Growth of mineral crystals in turkey tendon collagen fibers.

Bone and several other vertebrate mineralized tissues are formed by the organized growth of crystals of carbonated apatite within a matrix of type 1 collagen fibers. The development of this process in isolated fibrils of young turkey leg tendons has been studied by transmission electron microscopy. Collagen banding, presumably due to ion concentration, precedes the appearance of any crystals. The smallest crystals observed are short needles in bands near the surface of the fibrils. Longer needles, up to the length of the collagen gap regions, were also seen, and, evidently at a later stage, single crystal belts extending partly or wholly through the fibrils. Finally, in mature tendon crystal platelets, seemingly derived from the cracking of belts, extend partly into the collagen overlap zone. In the least mineralized tendon, extrafibrillar mineral-containing vesicles have occasionally been observed adjacent to regions of radiating needle crystal growth in the fibrils, and, more commonly, smaller particles adjacent to bands of very small needles.