Investigation of the regulation of bone mass by mechanical loading: from quantitative cytochemistry to gene array

From the 1860s to the early 1980s, the process that fitted bone architecture and mass to function had been investigated and characterized. It was known that increases in exercise were associated with increased bone mass, and that disuse caused osteopaenia, but the mechanisms by which those processes were regulated was not understood. The idea that osteocytes, the cells embedded in bone, were sensitive to the effects of mechanical loading was attractive, yet there was almost no experimental support for it, at least in part because the cells were considered inaccessible for study. In 1984, the techniques devised by Chayen and his co-workers were focused on this area. By analysis of the activity of the enzyme glucose 6-phosphate dehydrogenase in osteocytes in sections of avian bone that had been subjected to brief periods of applied mechanical loading, we showed for the first time that osteocytes could respond within a few minutes to mechanical stimulation. The lack of elevation of activity of other glycolytic enzymes led to the conclusion that this elevation was due to increased activity of the pentose shunt pathway, which was likely to be associated with increased production of reducing equivalents for biosynthesis, and ribose sugars for RNA synthesis. This was the first demonstration of an ability of osteocytes to respond to an external mechanical event and in effect provided a mechanistic link for the fundamental principle of what is known as Wolff's law of bone remodelling. These studies were dependent on several technical advances brought together in the Chayen Cellular Biology Laboratory at the Kennedy Institute. The ability to make cryosections of undecalcified bone, to perform cytochemical analysis of (soluble) enzyme activities by use of colloid stabilizers in the reaction medium, and finally to measure accurately the coloured reaction products by microdensitometry (which avoided optical heterogeneity errors) combined to provide a powerful way to explore bone cell function in situ. In the intervening years since then, similar studies have become routine, and the impact of molecular biological advances in hard tissues have remained dependent on techniques pioneered in the Chayen laboratory. During such studies, other advances have spun off, so that osteocyte gene expression has been analysed in samples taken from sections where the precise tissue characteristics were known, leading to advances in understanding of intercellular signalling mechanisms in bone by differential display, and the role of apoptosis in osteocytes in regulation of osteoclastic resorption. Still more recently, materials extracted from undecalcified sections have been used in gene array studies to discover new candidate genes with a role in the adaptive mechanism. Without Joe Chayen's involvement in this area, which now impacts on almost all bone biological science either directly or indirectly, our understanding of the pathophysiology of osteoporosis would have been very different.