Relationship between ultrastructure and "pin test" in osteons.

In the 3 types commonly recognizable under the polarizing microscope (longitudinally, alternately and transversally structured osteons), the lamellar appearance of osteons in electron micrographs (EM) is due to the presence of layers of paralleloriented collagen bundles, with the orientation of the bundles changing abruptly through about 90 degrees in successive layers. On the EM scale, lamellae appear as completely independent entities. All 3 osteon types can be considered as consisting of a series of complete or incomplete lamellae. The quantities of collagen bundles in each of the 2 types of lamella--those with longitudinally or concentrically oriented bundles--is a distinctive feature determining the specific appearance of each type of osteon. This view has interesting implications as regards the nature of the factors which instruct and regulate osteoblasts in determining the cross-parallel arrangement of lamellar bone, and the specific characteristics of the various types of osteon. The mechanical properties of osteons are closely related to their structure. The highest values for ultimate dilating strength were obtained with transversally and alternately structured osteons, and the lowest values with longitudinally structured ones.