maturation ameloblast contained immunogold particles. In control sections incubated with either protein A-gold complex alone, or antiserum preabsorbed with an excess of calmodulin and protein A-gold complex, only a few gold particles were observed to be randomly associated with the tissues. These results indicate that calmodulin is present in the cells of the enamel organ through all stages of amelogenesis. Its wide distribution is consistent with its involvement in various cytoplasmic functions.