Osteocyte density and histomorphometric parameters in cancellous bone of the proximal femur in five mammalian species.

The species-specific relationships between trabecular morphology and osteocyte density were investigated in the femoral heads of 30 adult animals of five mammalian species (rat, rabbit, Rhesus monkey, pig, and cow). Our hypothesis is that osteocytes are mechanosensory cells and are involved in the regulation of bone remodeling. According to the predictions from a simulation model, this hypothesis implies that the influencing distance of osteocytes, together with the magnitude of the mechanical loads, determines the thickness of trabeculae and that the number of osteocytes primarily affects the rate of bone remodeling. The number of osteocytes per bone volume ranged from 93,200 mm-3 in rat to 31,900 mm-3 in bovine cancellous bone. Osteocyte density was inversely related to the size of the species. Since basal metabolic output is related to body mass, we speculate that osteocyte density may be related to metabolic rates. Trabecular thickness was larger in the cow than in the other species, but the range of variation between species was relatively small. This agrees with the hypothesis that trabecular thickness is limited by the domain that can be regulated by an osteocyte and that this domain is of similar size regardless of the species. Only in the rat was trabecular thickness considerably smaller than in the other species. This is probably due to the presence of the cartilaginous growth plate in the femoral head of the rat. The relationships with species are different for osteocyte density than for morphometric parameters. Hence, our data support our hypothesis that osteocyte density is not directly associated with the macroscopic trabecular architecture.