Histomorphometric analysis of the effects of osteocyte density on osteonal morphology and remodeling

Osteocytes, the most abundant cells in the cortical bone matrix, are thought to have mechanosensory and chemosensory regulatory roles. Marotti theorized that osteocytes signal to osteoblasts to recruit them into the osteocyte lineage. Martin extended this theory, assuming that osteocytes display a general inhibitory effect on osteoblast function. The current study provides a quantitative analysis of the relationships between osteonal osteocyte density (Ot.N/BV), wall width (W.Wi), individual osteon porosity (IOP), and formation period (FP) in ulnar cortices from sheep labeled with tetracycline and calcein double labels. We postulated that osteocytes inhibit refilling so that the osteon wall width is thin enough, and the haversian canal is large enough, to allow adequate delivery of nutrients to the osteocytes throughout the forming and completed osteon. Therefore we tested the hypotheses that Ot.N/BV correlates negatively to FP and W.Wi, and positively to IOP, and that FP correlates positively with W.Wi. We found that Ot.N/BV correlated positively with IOP (P < 0.0001) and W.Wi correlated positively with FP (P < 0.0001). Significant negative correlations were observed between Ot.N/BV and both W.Wi (P < 0.0001) and FP (P = 0.006). These data support the general hypothesis that osteocytes contribute to the regulation of osteon morphology via the control of refilling rate and formation period, and the specific hypotheses that, for a given cement line diameter, high osteocyte density (1) reduces the rate of refilling and decreases the formation period and (2) decreases wall width and increases individual osteon porosity.