The locus of mineral crystallites in bone

The organic content of mineralized tissues has been found to decrease with increasing tissue density, from about 60% of the mineral weight in light bone like deer antler to 1 to 2% in hyperdense bone like porpoise petrosal. The ratio of the weight of mineral that can fill the collagen hole zones to the total mineral content can be no greater than 20% for deer antler and decreases to less than 5% for hyperdense bone. Moreover, the dimensions of hydroxyapatite crystallites have been determined by various investigators to be larger than the intermolecular spacing of collagen molecules. Such crystallites can only be fitted within the collagen fibril if collagen molecules are packed differently from the accepted models. Electron micrographs of fish dentin, at a very early stage of mineralization, show the needle-like crystallites lying in dense strips between collagen fibrils and practically no crystallites within the fibrils. A similar pattern of dense strips of crystallites between fibrils can be identified in examples from more advanced stages of mineralization, taken from fish dentin, cat dentin and cow tibia, even though some of the needle-like crystallites are superimposed on the fibril banded pattern. In every instance there are regions of the fibrils where there are no visible needle-like crystallites. Examination of the work of others shows a similar distribution of the mineral component, except that none exactly resemble the micrograph of the earliest stage of fish dentin provided in this report. The collagen banding is observed to be in spatial phase over many fibrils. The needle-like crystallites may be observed to be bunched in phase with the collagen banding and with the same spatial periodicity. The bunching is most obvious in the least densely mineralized specimens. This observation can account for the x-ray and neutron diffraction patterns which shown the axial period of the mineral to be like that of the collagen axial macroperiod and to be in phase with the hole zones of collagen fibrils. These prior studies were interpreted to show that the crystallites must be within the hole zones. Our images are interpreted to show that most of the mineral is outside of the collagen fibrils in the extrafibrillar volume. The interpretation is in agreement with neutron diffraction studies of various mineralized tissues as well as with earlier diffraction studies of mineralized turkey leg tendon and with the calculations of the amount of mineral that can be contained within the collagen of mineralized tissue.