Immunohistochemical detection of an enamel protein-related epitope in rat bone at an early stage of osteogenesis

Monoclonal antibody MI315 was produced against hamster tooth germ homogenate by in vitro immunization. It was found that MI315 reacted with enamel matrix, ameloblasts, and bone matrix at an early stage of osteogenesis. Decalcified tissues of rat femurs and mandibles were examined with MI315 using indirect immunofluorescence. In endochondral ossification of femurs, immunoreactivity was found in bone extracellular matrix (ECM) deposited on the surface of the cartilage core of primary spongiosa, but not in the cartilage core itself. In intramembranous ossification of 0-day-old rat mandibles, intense immunofluorescence was detected in bone ECM and a few young osteocytes, but not in osteoblasts. Immunoreactivity in bone ECM of 2-day-old rats decreased and almost disappeared from bone ECM of 4-day-old rats. Although in nondecalcified sections of 0-day-old rats, negligible immunofluorescence was detected in bone ECM which showed positive staining in decalcified tissues, the immunostaining appeared after decalcification using ethylenediaminetetraacetic acid (EDTA). These results indicate that a substance(s), which had a common epitope with an enamel-derived protein(s), existed in immature bone ECM of both endochondral and intramembranous ossification, and that it might be masked by bone mineral. Monoclonal antibody MI315 is a useful tool to investigate the time- and position-specific changes in osteogenesis and amelogenesis.