Twisted plywood architecture of collagen fibrils in human compact bone osteons

Ultrathin sections of decalcified human compact bone, observed by transmission electron microscopy, reveal that collagen fibrils can be distributed in the form of a superimposed series of nested arcs. This characteristic pattern has never been interpreted in previous works on compact bone structure. We demonstrate, by goniometric observations at the ultrastructural level, that such series of nested arcs are a consequence of the "twisted plywood" architecture of collagen fibrils in the compact bone matrix. In the same specimens, an "orthogonal plywood" disposition of collagen fibrils is also observed; a transition exists between these two types of orders. We show that the "twisted plywood structure" accounts well for certain optical properties of osteons, observed in polarizing microscopy, described as "intermediate osteons." The particular geometry of collagen fibrils, leading to nested arcs in oblique sections, is analogous to the distribution of molecules in certain liquid crystals (called cholesteric liquid crystals). The principle of a liquid crystalline self-assembly of the collagen matrix in bone is therefore discussed.