A continuous model of osteocyte generation in bone matrix
Author: P R Buenzli
The formation of new bone involves both the deposition of bone matrix by cells called osteoblasts, and the formation of a network of cells embedded within the bone matrix, called osteocytes. Osteocytes derive from osteoblasts that become buried in bone matrix during bone deposition. There has been a growing interest in osteocytes in recent years with the realisation that these cells are essential to the detection of micro-damage in bone, and that they participate in the orchestration of local bone renewal. However, the generation of osteocytes is a complex process that remains incompletely understood. Whilst osteoblast burial determines the density of osteocytes, the expanding network of osteocytes regulates in turn osteoblast activity and osteoblast burial through their interconnected cell processes. In this contribution, a spatiotemporal continuous model is proposed to investigate the osteoblast-to- osteocyte transition. The model elucidates the interplays between matrix secretory rate, rate of entrapment, and curvature of the bone substrate in determining the density of osteocytes in the new bone matrix. We find that the density of osteocytes generated at the moving deposition front depends solely on the ratio of the instantaneous burial rate and matrix secretory rate. It is remarkably independent of osteoblast density and substrate curvature. This mathematical result is used with experimental measurements of osteocyte lacuna distributions in a human cortical bone sample to determine for the first time the rate of burial of osteoblasts in bone matrix. Our results suggest that in the bone specimen analysed: (i) burial rate decreases during osteonal infilling, and (ii) the control of osteoblast burial by osteocytes is likely to emanate as a collective signal from a large group of osteocytes, rather than from the osteocytes closest to the bone deposition front.