Rotated plywood structure of primary lamellar bone in the rat: orientations of the collagen fibril arrays.

A basic structural motif of lamellar bone is the arrays of parallel collagen fibrils, with successive arrays having different orientations to form a plywood-like structure. Measurements of the angles between adjacent arrays from cryomicrotomed and vitrified thin sections of demineralized rat bone, cut approximately parallel to the lamellar boundary plane, show that most angles are around 30 degrees, although a subset are around 70 degrees. A structural model for collagen organization based on these measurements is proposed in which an individual lamellar unit (thick and thin lamellae together with transition zones) is composed of five arrays of parallel collagen fibrils, each offset by 30 degrees.