Skeletal adaptation to mechanical stimuli in the absence of formation or resorption of bone.
Too often, unique loading environments fail to alter bone mass and morphology, calling to question the validity of Wolff's Law; the skeleton's plasticity to mechanical signals(1). We propose that bone can accommodate new loading environments without the need to form or resorb tissue, and that a critical aspect of bone tissue's ability to adapt to mechanical stimuli is first achieved via the plasticity of the osteocyte. We suggest that the osteocyte is capable of "normalizing" its local mechanical environment by modulating its cytoskeletal architecture, attachment to the matrix, configuration of the periosteocytic space, and communication channels to surrounding cells. We believe that through this local adaptive mechanism the osteocyte can accommodate the majority of changes in the mechanical milieu without altering the tissue architecture. It is only when bone tissue is subject to more severe (albeit rare) increases or decreases in the functional environment, the osteocyte participates in the formation and/or resorption of bone by coordinating site-specific recruitment of osteoblasts and/or osteoclasts. In vivo models of bone adaptation, combined with in situ reverse transcriptase-PCR, semi-quantitative RT-PCR, Northern analysis, immuno-cytochemistry and histomorphometry, can demonstrate how distinct mechanical stimuli influence the osteocyte's cytoskeletal and lacunar architecture, coupling (and uncoupling) of the osteocyte to the matrix and neighboring cells, and the osteocyte's participation in the recruitment and differentiation of osteoblasts and osteoclasts. Thus, the osteocyte controls three strategies to modulate its local and global environment in response to three distinct functional stimuli: 1) exogenous mechanical stimuli which are distinct from normal but sufficient to maintain bone mass, 2) mechanical stimuli which are osteogenic, and 3) disuse. If it is true that the resident cell population is capable of accommodating subtle changes in the functional milieu before modification of tissue morphology is deemed necessary, a novel strategy for the development of prophylaxes for osteopenia, osseointegration and fracture healing may become apparent.