Osteocytes looking upwards (OR bone structure watched from below)

Abstract

The mechanistic investigation of the complex molecular and cellular pathways that underlie the manifestations of all biological systems is plausible; however, such systems are better understood - especially by clinicians - when they are studied at the highest possible (ideally, individual) level of structural organization. Bones are born, grow and dead biomechanically. This way they will be described and interpreted in this article, as needed to understand the mechanisms by which many weakening diseases affect their behavior as struts or levers.
The unusual aim of this article is to review the current knowledge on osteocytes as sensitive headquarters of the mechanisms which seem to govern the whole-bone (structural) stiffness, toughness and strength at the organ level of integration. Thus, use is made of creative suppositions, risky hypotheses, and unavoidable paradigms, in the hope to help understand the pathophysiology and the real nature of all bone-weakening diseases from a dynamic point of view, close to the clinical and therapeutic interest of the osteologists. No less than three distinctive features of this article support that philosophy: 1) Osteocytes are regarded as parts of a system organized at the cell, tissue and organ levels of biological complexity, which involves the supporting structure of an ideal bone - mineralized bone matrix, vessels and nerves, intrinsic (osteocytes) and extrinsic (blasts, clasts) cells and their precursors -, integrated in a vertebrates skeleton at the systemic and individual levels, and subjected to both mechanical and metabolic interactions; 2) The leading paradigm proposes that all living supporting structures are biologically regulated, in such a way that the stresses and strains provoked by the maximal loads they usually stand result much lesser than those that will break them (i.e. maintaining reasonable safety factors); 3) The organization of the proposed structure is based on a feedback mechanism that would control its stiffness at both tissue and organ levels of complexity, of which osteocytes (its cellular units) will be the sensor elements. Authors hope this revision - which they pretend to have supported with a reasonable amount of original evidences will help osteologists to get familiar with bone structural and biomechanical analysis, to understand, diagnose, treat, and monitor all bone-weakening condition as resulting from changes in the true, structural determinants of bone strength.