Osteocytes, the cells of established bone, are derived from osteoblasts. In fact, an osteocyte is simply an osteoblast that has surrounded itself with bone matrix. Of course, the morphology of the cell changes: because the osteocyte is less metabolically active, it contains fewer organelles, and the nucleus occupies a greater proportion of the entire cell body.
Although osteocytes are, by definition, surrounded by osteoid, they are able to communicate with other distant osteocytes through long cell processes traveling through the canaliculi of bone. This network of osteocytes can regulate its metabolic environment and respond to signals applied to the surface of the bone.