Osteocyte density in woven bone.

Woven bone forms rapidly during tissue growth, following injury and in response to certain anabolic stimuli. Functional differences between woven and lamellar bone may be due, in part, to differences in osteocyte density (cells per unit tissue). Woven bone has been estimated to contain four to eight times more osteocytes than lamellar bone, although primary data to support this assertion are limited. Given recent findings implicating osteocytes as regulators of bone remodeling, bone formation and bone volume, such large differences in osteocyte density between woven and lamellar bone may have important consequences. In this study, we compared the density of osteocyte lacunae (lacunae/mm(2) tissue) in rat lamellar bone with that in woven bone formed under several different circumstances. We found that the lacunar density of lamellar cortical bone in the rat (834+/-83 cells/mm2, mean+/-SD) did not differ significantly from that of periosteal woven bone formed via intramembranous osteogenesis, either in response to mechanical loading (921+/-204 cells/mm2) or in the periosteal buttressing region of the fracture callus (1138+/-168 cells/mm2). In contrast, lacunar density of endochondrally derived woven bone in the center (gap) region of fracture callus was nearly 100% greater (1875+/-270 cells/mm2) than in lamellar cortical bone while lacunar density of primary spongiosa of the growth plate was 40% greater (1674+/-228 cells/mm2) than that in lamellar cancellous bone (1189+/-164). These findings demonstrate that lacunar density in woven bone varies depending on skeletal site and developmental history and appears to be elevated in endochondrally derived woven bone adjacent to marrow space. Given the considerable evidence supporting osteocytes as local initiators of bone remodeling, we suggest that woven bone with increased lacunar density may undergo remodeling at an accelerated rate.