Morphometry and patterns of lamellar bone in human Haversian systems.

The lamellar architecture of secondary osteons (Haversian systems) has been studied with scanning electron microscopy (SEM) in transverse sections of human cortical bone. Na(3) PO(4) etching was used to improve the resolution of the interface between neighboring lamellae and the precision of measurements. These technical improvements permitted testing of earlier morphometry assumptions concerning lamellar thickness while revealing the existence of different lamellar patterns. The mean lamellar thickness was 9.0 ± 2.13 µm, thicker and with a wider range of variation with respect to earlier measurements. The number of lamellae showed a direct correlation with the lamellar bone area, and their thickness had a random distribution for osteonal size classes. The circular, concentrical pattern was the more frequently observed, but spiral and crescent-moon-shaped lamellae were also documented. Selected osteons were examined by either SEM or SEM combined with polarized light microscopy allowing comparisons of corresponding sectors of the osteon. The bright bands observed with polarized light corresponded to the grooves observed in etched sections by SEM. The dark bands corresponded to the lamellar surface with the cut fibrils oriented approximately longitudinally along the central canal axis. However, lamellae with large and blurred bright bands could be observed, which did not correspond to a groove observed by SEM. These findings are in contrast with the assumption that all the fibril layers within a lamella are oriented along a constant and unchangeable angle. The different lamellar patterns may be explained by the synchronous or staggered recruitment and activation of osteoblasts committed to the osteon's completion.

A model of osteoblast-osteocyte kinetics in the development of secondary osteons in rabbits.

The kinetics of osteogenic cells within secondary osteons have been examined within a 2-D model. The linear osteoblast density of the osteons and the osteocyte lacunae density were compared with other endosteal lamellar systems of different geometries. The cell density was significantly greater in the endosteal appositional zone and was always flatter than the central osteonal canals. Fully structured osteons compared with early structuring (cutting cones) did not show any significant differences in density. The osteoblast density may remain constant because some of them leave the row and become embedded within matrix. The overall shape of the Haversian system represented a geometrical restraint and it was thought to be related to osteoblast-osteocyte transformation. To test this hypothesis of an early differentiation and recruitment of the osteoblast pool which completes the lamellar structure of the osteon, the number and density of osteoblasts and osteocyte lacunae were evaluated. In the central canal area, the mean osteoblast linear density and the osteocyte lacunae planar density were not significantly different among sub-classes (with the exclusion of the osteocyte lacunae of the 300-1000 µm(2) sub-class). The mean number of osteoblasts compared with osteocyte lacunae resulted in significantly higher numbers in the two sub-classes, no significant difference was seen in the two middle sub-classes with the larger canals, and there were significantly lower levels in the smallest central canal sub-class. The TUNEL technique was used to identify the morphological features of apoptosis within osteoblasts. It was found that apoptosis occurred during the late phase of osteon formation but not in osteocytes. This suggests a regulatory role of apoptosis in balancing the osteoblast-osteocyte equilibrium within secondary osteon development. The position of the osteocytic lacunae did not correlate with the lamellar pattern and the lacunae density in osteonal radial sectors was not significantly different. These findings support the hypothesis of an early differentiation of the osteoblast pool and the independence of the fibrillar lamellation from osteoblast-osteocyte transformation.