The nature and distribution of large hyaluronate-binding proteoglycans (PGs) in early bone formation bf developing rat mandible were biochemically and immunohistochemically investigated with a monoclonal antibody (MAb) 5D5, which is derived from bovine sciera and specifically recognizes the core protein of the large PGs such as versican, neurocan and brevican, but not that of aggrecan.

The first sign of mandibular bone formation is evident in the embryonic day 15 (Day 15). At the site of initiation of bone formation, a center of ossification was seen in the close proximity to Meckel’s cartilage. At the light microscopic level, these centers contained fine strips of the intercellular matrix surrounded by osteoblasts (Fig. 1). At the EM level, these matrices contained very fine fibrillar material, numerous, osteoblastic cell processes, and vesicular structures, but did not show any typical mineralization pattern. In Day 16, clusters of needle-like crystals or bone nodules, which varied considerably in size and shape, were seen in the intercellular matrix among osteoblasts (Fig. 2).