Drifting osteons were followed longitudinally through the cortex of human a
nd baboon long bones using serial sections, Direction of transverse drift w
as recorded at different cross-sectional levels of the same systems, and ma
ximum angular change in drift direction was measured for each system. Most
drifting osteons exhibit: (1) substantial (similar to 90 degrees) variation
in the direction of transverse drift along their longitudinal axes, (2) in
termittent regions of concentric (type I) morphology, and (3) change in dri
ft direction over time, evident at single cross-sectional levels. Additiona
lly, 3-dimensional reconstruction reveals that the basic multicellular unit
s (BMUs) responsible for creating drifting osteons are morphologically dist
inct from the cutting-cone-closing-cone model BMUs that produce other types
of osteons. The stimulus involved in the activation and guidance of drifti
ng BMUs is unclear, but it is likely that the complex strain environment ex
perienced by long bone cortices exerts a significant influence on their mor
phology.