Junctions between early developing osteoblasts of rat calvaria as revealed by freeze-fracture and ultrathin section electron microscopy.

Although intercellular junctions have been described between mature lamellar bone cells, little has been known about junctions between osteoblasts in early developing bone. We therefore conducted a freeze-fracture and ultrathin section study on developing calvaria of rat embryos aged 17-19 days to determine what types of intercellular junctions appear between osteoblasts in early osteogenesis. We observed that three main types of junctional structures, i.e., adherens of the macular type, gap, and focal tight junctions, coexist between osteoblasts in early developing bone. Their possible involvement in early morphogenetic events is discussed. Tight junctions are considered to be involved in compartmentalization of the early matrix and final polarization of osteoblasts.