Essential Concepts of Endosseous healing
1) Osteoblast is the prime bone matrix synthesizing cell. Like most secretory cells, osteoblasts are polarized cells, and the direction of their secretory activity is away from the nuclear end of the cell. The cell processes of osteoblasts become surrounded by mineralized matrix and, with their canaliculi, form the only means of vital communication between surface osteoblasts and those that have become completed surrounded by matrix as osteocytes. Thus the osteoblast is irrevocably attached to the bone-forming surface.
2) Osteoblasts have polarized synthetic activity. Thus bone can only be deposited by laying down matrix on a pre-existing solid surface i.e., bone only grows by apposition. Since each osteoblast may become a completely entombed osteocyte, the osteoblast is incapable of migrating away from the bone surface, and the only method by which this surface can receive further additions (beyond the synthetic capacity of a single osteoblast) is by the recruitment of more osteogenic cells to the surface, which then differentiate into secretory active osteoblasts.
3) Bone matrix mineralizes and has no inherent capacity to "grow." This is quite different from other connective tissue, for example, cartilage, which can grow both interstitially and by apposition. Thus, once bone formation has been initiated, the matrix and the cells that have synthesized that matrix have almost no ability to govern the ongoing pattern of bone growth on the implant surface