Different fibrillar architectures coexisting in Haversian bone.

Samples of compact bone were deproteinated by heat treatment and analysed by SEM. This technique removes very effectively cells and vascular structures and brings into full view the mineralization front along the wall of the Havers canal. The present study was confined to samples of equine bone that are known to be subjected to different functional requirements. Bone subjected to high tensile stress exhibited collagen fibrils substantially aligned with the stress direction, and the vast majority of its osteons appeared made of almost parallel fibrils crossing at very narrow angles. On the contrary, bone subjected to prevalent compressive forces showed either an orthogonal alternation of collagen lamellae, or a multidirectional arrangement corresponding to the twisted plywood described by other authors. Our observations substantiate the classical concept of the osteon structure as well as the twisted plywood; they indicate that several osteon architectures coexist in the same specimen, and that the different structures have a different preferential distribution in different parts of the same bone; and suggest that the relative distribution of the osteon ultrastructure across the bone matrix is modulated by mechanical factors.