The morphology, structure and crystal growth of apatite crystals isolated from calcified turkey tendon and synthetic carbonated apatites have been examined using high resolution transmission electron microscopy. The biogenic apatite consisted of small (35 × 20 × 5 nm) platelike crystals. Despite their irregular shape and ill-defined edges, individual particles were single domain crystals. Lattice images recorded from isolated turkey tendon crystals indicated that the crystallographic oaxis (0001) of apatite lies in the plane of the pjate and parallel to the length of the crystallites. Lattice images suggested that the top face corresponds to the (1100) face of carbonated apatite. Lattice fringes observed in platelike crystallites viewed from the side corresponded to the projection of the apatite structure viewed along the  direction. Thus, it can be argued that crystal growth is constrained along the  direction, extends laterally along the  direction, and is maximal along the  direction. This latter direction is aligned with the collagen fiber axis. A mean length to width ratio (1.7) was determined by systematically measuring the maximum distances parallel and perpendicular to the c-axis identified from lattice images of the crystals. Similar information was obtained from lattice images of crystals located in collagen fibres. This confirmed that the morphological and structural features of isolated turkey tendon apatite crystals correlate directly with the in vivo crystallochemical characteristics of apatite. Crystals of synthetic carbonated apatite prepared at 37°C were also platelike and, although generally much larger, had length to width ratios comparable with the turkey tendon apatite. The synthetic carbonated apatites were noticeably more sensitive to radiolytic damage than the turkey tendon crystals. The crystallographic c-axis of the inorganic particles was aligned parallel with the long, physical axis of the plate and the top face was identified as (1100). Similar data were also obtained from non-carbonated synthetic apatite samples. The results of the present study offer critical information about the crystal growth of individual carbonated apatite crystals in calified turkey tendon and its relationship to the morphology of the crystallites. As similar growth characteristics are expressed in synthetic analogues, the data bring into question the putative regulatory role of the collagen-based matrix upon the nucleation and growth of biogenic apatite. It is suggested that the crystallographic co-alignment of the crystals and collagen fibers may be indicative of interactions between the organic matrix and the nascent crystal lattice during nucleation while the later growth of the crystals is regulated only by the imposition of spatial constraints rather than through specific matrix-crystal interactions.
Sparks, N., Heywood, BR., Shellis, R. P., Weiner, S., & Mann, S. (1990). Ultrastructure, Morphology and Crystal Growth of Biogenic and Synthetic Apatites. Connective Tissue Research, 25(2), 103-119. https://doi.org/10.3109/03008209009006985