Cell membrane specializations occur at contact sites between adjacent osteoblasts and osteoblasts and osteocytes. These junctions have been described by other investigators as being important in preventing the extracellular movement of material around bone cells. Previously we described how certain small proteins circumvented the osteoblast population and rapidly penetrated the canalicular-osteocyte system. In the present study we used lanthanum colloid as an extracellular marker, the lanthanum readily penetrated the bone cell junctions and the extracellular space of bone. Morphologically, these junctions were not "tight" or "occluding" structures, but resembled "gap" junctions. These gap junctions contained elements which formed intercellular bridges between adjacent cells but also maintained a 2 nm space between cells that contained extracellular fluid. These gap junctions may have an important function in the control or coordination of bone cell activity throughout a given volume of bone.