A Scanning Electron Microscope Study of the Ultrastructural Organization of Bone Mineral

Scanning electron microscopy (SEM) was used to study samples of lamellar bone at magnifications typical for the published transmission electron micrographs, to gain more insight into the three-dimensional ultrastructure of bone mineral. Untreated (whole bone) samples allowed an assessment of the degree of mineralization. Deproteinized samples revealed the ultrastructural form and organization of bone apatite to be a function of the extent to which collagen fibers were imbibed with mineral. Numerous parallel formations reminiscent of troughs, gutters, or furrows, pierced and traversed the mineralization front. These troughs showed varying diameters identifiable with collagen fibers, and were separated from one another by an elaborate system of thin platy septa and flakes. The troughs were interpreted as impressions of dissolved collagen fibers and bundles. Confluencing calcospherites were characterized by rod-shaped and fusiform mineral aggregates with diameters complying with collagen fibers and bundles. Mature mineral showed no plates; it was characterized by bundles of vermiform particles. It is suggested that in newly deposited bone mineral, mineral aggregates incompletely encrust collagen fibers and bundles, forming perifibrillar and interfibrillar plates and sheaths. During further mineralization these aggregates appears to coalesce to a continuous mineral phase.